Geominerals/Silicates

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Crystals of perovskite occur on a matrix. Credit: Robert M. Lavinsky.{{free media}}
An optical image of the Tenham L6 chondrite in thin section USNM 7703 where bridgmanite is identified in a shock vein by the arrow. Credit: Oliver Tschauner, Chi Ma, John R. Beckett, Clemens Prescher, Vitali B. Prakapenka, George R. Rossman.{{fairuse}}

The geominerals of silicates is an effort to determine which silicates are on Earth and the geochemical reason why from a thermodynamics perspective.

Silicate perovskite is either (Mg,Fe)SiO
3
(the magnesium end-member is called bridgmanite[1]) or CaSiO
3
(calcium silicate) when arranged in a perovskite structure. Silicate perovskites are not stable at Earth's surface, and mainly exist in the lower part of Earth's mantle, between about 670 and 2,700 km (420 and 1,680 mi) depth. They are thought to form the main mineral phases, together with ferropericlase.

The existence of silicate perovskite in the mantle was first suggested in 1962, and both MgSiO
3
and CaSiO
3
had been synthesized experimentally before 1975. By the late 1970s, it had been proposed that the seismic discontinuity at about 660 km in the mantle represented a change from spinel structure minerals with an olivine composition to silicate perovskite with ferropericlase.

Natural silicate perovskite was discovered in the heavily shocked Tenham meteorite.[2][3] In 2014, the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA) approved the name bridgmanite for perovskite-structured (Mg,Fe)SiO
3
,[1] in honor of physicist Percy Williams Bridgman, who was awarded the Nobel Prize in Physics in 1946 for his high-pressure research.[4]

The perovskite structure (first identified in the mineral perovskite occurs in substances with the general formula ABX
3
, where A is a metal that forms large cations, typically magnesium, ferrous iron, or calcium. B is another metal that forms smaller cations, typically silicon, although minor amounts of ferric iron and aluminum can occur. X is typically oxygen. The structure may be cubic, but only if the relative sizes of the ions meet strict criteria. Typically, substances with the perovskite structure show lower symmetry, owing to the distortion of the crystal lattice and silicate perovskites are in the orthorhombic crystal system.[5]

Bridgmanite is a high-pressure polymorph of enstatite, but in the Earth predominantly forms, along with ferropericlase, from the decomposition of ringwoodite (a high-pressure form of olivine) at approximately 660 km depth, or a pressure of ~24 GPa.[5][6] The depth of this transition depends on the mantle temperature; it occurs slightly deeper in colder regions of the mantle and shallower in warmer regions.[7] The transition from ringwoodite to bridgmanite and ferropericlase marks the bottom of the mantle transition zone and the top of the lower mantle. Bridgmanite becomes unstable at a depth of approximately 2700 km, transforming isochemically to post-perovskite.[8]

Calcium silicate perovskite is stable at slightly shallower depths than bridgmanite, becoming stable at approximately 500 km, and remains stable throughout the lower mantle.[8]

Bridgmanite is the most abundant mineral in the mantle. The proportions of bridgmanite and calcium perovskite depends on the overall lithology and bulk composition. In pyrolitic and harzburgitic lithogies, bridgmanite constitutes around 80% of the mineral assemblage, and calcium perovskite < 10%. In an eclogitic lithology, bridgmanite and calcium perovskite comprise ~30% each.[8]

Calcium silicate perovskite has been identified at Earth's surface as inclusions in diamonds.[9] The diamonds are formed under high pressure deep in the mantle. With the great mechanical strength of the diamonds a large part of this pressure is retained inside the lattice, enabling inclusions such as the calcium silicate to be preserved in high-pressure form.

Experimental deformation of polycrystalline MgSiO
3
under the conditions of the uppermost part of the lower mantle suggests that silicate perovskite deforms by a dislocation creep mechanism. This may help explain the observed seismic anisotropy in the mantle.[10]

Cyclosilicates[edit | edit source]

Def. any group of silicates that have a ring of linked tetrahedra is called a cyclosilicate.

Abenakiite-(Ce)[edit | edit source]

Abenakiite-(Ce) is a cyclosilicate. Credit: Salah Rashad Zaqzoq.{{free media}}

Abenakiite-(Ce) has the chemical formula Na
26
Ce
6
(SiO
3
)
6
(PO
4
)
6
(CO
3
)
6
(S4+
O
)O
. Abenakiite-(Ce) (IMA1991-054; IMA Symbol Abk-Ce[11]) is a mineral of sodium, cerium, neodymium, lanthanum, praseodymium, thorium, samarium, oxygen, sulfur, carbon, phosphorus, and silicon. The silicate groups may be given as the cyclic Si
6
O
18
grouping. Its Mohs scale rating is 4 to 5.[12]

Abenakiite-(Ce) was discovered in a sodalite syenite xenolith at Mont Saint-Hilaire, Québec, Canada, together with aegirine, eudialyte, manganoneptunite, polylithionite, serandite, and steenstrupine-(Ce).[12][13]

Combination of elements in abenakiite-(Ce) is unique. Somewhat chemically similar mineral is steenstrupine-(Ce).[13][14] The hyper-sodium abenakiite-(Ce) is also unique in supposed presence of sulfur dioxide ligand. With a single grain (originally) found, abenakiite-(Ce) is extremely rare.[12]

In the crystal structure, described as a hexagonal net, of abenakiite-(Ce) there are:[12]

  • chains of NaO
    7
    polyhedra, connected with PO
    4
    groups
  • columns with six-membered rings of NaO
    7
    , and NaO
    7
    -REEO
    6
    , and SiO
    4
    polyhedra (REE - rare earth elements)
  • CO
    3
    groups, NaO
    6
    octahedra, and disordered SO
    2
    ligands within the columns

Beryls[edit | edit source]

Colorless beryl, a cyclosilicate, is called goshenite. Credit: Piotr Menducki
Photograph is of a dioptase specimen from Renéville Republic of Congo. Credit: Didier Descouens.
Aquamarine is a blue or turquoise variety of beryl. Credit: .

"Beryl of various colors is found most commonly in granitic pegmatites, but also occurs in mica schists ... Goshenite [a beryl clear to white cyclosilicate] is found to some extent in almost all beryl localities."[15]

The gem-gravel placer deposits of Sri Lanka contain aquamarine.

The deep blue version of aquamarine is called maxixe. Maxixe is commonly found in the country of Madagascar. Its color fades to white when exposed to sunlight or is subjected to heat treatment, though the color returns with irradiation.

The pale blue color of aquamarine is attributed to Fe2+. The Fe3+ ions produce golden-yellow color, and when both Fe2+ and Fe3+ are present, the color is a darker blue as in maxixe. Decoloration of maxixe by light or heat thus may be due to the charge transfer Fe3+ and Fe2+.[16][17][18][19] Dark-blue maxixe color can be produced in green, pink or yellow beryl by irradiating it with high-energy particles (gamma rays, neutrons or even X-rays).[20]

Breyites[edit | edit source]

Breyite has the chemical formula Ca
3
Si
3
O
9
, crystallizes in the triclinic system, is a member of the margarosanite group of minerals, is a polymorph of pseudowollastonite and Wollastonite, and isostructural with margarosanite and walstromite.[21]

Breyite is a cyclosilicate.[22]

Eudialytes[edit | edit source]

Eudialyte is a somewhat rare, red silicate mineral. Credit: Baryonyx.

Eudialyte is a somewhat rare, red silicate mineral, which forms in alkaline igneous rocks, such as nepheline syenites.

Margarosanites[edit | edit source]

Walstromites[edit | edit source]

Inosilicates[edit | edit source]

Metasilicates[edit | edit source]

Def. the "oxyanion of silicon SiO32- or any salt or mineral containing this ion"[23] is called a metasilicate.

Pyroxferroites[edit | edit source]

These black crystals of pyroxferroite are from the famous Eifel quarry field, Bellerberg, Ettringen, Eifel Mountains, Germany. Credit: Giuseppe Siccardi.{{fairuse|permission:Giuseppe Siccardi}}

The pyroxferroite crystals in the image on the right are 0.6 x 1.1 x 0.7 cm in dimensions.

Raites[edit | edit source]

"This hydrated sodium-manganese silicate [raite] extends the already wide range of manganese crystal chemistry (3), which includes various complex oxides in ore deposits and nodules from the sea floor and certain farming areas, the pyroxmangite analog of the lunar volcanic metasilicate pyroxferroite, the Mn analog yofortierite of the clay mineral palygorskite, and the unnamed Mn analog of sepiolite."[24]

Wollastonites[edit | edit source]

Wollastonite is from the USGS and the Minerals Information Institute. Credit: Jeff Evarts.{{free media}}
White acicular crystals of wollastonite (field of view 8 mm) are from the Central Bohemia Region, Czech Republic. Credit: Leon Hupperichs.{{free media}}
Wollastonite skarn is with diopside (green), andradite garnet (red) and vesuvianite (dark brown) from the Stanisław mine near Szklarska Poręba, Izerskie Mountains, Lower Silesia, Poland. Credit: Piotr Sosnowski.{{free media}}
Unit cell is of triclinic wollastonite-1A. Credit: Solid State.{{free media}}
Tetrahedra arrangement within the chains in pyroxenes is compared to wollastonite. Credit: Solid State.{{free media}}

Wollastonite is a calcium metasilicate with the formula CaSiO
3
.

Wollastonite may contain small amounts of iron, magnesium, and manganese substituting for calcium that is usually white, forms when impure limestone or dolomite is subjected to high temperature and pressure, which sometimes occurs in the presence of silica-bearing fluids as in skarns,[25] or in contact with metamorphic rocks, named after the English chemist and mineralogist William Hyde Wollaston (1766–1828).

Despite its chemical similarity to the compositional spectrum of the pyroxene group of minerals—where magnesium and iron substitution for calcium ends with diopside and hedenbergite respectively—it is structurally very different, with a third SiO
4
tetrahedron[26] in the linked chain (as opposed to two in the pyroxenes).

In a pure CaSiO
3
, each component forms nearly half of the mineral by weight: 48.3% of CaO and 51.7% of SiO
2
. In some cases, small amounts of iron (Fe), and manganese (Mn), and lesser amounts of magnesium (Mg) substitute for calcium (Ca) in the mineral formula (e.g., rhodonit]).[27] Wollastonite can form a series of solid solutions in the system CaSiO
3
-FeSiO
3
, or hydrothermal synthesis of phases in the system MnSiO
3
-CaSiO
3
.[26]

Wollastonite usually occurs as a common constituent of a thermally metamorphosed impure limestone, it also could occur when the silicon is due to metamorphism in contact altered calcareous sediments, or to contamination in the invading igneous rock. In most of these occurrences it is the result of the following reaction between calcite and silica with the loss of carbon dioxide:[26]

CaCO3 + SiO2 → CaSiO3 + CO2

Wollastonite may also be produced in a diffusion reaction in skarn, it develops when limestone within a sandstone is metamorphosed by a dike, which results in the formation of wollastonite in the sandstone as a result of outward migration of Ca.[26]

Associated minerals: garnets, vesuvianite, diopside, tremolite, epidote, plagioclase feldspar, pyroxene and calcite. It is named after the English chemist and mineralogist William Hyde Wollaston (1766–1828).

Wollastonite crystallizes triclinically in space group P1 with the lattice constants a = 7.94 Å, b = 7.32 Å, c = 7.07 Å; α = 90,03°, β = 95,37°, γ = 103,43° and six formula units per unit cell.[28] Wollastonite was once classed structurally among the pyroxene group, because both of these groups have a ratio of Si:O = 1:3. In 1931, Warren and Biscoe showed that the crystal structure of wollastonite differs from minerals of the pyroxene group, and they classified this mineral within a group known as the pyroxenoids.[26] It has been shown that the pyroxenoid chains are more kinked than those of pyroxene group, and exhibit longer repeat distance. The structure of wollastonite contains infinite chains of [SiO
4
] tetrahedra sharing common vertices, running parallel to the b-axis. The chain motif in wollastonite repeats after three tetrahedra, whereas in pyroxenes only two are needed. The repeat distance in the wollastonite chains is 7.32 Å and equals the length of the crystallographic b-axis.

Molten CaSiO3, maintains a tetrahedral SiO4 local structure, at temperatures up to 2000 ˚C.[29] The nearest neighbour Ca-O coordination decreases from 6.0(2) in the room temperature glass to 5.0(2) in the 1700 ˚C liquid, coincident with an increasing number of longer Ca-O neighbors.[30][31]

"Primary silicate–melt and carbonate–salt inclusions occur in the phenocrysts (nepheline, fluorapatite, wollastonite, clinopyroxene) in the 1917 eruption combeite–wollastonite nephelinite at Oldoinyo Lengai."[32]

Large deposits of wollastonite have been identified in China, Finland, India, Mexico, and the United States. Smaller, but significant, deposits have been identified in Canada, Chile, Kenya, Namibia, South Africa, Spain, Sudan, Tajikistan, Turkey, and Uzbekistan.[33]

Nesosilicates[edit | edit source]

Orthosilicates[edit | edit source]

Phyllosilicates[edit | edit source]

Def. any "silicate mineral having a crystal structure of parallel sheets of silicate tetrahedra"[34] is called a phyllosilicate.

Ajoites[edit | edit source]

Bluish ajoite inclusion is in a colourless quartz crystal. Credit: Robert M. Lavinsky.{{free media}}
Ajoite is in quartz from Messina District, South Africa (size: 16.9 x 3.7 x 3 cm.) Credit: Robert M. Lavinsky.{{free media}}

Ajoite has the chemical formula (Na,K)Cu
7
AlSi
9
O
24
(OH)
6
·3H
2
O
,[35] and minor Mn, Fe and Ca are usually also present in the structure.[36] Ajoite is used as a minor ore of copper.

Ajoite (International Mineralogical Association (IMA) symbol Aj[11]) is a hydrated sodium potassium copper aluminium silicate hydroxide mineral.

Ajoite is a secondary mineral that forms from the oxidation of other secondary copper minerals in copper-rich base metal deposits in massive fracture coatings, in vein fillings, and in vugs. It may form from shattuckite and also it may be replaced by shattuckite.[36]

At the type locality it is associated with shattuckite, conichalcite, quartz, muscovite and pyrite.[37][35]

Ajoite is named after its type locality, the New Cornelia Mine in the Ajo District of Pima County, Arizona. Type specimen material is conserved at the National Museum of Natural History, Washington DC, USA, reference number 113220.

Other localities include Wickenburg and Maricopa County, Arizona, within the United States, and the Messina (Musina) District in South Africa. Quartz specimens from the defunct Messina Mines on the border between Zimbabwe and South Africa are well known for their inclusions of blue copper silicate minerals such as shattuckite, papagoite and ajoite,[38] but ajoite from American localities does not occur like this.

Biotites[edit | edit source]

Biotite is a black phyllosilicate mineral. Credit: United States Geological Survey and the Mineral Information Institute.

Biotite has the chemical formula "K(Mg, Fe)3(Al, Fe)Si3O10(OH, F)2".[39]

Def. a "dark brown mica; it is a mixed aluminosilicate and fluoride of potassium, magnesium and iron"[40] is called a biotite.

Garnierites[edit | edit source]

Garnierite is a general term for hydrous nickel silicates. Credit: Didier Descouens.{{free media}}

Chemical analysis of garnierite samples yields non-stoichiometric formulae that can be reduced to formulas like those of talc and serpentine suggesting a talc monohydrate formula of H
2
O
(Mg,Ni)
3
Si
4
O
10
(OH)
2
for the talc-like garnierite.[41]

The main difference between the serpentine-like and talc-like variants of garnierite is the spacing between layers in the structure, seen in x-ray powder diffraction studies. The serpentine-like variants have 7 Å basal spacings while the talc-like variants have a basal spacing of 10 Å.[41]

Garnierite is a layer silicate.[41]

7 Å type garnierites usually resemble chrysotile or lizardite in their structures, while 10 Å types usually resemble pimelite.[41]

The color comes from the presence of nickel in the mineral structure for magnesium.[41]

Kaolinites[edit | edit source]

Kaolin is a white phyllosilicate. Credit: USGS and the Minerals Information Institute.
Kaolinite is from Twiggs County, Georgia, USA. Credit: James St. John.{{free media}}

Kaolinite has the chemical formula Al
2
Si
2
O
5
(OH)
4
.

Kaolinite is a clay mineral, a layered silicate mineral, with one tetrahedral sheet of silica (SiO
4
) linked through oxygen atoms to one octahedral sheet of alumina (AlO
6
) octahedra.[26] Rocks that are rich in kaolinite are known as kaolin or porcelain (china) clay.[42]

The chemical formula for kaolinite as used in mineralogy is Al
2
Si
2
O
5
(OH)
4
,[43] however, in ceramics applications the formula is typically written in terms of oxides, thus the formula for kaolinite is Al
2
O
3
*2SiO
2
*2H
2
O
.[44]

As the most numerous element is oxygen at 9 for 52.9 at %, kaolin is an oxide.

Micas[edit | edit source]

Here is mica in a rock. Credit: Rpervinking.{{free media}}

Def. a group of monoclinic phyllosilicates with the general formula[45]

X2Y4–6Z8O20(OH,F)4
in which X is K, Na, or Ca or less commonly Ba, Rb, or Cs;
Y is Al, Mg, or Fe or less commonly Mn, Cr, Ti, Li, etc.;
Z is chiefly Si or Al, but also may include Fe3+ or Ti;
dioctahedral (Y = 4) and trioctahedral (Y = 6)

is called a mica.

Muscovites[edit | edit source]

The Nikon image shows discrete flakes of muscovite. Credit: Luis Miguel Bugallo Sánchez.{{free media}}

Def. a "pale brown mineral of the mica group, being a basic potassium aluminosilicate[46] with the chemical formula KAl2(Si3Al)O10(OH],F)2"[47] is called a muscovite.

"The strongly peraluminous tuffs contain phenocrysts of andalusite, sillimanite, and muscovite and have high 87Sr/86Sri (0.7258 and 0.7226) and δ18O (+11‰). Elevated concentrations of Li, Cs, Be, Sn, B, and other minor elements compare with those in “tin granites.”"[48]

Serpentines[edit | edit source]

Serpentine is from Poland. Credit: Kluka.{{free media}}
A sample of serpentinite rock is partially made up of chrysotile, from Slovakia. Credit: Helix84.{{free media}}
A rock of serpentinite is from the Maurienne valley, Savoie, French Alps. Credit: Gabriel HM.{{free media}}
Sample of serpentinite from the Golden Gate National Recreation Area, California, United States. Credit: Zimbres|Eurico Zimbres.{{free media}}
Chromitic serpentinite (7.9 cm (3.1 in) across), Styria Province, Austria. Protolith was a Proterozoic-Early Paleozoic upper mantle dunite peridotite that has been multiply metamorphosed during the Devonian, Permian, and Mesozoic. Credit: James St. John.{{free media}}
Tightly folded serpentinite is from the Tux Alps, Austria, closeup view about 30 cm × 20 cm (11.8 in × 7.9 in). Credit: Hermann Hammer.{{free media}}

The serpentine subgroup (part of the kaolinite-serpentine group)[49] are greenish, brownish, or spotted minerals commonly found in serpentinite rocks, used as a source of magnesium and asbestos, and as a decorative stone.[50] The name is thought to come from the greenish color being that of a serpent.[51]

The serpentine subgroup are a set of common rock-forming hydroxyl magnesium iron phyllosilicates (Mg,Fe)
3
Si
2
O
5
(OH)
4
minerals, resulting from the metamorphism of the minerals that are contained in ultramafic rocks.[52] They may contain minor amounts of other elements including chromium, manganese, cobalt or nickel. In mineralogy and gemology, serpentine may refer to any of 20 varieties belonging to the serpentine subgroup. Owing to admixture, these varieties are not always easy to individualize, and distinctions are not usually made. There are three important mineral polymorphs of serpentine: antigorite, chrysotile and lizardite.

The serpentine subgroup of minerals are polymorphous, meaning that they have the same chemical formulae, but the atoms are arranged into different structures, or crystal lattices.[53] Chrysotile, which has a fiberous habit, is one polymorph of serpentine and is one of the more important asbestos minerals. Other polymorphs in the serpentine subgroup may have a platy habit. Antigorite and lizardite are the polymorphs with platy habit.

Serpentinization is a form of low-temperature metamorphism of ultramafic rocks, such as dunite, harzburgite, or lherzolite. These are rocks low in silica and composed mostly of olivine, pyroxene, and chromite. Serpentinization is driven largely by hydration and oxidation of olivine and pyroxene to serpentine minerals, brucite, and magnetite.[54] Under the unusual chemical conditions accompanying serpentinization, water is the oxidizing agent, and is itself reduced to hydrogen, H
2
. This leads to further reactions that produce rare iron group native element minerals, such as awaruite (Ni
3
Fe
) and native iron;[55] methane and other hydrocarbon compounds; and hydrogen sulfide.[56]

During serpentinization, large amounts of water are absorbed into the rock, increasing the volume, reducing the density and destroying the original structure.[57][58] The density changes from 3.3 to 2.5 g/cm3 (0.119 to 0.090 lb/cu in) with a concurrent volume increase on the order of 30-40%.[59] The reaction is highly exothermic and rock temperatures can be raised by about 260 °C (500 °F),[58] providing an energy source for formation of non-volcanic hydrothermal vents.[60] The hydrogen, methane, and hydrogen sulfide produced during serpentinization are released at these vents and provide energy sources for deep sea chemotroph microorganisms.[61][58]

The final mineral composition of serpentinite is usually dominated by lizardite, chrysotile, and magnetite. Brucite and antigorite are less commonly present. Lizardite, chrysotile, and antigorite are serpentine minerals. Accessory minerals, present in small quantities, include awaruite, other native metal minerals, and sulfide minerals.[62]

Olivine is a solid solution of forsterite, the magnesium-endmember, and fayalite, the iron-endmember, with forsterite typically making up about 90% of the olivine in ultramafic rocks.[63] Serpentinite can form from olivine via several reactions:

Forsterite3 Mg
2
SiO
4
+ silicon dioxideSiO
2
+ 4 H
2
O
serpentine2 Mg
3
Si
2
O
5
(OH)
4

 

 

 

 

(Reaction 1a)

Forsterite2 Mg
2
SiO
4
+ water3 H
2
O
serpentineMg
3
Si
2
O
5
(OH)
4
+ bruciteMg(OH)
2

 

 

 

 

(Reaction 1b)

Reaction 1a tightly binds silica, lowering its chemical activity to the lowest values seen in common rocks of the Earth's crust.[64] Serpentinization then continues through the hydration of olivine to yield serpentine and brucite (Reaction 1b).[65] The mixture of brucite and serpentine formed by Reaction 1b has the lowest silica activity in the serpentinite, so that the brucite phase is very important in understanding serpentinization.[64] However, the brucite is often blended in with the serpentine such that it is difficult to identify except with X-ray diffraction, and it is easily altered under surface weathering conditions.[66]

A similar suite of reactions involves pyroxene-group minerals:

Enstatite3 MgSiO
3
+ silicon dioxideSiO
2
+ H
2
O
talcMg
3
Si
4
O
10
(OH)
2

 

 

 

 

(Reaction 2a)

Enstatite6 MgSiO
3
+ 3 H
2
O
serpentineMg
3
Si
2
O
5
(OH)
4
+ talcMg
3
Si
4
O
10
(OH)
2

 

 

 

 

(Reaction 2b)

Reaction 2a quickly comes to a halt as silica becomes unavailable, and Reaction 2b takes over.[67] When olivine is abundant, silica activity drops low enough that talc begins to react with olivine:

Forsterite6 Mg
2
SiO
4
+ talcMg
3
Si
4
O
10
(OH)
2
+ water9 H
2
O
serpentine5 Mg
3
Si
2
O
5
(OH)
4

 

 

 

 

(Reaction 3)

This reaction requires higher temperatures than those at which brucite forms.[68]

The final mineralogy depends both on rock and fluid compositions, temperature, and pressure. Antigorite forms in reactions at temperatures that can exceed 600 °C (1,112 °F) during metamorphism, and it is the serpentine group mineral stable at the highest temperatures. Lizardite and chrysotile can form at low temperatures very near the Earth's surface.[69]

Ultramafic rocks often contain calcium-rich pyroxene (diopside), which breaks down according to the reaction

Diopside3 CaMgSi
2
O
6
+ 6 H+
serpentineMg
3
Si
2
O
5
(OH)
4
+ 3 Ca2+
+ H
2
O
+ silicon dioxide4 SiO
2

 

 

 

 

(Reaction 4)

This raises both the pH, often to very high values, and the calcium content of the fluids involved in serpentinization. These fluids are highly reactive and may transport calcium and other elements into surrounding mafic rocks. Fluid reaction with these rocks may create metasomatic reaction zones enriched in calcium and depleted in silica, called rodingites.[70]

In most crustal rock, the chemical activity of oxygen is prevented from dropping to very low values by the fayalite-magnetite-quartz (FMQ) buffer.[71] The very low chemical activity of silica during serpentinization eliminates this buffer, allowing serpentinization to produce highly reducing conditions.[64] Under these conditions, water is capable of oxidizing ferrous (Fe2+
) ions in fayalite. The process is of interest because it generates hydrogen gas:[72][73]

Fayalite3 Fe
2
SiO
4
+ water2 H
2
O
magnetite2 Fe
3
O
4
+ silicon dioxide3 SiO
2
+ hydrogen2 H
2

 

 

 

 

(Reaction 5)

However, studies of serpentinites suggest that iron minerals are first converted to ferroan brucite, Fe(OH)
2
,[74] which then undergoes the Schikorr reaction in the anaerobic conditions of serpentinization:[75][76]

6 Fe(OH)
2
ferrous hydroxide
2 Fe
3
O
4
magnetite
+ 4 H
2
O
water
+ 2 H
2
hydrogen

 

 

 

 

(Reaction 6)

Maximum reducing conditions, and the maximum rate of production of hydrogen, occur when the temperature of serpentinization is between 200 and 315 °C (392 and 599 °F).[77] If the original ultramafic rock (the protolith) is peridotite, which is rich in olivine, considerable magnetite and hydrogen are produced. When the protolith is pyroxenite, which contains more pyroxene than olivine, iron-rich talc is produced with no magnetite and only modest hydrogen production. Infiltration of silica-bearing fluids during serpentinization can suppress both the formation of brucite and the subsequent production of hydrogen.[78]

Chromite present in the protolith will be altered to chromium-rich magnetite at lower serpentinization temperatures. At higher temperatures, it will be altered to iron-rich chromite (ferrit-chromite).[79] During serpentinization, the rock is enriched in chlorine, boron, fluorine, and sulfur. Sulfur will be reduce to hydrogen sulfide and sulfide minerals, though significant quantities are incorporated into serpentine minerals, and some may later be reoxidized to sulfate minerals such as anhydrite.[59] The sulfides produced include nickel-rich sulfides, such as mackinawite.[80]

Laboratory experiments have confirmed that at a temperature of 300 °C (572 °F) and pressure of 500 bars, olivine serpentinizes with release of hydrogen gas. In addition, methane and complex hydrocarbons are formed through reduction of carbon dioxide. The process may be catalyzed by magnetite formed during serpentinization.[56] One reaction pathway is:[75]

forsterite18 Mg
2
SiO
4
+ fayalite6 Fe
2
SiO
4
+ 26 H
2
O
+ CO
2
serpentine12 Mg
3
Si
2
O
5
(OH)
4
+ magnetite4 Fe
3
O
4
+ methaneCH
4

 

 

 

 

(Reaction 7)

Lizardite and chrysotile are stable at low temperatures and pressures, while antigorite is stable at higher temperatures and pressure. Its presence in a serpentinite indicates either that serpentinization took place at unusually high pressure and temperature or that the rock experienced higher grade metamorphism after serpentinization was complete.[81]

Infiltration of CO
2
-bearing fluids into serpentinite causes distinctive talc-carbonate alteration. Brucite rapidly converts to magnesite and serpentine minerals (other than antigorite) are converted to talc. The presence of pseudomorphs of the original serpentinite minerals shows that this alteration takes place after serpentinization.[82]

Serpentinite may contain chlorite, tremolite, and metamorphic olivine and diopside. This indicates that the serpentinite has been subject to more intense metamorphism, reaching the upper greenschist or amphibolite metamorphic facies.[83]

Above about 450 °C (842 °F), antigorite begins to break down. Thus serpentinite does not exist at higher metamorphic facies.[61]

Sorosilicates[edit | edit source]

Def. any group of silicates that have structurally isolated double tetrahedra (the dimeric anion Si2O76-)[84] is called a sorosilicate.

"There are four distinct Si positions, all of which are tetrahedrally coordinated; one of the silicate tetrahedra is an acid silicate group: SiO
3
(OH). The four Si tetrahedra form a linear [Si
4
O
12
(OH)] cluster [that is] a sorosilicate."[85]

Let 𝞍 be an unspecified anion.

"The silicate tetrahedra polymerize to form an [Si
4
𝞍13, 𝞍:O2−
, OH
] sorosilcate fragment. These sorosilicate fragments matchup with the large interstices between the strips of edge-sharing octahedra [...], linking to both the octahedra of adjacent strips and to the shared edge of the [Mn
2
O
6
] dimer of tetrahedra."[85]

"The detailed stereochemistry of extended [T
n
𝞍(3n+1)], n > 2 sorosilicate clusters was considered by Hawthorne (1984)".[85]

All "sorosilicate clusters with n > 3 have acid tetrahedral groups."[85]

"Sorosilicate structures with short clusters ([Si
2
𝞍7], [Si
3
𝞍10]) are based on sheets or walls of polyhedra (octahedra or octahedra and tetrahedra) that are cross-linked by sorosilicate clusters that extend perpendicular to the sheets. Sorosilicate structures with longer clusters ([T
6
𝞍19] in medaite, Mn
6
[VSi
5
O
18
(OH)], Gramaccioli et al. 1981) consist of sheets or ribbons of octahedra in which the sorosilicate cluster extends parallel to the sheets or ribbons. In ruizite (Ca
2
Mn
2
[Si
4
O
11
(OH)
2
](OH)
2
(H
2
O)
2
: Hawthorne 1984), discontinuous sheets of edge-sharing octahedra are linked by [Si
4
𝞍13] clusters orthogonal to the sheets. On the other hand, in tiragalloite [Mn
4
AsSi
3
O
12
(OH): Gramacciolei et al. 1979] and akatoreite, the sorosilicate clusters are oriented parallel to the sheets of octahedra, and show more structural affinity with the inosilicate structures, in which infinite (T𝞍3) chains generally extend parallel to sheets of edge-sharing octahedra. Thus the sorosilicate composition [T
4
𝞍13] seems to be a transitional point between these two structural themes."[85]

Åkermanites[edit | edit source]

Åkermanite is yellowish brown, with calcite (blue), foshagite (hillebrandite variety, fibrous), and tilleyite (creamy). Credit: Ra'ike.{{free media}}

Akatoreites[edit | edit source]

Detail shows akatoreite mineral in rhodonite matrix. Credit: David Hospital.{{free media}}

Akatoreite has the chemical formula Mn2+
9
Al
2
Si
8
O
24
(OH)
8
and crystallizes in the triclinic system.[86][87][88][85]

Empirical formula: Mn2+
8.61
Fe2+
0.19
Mg
0.09
Ca
0.05
Al
2.09
Si
7.75
O
23.17
(OH)
8.83
.[87]

"Results of chemical analyses reported by Read & Reay (1971) indicate that akatoreite has the formula (Mn
8.61
Fe
0.19
Mg
0.08
Ca
0.05
)Al
2.09
Si
7.75
O
23.17
(OH)
8.83
."[85]

Common impurities: Ti,Fe,Mg,Ca.[86]

The International Mineralogical Association (IMA) symbol is Aka.[11]

Akatoreite is a sorosilicate.[86]

Geological setting: "in manganiferous potassium-rich felsic metavolcanics (Norberg, Sweden)."[86]

Occurrence: "as vitreous, orange-brown, massive to radiating twinned columnar aggregates in a manganiferous metachert and carbonate lens on the southeastern margin of the Haast Schist Group, South Island, New Zealand."[85]

Environment: "In a manganiferous metachert and carbonate lens in schists (Akatore Creek, New Zealand)".[87]

"Locality: In New Zealand, three km south of Akatore Creek, east Otago, South Island."[87]

Associated minerals at type locality: Alabandite, Apatite, Pyroxmangite, Quartz, Rhodochrosite, Rhodonite, Spessartine, Tinzenite, Todorokite.[86]

"Akatoreite is associated with rhodochrosite, rhodonite, spessartine, quartz, hübnerite, alabandite, pyroxmangite, tinzenite, apatite, psilomelane, pyrolusite and todorokite."[85]

Common associates: Ganophyllite - (K,Na,Ca)
2
Mn
8
(Si,Al)
12
(O,OH)
32
· 8H
2
O
, Pyrolusite - Mn4+
O
2
and Rhodochrosite - MnCO
3
.[86]

Clinozoisites[edit | edit source]

Clinozoisite is from Quiruvilca Mine (La Libertad Mine; ASARCO Mine), Quiruvilca District, Santiago de Chuco Province, La Libertad Department, Peru. Credit: John Sobolewski.{{free media}}

"Epidote and clinozoisite are widely distributed in the andesitic lavas and tufts of the Borrowdale Volcanic Series. Four main types of occurrence may be distinguished, namely quartz-epidote vein fillings; 'Shap type' veins, bordered by pink reaction zones rich in SiO2, Na2O, and K2O; autometasomatized lavas and tufts; and tufts subjected to alkali metasomatism. Epidote alone formed in the quartz-epidote veins; clinozoisite is found with epidote in the other types of occurrence."[89]

Def. a sorosilicate, basic calcium aluminosilicate "mineral found in crystalline schists, a metamorphic product of calcium feldspar"[90] is called a clinozoisite.

Epidotes[edit | edit source]

This is a volcanic epidote crystal from Falin Lake, Usseglio, Viù Valley, Lanzo Valleys, Torino Province, Piedmont, Italy. Credit: Rob Lavinsky.{{free media}}

Epidote is another usually metamorphic mineral that can occur in volcanic rocks.

Def. any "of a class of mixed calcium iron aluminium sorosilicates found in metamorphic rocks"[91] is called an epidote.

Leucophanites[edit | edit source]

At the center of this image are yellowish crystals of the sorosilicate mineral leucophanite. Credit: Parent Géry.{{free media}}

Medaites[edit | edit source]

A thin drusy crust on matrix is the very rare mineral medaite: a manganese, calcium, vanadium arsenic, silicate hydroxide. Credit: Robert M. Lavinsky.{{free media}}

The medaite in the image on the right has sparkling luster and a rose red color, and is from Molinello Mine, Graveglia Valley, Ne, Genova Province, Liguria, Italy.

"Sorosilicate structures with longer clusters ([T
6
𝞍19] in medaite, Mn
6
[VSi
5
O
18
(OH)], Gramaccioli et al. 1981) consist of sheets or ribbons of octahedra in which the sorosilicate cluster extends parallel to the sheets or ribbons."[85]

Formula: Mn2+
6
V5+
Si
5
O
18
(OH).[92]

Ruizites[edit | edit source]

Clusters of ruizite are on calcite from South Africa. Credit: Robert M. Lavinsky.{{free media}}
Ruizite is from Arizona. Credit: Leon Hupperichs.{{free media}}

Ruizite has the formula Ca
2
Mn
2
Si
4
O
11
(OH)
4
·2H
2
O
.[93]

Ruizite is a sorosilicate mineral discovered at the Christmas mine in Christmas, Arizona, and described in 1977.[93] The mineral is named for discoverer Joe Ana Ruiz.[93]

Ruizite is translucent and orange to red-brown in color with an apricot yellow streak.[94] The mineral occurs as euhedral prisms up to 1 mm (0.039 in) or as radial clusters of acicular (needle-like) crystals.[93]

Ruizite is common at the Christmas mine.[95] The mineral is known from Arizona, Pennsylvania, and Northern Cape Province, South Africa.[94]

Association: Apophyllite, kinoite, junitoite, wollastonite, grossular, diopside, vesuvianite, calcite, chalcopyrite, bornite, sphalerite, smectite (Christmas, Arizona, USA); apophyllite, pectolite, datolite, inesite, orientite, quartz (Wessels mine, South Africa).[93]

Occurrence: In veinlets and on fracture surfaces in metalimestone, formed during cooling of a high-temperature calc-silicate assemblage under oxidizing conditions (Christmas, Arizona, USA).[93] The mineral formed by retrograde metamorphism during cooling of a calc–silicate skarn assemblage in an oxidizing environment.[93][95]

Ruizite crystallizes in the monoclinic crystal system and twinning is common along the miller index {100} plane between exactly two crystals.[93] Ruizite's formula was originally identified as CaMn(SiO
3
)
2
(OH)
2
·2H
2
O
in 1977.[95] The formula was revised to Ca
2
Mn3+
2
Si
4
O
11
(OH)
4
·2H
2
O
.[96]

Ruizite's structure consists of edge-sharing Mnφ6 octahedra, connected at corners into sheets and together into a lattice by clusters of Si
4
O
11
(OH)
2
.[97]

Nitric acid, hydrochloric acid, and potassium hydroxide have little effect on ruizite at low temperatures but readily dissolve the mineral at elevated temperatures.[95]

During the investigation of junitoite at the Christmas mine in Christmas, Arizona, Joe Ana Ruiz and Robert Jenkins discovered an unknown brown mineral.[95] Mine geologist Dave Cook located better specimens, and it was determined to be a new mineral species.[95] The mineral was named ruizite in honor of Joe Ruiz as discoverer.[95] Ruizite's properties were analyzed using a sample provided by Joseph Urban, and it was described in the journal Mineralogical Magazine in December 1977.[95] The International Mineralogical Association approved the mineral as IMA 1977-077.[98] Type specimens are housed in the University of Arizona, Harvard University, the National Museum of Natural History, and The Natural History Museum, London.[93]

Tiragalloites[edit | edit source]

Tiragalloite is from Molinello Mine, Graveglia Valley, Ne, Genova Province, Liguria, Italy. Credit: Leon Hupperichs.{{free media}}

"Tiragalloite belongs to a rare class of compounds with As atoms replacing Si atoms in silicate anions, the so-called arsenosilicates (analoguous to the more common alumosilicates and borosilicates, in which Si atoms are replaced by Al atoms and B atoms, respectively). It contains isolated chains consisting of one (AsO
4
) tetrahedron and three (SiO
4
) tetrahedra, which are linked by sharing corners. One oxygen atom is protonated, resulting in the anion formula (HAsSi
3
O
13
)8−
. The tetrahedral sites in the anion are ordered in that the As atoms are only found at the end of the chains, and there is exactly one As atom in each anion."[99]

Tiragalloite is a sorosilicate with an orange, brownish orange color.[99]

"Named after Paolo Onofrio Tiragallo (1905, Carro, La Spezia, Liguria, Italy - 1987), distinguished amateur mineralogist. Conservator and curator of the mineralogical collection of the University of Genoa."[99]

Tiragalloite has the formula Mn2+
4
As5+
Si
3
O
12
(OH).[99]

Common impurities: Ti,Al,V,Fe,Ca,Na,K.[99]

Common associates: areniopleites NaCaMn2+
Mn2+
2
(AsO
4
)
3
and rhodonites CaMn2+
3
Mn2+
[Si
5
O
15
].[99]

The end member compositions are areniopleites NaCaMn2+
Mn2+
2
(AsO
4
)
3
and caryinites CaNaCa2+
Mn2+
2
As
3
O
12
.[100]

Vesuvianites[edit | edit source]

These are relatively large vesuvianite crystals in a volcanic rock. Credit: Dave Dyet.{{free media}}

"Minerals such as harkerite, wilkeite, cuspidine, cancrinite, vesuvianite and phlogopite indicate the intrusive melt had a high volatile content which is in agreement with the very high explosivity index of this volcanic district."[101]

Def. a "yellow, green or brown mineral, a mixed calcium, magnesium and aluminium silicate sometimes used as a gemstone"[102] is called a vesuvianite.

Tectosilicates[edit | edit source]

Def. any "of various silicate minerals ... with a three-dimensional framework of silicate tetrahedra"[103] is called a tectosilicate.

Def. type "of silicate crystal structure characterized by the sharing of all SiO4 tetrahedral oxygens resulting in three-dimensional framework structures"[39] is called a tektosilicate.

Quartzes[edit | edit source]

This is a sample from a hydrothermal quartz (white)-gold vein (Precambrian) of the Archean Cadillac Group in southwestern Quebec, Canada. Credit: James St. John.{{free media}}

Red "thermoluminescence (RTL) emission from quartz, as a dosimeter for baked sediments and volcanic deposits, [from] older (i.e., >1 Ma), quartz-bearing known age volcanic deposits [can use as standards] independently-dated silicic volcanic deposits from New Zealand, ranging in age from 300 ka through to 1.6 Ma."[104]

Alpha quartzes[edit | edit source]

This image shows a piece of alpha quartz with many crystals. Credit: Basham Jewelry.{{fairuse}}
The photomicrographs show of a sand grain held in an amorphous matrix, in plane-polarized light on top, cross-polarized light on bottom. Scale box in mm. Credit: Qfl247.
This is a thin section with cross-polarized right through a sand-sized quartz grain of 0.13 mm diameter. Credit: Glen A. Izett, USGS.
This is a thin section of a shocked quartz grain. Credit: Martin Schmieder.

Def. "a continuous framework [tectosilicate] of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall [chemical] formula [of] SiO2 ... [of] trigonal trapezohedral class 3 2"[105], usually with some substitutional or interstitial impurities, is called α-quartz.

Alpha-quartz (space group P3121, no. 152, or P3221, no. 154) under a high pressure of 2-3 gigapascals and a moderately high temperature of 700°C changes space group to monoclinic C2/c, no. 15, and becomes the mineral coesite. It is "found in extreme conditions such as the impact craters of meteorites.

When the concentration of interstitial or substitutional impurities becomes sufficient to change the space group of a mineral such as α-quartz, the result is another mineral. When the physical conditions are sufficient to change the solid space group of α-quartz without changing the chemical composition or formula, another mineral results.

Referring to the image on the right: "Lake County experienced incredible volcanic activity. The heat melted the quartz but temperatures and pressures were just right so it was not destroyed. Rather, the melted quartz was carried along with the lava flows."[106]

"Shocked quartz is associated with two high pressure polymorphs of silicon dioxide: coesite and stishovite. These polymorphs have a crystal structure different from standard quartz. Again, this structure can only be formed by intense pressure, but moderate temperatures. High temperatures would anneal the quartz back to its standard form."[107]

"Short-lived bottle-green or blue luminescence colours with zones of non-luminescing bands are very common in authigenic quartz overgrowths, fracture fillings or idiomorphic vein crystals. Dark brown, short-lived yellow or pink colours are often found in quartz replacing sulphate minerals. Quartz from tectonically active regions commonly exhibits a brown luminescence colour. A red luminescence colour is typical for quartz crystallized close to a volcanic dyke or sill."[108]

So far several of the polymorphs of α-quartz formed at high temperature and pressure occur with rock types away from meteorite impact craters.

At lower right is a thin section through a sand-sized quartz grain "from the USGS-NASA Langley core showing two well-developed, intersecting sets of shock lamellae produced by the late Eocene Chesapeake Bay bolide impact. This shocked quartz grain is from the upper part of the crater-fill deposits at a depth of 820.6 ft in the core. The corehole is located at the NASA Langley Research Center, Hampton, VA, near the southwestern margin of the Chesapeake Bay impact crater."[109] "Very high pressures produced by strong shock waves cause dislocations in the crystal structure of quartz grains along preferred orientations. These dislocations appear as sets of parallel lamellae in the quartz when viewed with a petrographic microscope. Bolide impacts are the only natural process known to produce shock lamellae in quartz grains."[109]

At third down on the right shows another thin section in plane polarized light of a shocked quartz grain with two sets of decorated planar deformation features (PDFs) surrounded by a cryptocrystalline matrix from the Suvasvesi South impact structure, Finland.

In a specimen of shocked quartz, stishovite can be separated from quartz by applying hydrogen fluoride (HF); unlike quartz, stishovite will not react.[110]

The major evidence for a volcanic origin for tektites "includes: close analogy between shaped tektites and small volcanic bombs, and between layered tektites and lava or tuff-lava flows or huge bombs; analogy between flanged tektites and volcanic bombs ablated by gasjets: long-time, multistage formation of some tektites that corresponds to wide variations in their radiometric ages; well-ordered long compositional trends (series) typical of magmatic differentiation; different compositional tektite families (subseries) comparable to different stages (phases) of the volcanic process."[111]

"As with the North American microtektite-bearing cores, all the Australasian microtektite-bearing cores containing coesite and shocked quartz also contained volcanic ash, which complicated the search."[112]

Beta quartzes[edit | edit source]

Beta quartz (β-Quartz) is stable "between 573° and 870°C".[39]

Coesites[edit | edit source]

Crossed-polars image of coesite grain (gray) ~1 mm across is in eclogite, small colored inclusion is pyroxene, polycrystalline rim is quartz. Credit: J. Smyth.{{free media}}
Atomic structure is of coesite. Credit: Materialscientist.{{free media}}

Coesite is a polymorph of silicon dioxide SiO
2
that is formed when very high pressure (2–3 gigapascals), and moderately high temperature (700 °C, 1,300 °F), are applied to quartz. Coesite was first synthesized by Loring Coes Jr., a chemist at the Norton Company, in 1953.[113][114]

In 1960, a natural occurrence of coesite was reported by Edward C. T. Chao,[115] in collaboration with Eugene Shoemaker, from the Barringer Crater, in Arizona, US, which was evidence that the crater must have been formed by an impact. After this report, the presence of coesite in unmetamorphosed rocks was taken as evidence of a meteorite impact event or of an atomic bomb explosion. It was not expected that coesite would survive in high pressure metamorphic rocks.

In metamorphic rocks, coesite was initially described in eclogite xenoliths from the mantle of the Earth that were carried up by ascending magmas; kimberlite is the most common host of such xenoliths.[116] In metamorphic rocks, coesite is now recognized as one of the best mineral indicators of metamorphism at very high pressures (UHP, or ultrahigh-pressure metamorphism).[117] Such UHP metamorphic rocks record subduction or continental collisions in which crustal rocks are carried to depths of 70 km (43 mi) or more. Coesite is formed at pressures above about 2.5 GPa (25 kbar) and temperature above about 700 °C. This corresponds to a depth of about 70 km in the Earth. It can be preserved as mineral inclusions in other phases because as it partially reverts to quartz, the quartz rim exerts pressure on the core of the grain, preserving the metastable grain as tectonic forces uplift and expose these rock at the surface. As a result, the grains have a characteristic texture of a polycrystalline quartz rim.

Coesite has been identified in UHP metamorphic rocks around the world, including the western Alps of Italy at Dora Maira,[117] the Erzgebirge of Germany,[118] the Lanterman Range of Antarctica,[119] in the Kokchetav Massif of Kazakhstan,[120] in the Western Gneiss region of Norway,[121] the Dabie-Shan Range in Eastern China,[122] the Himalayas of Eastern Pakistan,[123] and the Vermont Appalachian Mountains.[124][125]

Coesite is a tectosilicate with each silicon atom surrounded by four oxygen atoms in a tetrahedron. Each oxygen atom is then bonded to two Si atoms to form a framework. There are two crystallographically distinct Si atoms and five different oxygen positions in the unit cell. Although the unit cell is close to being hexagonal in shape ("a" and "c" are nearly equal and β nearly 120°), it is inherently monoclinic and cannot be hexagonal. The crystal structure of coesite is similar to that of feldspar and consists of four silicon dioxide tetrahedra arranged in Si
4
O
8
and Si
8
O
16
rings. The rings are further arranged into chains. This structure is metastable within the stability field of quartz: coesite will eventually decay back into quartz with a consequent volume increase, although the metamorphic reaction is very slow at the low temperatures of the Earth's surface. The crystal symmetry is monoclinic C2/c, No.15, Pearson symbol mS48.[126]

Cristobalites[edit | edit source]

Cristobalite spheres appear within obsidian. Credit: Rob Lavinsky.{{free media}}

Def. a high-temperature (above 1470°C) polymorph of α-quartz with cubic, Fd3m, space group no. 227, and a tetragonal form (P41212, space group no. 92) is called cristobalite.

Def. a "mineral of volcanic rocks that solidified at a high temperature [...] chemically identical to quartz, with the chemical formula SiO2, but has a different crystal structure"[127] is called cristobalite.

There is more than one form of the cristobalite framework. At high temperatures, the structure is called β-cristobalite. It is in the cubic crystal system, space group Fd3m (No. 227, Pearson symbol cF104).[128] It has the diamond structure but with linked tetrahedra of silicon and oxygen where the carbon atoms are in diamond. A chiral tetragonal form called α-cristobalite (space group either P41212, No. 92,[129] or P43212, No. 96, at random) occurs on cooling below about 250 °C at ambient pressure and is related to the cubic form by static tilting of the silica tetrahedra in the framework. This transition is variously called the low-high or transition. It may be termed "displacive"; i.e., it is not generally possible to prevent the cubic β-form from becoming tetragonal by rapid cooling. Under rare circumstances the cubic form may be preserved if the crystal grain is pinned in a matrix that does not allow for the considerable spontaneous strain that is involved in the transition, which causes a change in shape of the crystal. This transition is highly discontinuous. Going from the α form to the β form causes an increase in volume of 3[130] or 4[131] percent. The exact transition temperature depends on the crystallinity of the cristobalite sample, which itself depends on factors such as how long it has been annealed at a particular temperature.

The cubic β phase consists of dynamically disordered silica tetrahedra. The tetrahedra remain fairly regular and are displaced from their ideal static orientations due to the action of a class of low-frequency phonons called rigid unit modes. It is the "freezing" of one of these rigid unit modes that is the soft mode for the α–β transition.

In β-cristobalite, there are right-handed and left-handed helices of tetrahedra (or of silicon atoms) parallel to all three axes. But in the α–β phase transition, only the right-handed or the left-handed helix in one direction is preserved (the other becoming a two-fold screw axis), so only one of the three degenerate cubic crystallographic axes retains a fourfold rotational axis (actually a screw axis) in the tetragonal form. (That axis becomes the "c" axis, and the new "a" axes are rotated 45° compared to the other two old axes. The new "a" lattice parameter is shorter by approximately the square root of 2, so the α unit cell contains only 4 silicon atoms rather than 8.) The choice of axis is arbitrary, so that various twins can form within the same grain. These different twin orientations coupled with the discontinuous nature of the transition (volume and slight shape change) can cause considerable mechanical damage to materials in which cristobalite is present and that pass repeatedly through the transition temperature, such as refractory bricks.

When devitrifying silica, cristobalite is usually the first phase to form, even when well outside its thermodynamic stability range. This is an example of Ostwald's step rule. The dynamically disordered nature of the β-phase is partly responsible for the low enthalpy of fusion of silica.

The micrometre-scale spheres that make up precious opal exhibit some x-ray diffraction patterns that are similar to that of cristobalite, but lack any long-range order so they are not considered true cristobalite. In addition, the presence of structural water in opal makes it doubtful that opal consists of cristobalite.[132][133]

Stishovites[edit | edit source]

Crystal structure is of stishovite. Credit: Materialscientist.{{free media}}

Stishovite is an extremely hard, dense tetragonal polymorph of silicon dioxide. It is very rare on the Earth's surface; however, it may be a predominant form of silicon dioxide in the Earth, especially in the lower mantle.[134]

Stishovite was named after Sergey M. Stishov, a Russian high-pressure physicist who first synthesized the mineral in 1961. It was discovered in Meteor Crater in 1962 by Edward C. T. Chao.[135]

Unlike other silica polymorphs, the crystal structure of stishovite resembles that of rutile TiO
2
. The silicon in stishovite adopts an octahedral coordination geometry, being bound to six oxides. Similarly, the oxides are three-connected, unlike low-pressure forms of SiO2. In most silicates, silicon is tetrahedral, being bound to four oxides.[136] It was long considered the hardest known oxide (~30 GPa Vickers[137]); however, boron suboxide has been discovered[138] in 2002 to be much harder. At normal temperature and pressure, stishovite is metastable.

Stishovite can be separated from quartz by applying hydrogen fluoride (HF); unlike quartz, stishovite will not react.[135]

Large natural crystals of stishovite are extremely rare and are usually found as clasts of 1 to 2 mm in length. When found, they can be difficult to distinguish from regular quartz without laboratory analysis. It has a vitreous luster, is transparent (or translucent), and is extremely hard. Stishovite usually sits as small rounded gravels in a matrix of other minerals.

Until recently, the only known occurrences of stishovite in nature formed at the very high shock pressures (>100 kbar, or 10 GPa) and temperatures (> 1200 °C) present during hypervelocity meteorite impact into quartz-bearing rock. Minute amounts of stishovite have been found within diamonds,[139] and post-stishovite phases were identified within ultra-high-pressure mantle rocks.[140] Stishovite may also be synthesized by duplicating these conditions in the laboratory, either isostatically or through shock (see shocked quartz).[141] At 4.287 g/cm3, it is the second densest polymorph of silica, after seifertite. It has tetragonal crystal symmetry, P42/mnm, No. 136, Pearson symbol tP6.[142]

Def. a polymorph of α-quartz formed by pressures > 100 kbar or 10 GPa and temperatures > 1200 °C is called stishovite.[143]

Stishovite may be formed by an instantaneous over pressure such as by an impact or nuclear explosion type event.[107]

Minute amounts of stishovite has been found within diamonds.[139]

Tridymites[edit | edit source]

Tabular tridymite crystals are from Ochtendung, Eifel, Germany. Credit: Fred Kruijen.{{free media}}
Specimen consists of "porcelainite" - a semivitrified chert- or jasper-like rock composed of cordierite, mullite and tridymite, admixture of corundum, and subordinate K-feldspar. Credit: John Krygier.{{free media}}
Crystal structure is of α-tridymite. Credit: Andel.{{free media}}
Crystal structure is of β-tridymite. Credit: Andel.{{free media}}

Def. a polymorph of α-quartz formed at temperatures from 22-460°C with at least seven space groups for its forms with tabular crystals is called tridymite.[144]

Def. a "rare [tektosilicate] mineral of volcanic rocks that solidified at a high temperature, [with the chemical composition of silicon dioxide, SiO22,] chemically identical to quartz, but has a different crystal structure"[145] is called a tridymite.

Tridymite is a high-temperature polymorph of silica and usually occurs as minute tabular white or colorless pseudo-hexagonal crystals, or scales, in cavities in felsic volcanic rocks. Its chemical formula is SiO
2
. Tridymite was first described in 1868 and the type location is in Hidalgo, Mexico. The name is from the Greek tridymos for triplet as tridymite commonly occurs as twinned crystal trillings[146] (compound crystals comprising three twinned crystal components).

α-tridymite is orthorhomic and β-tridymite is hexagonal.[39]

Tridymite can occur in seven crystalline forms. Two of the most common at standard pressure are known as α and β. The α-tridymite phase is favored at elevated temperatures (>870 °C) and it converts to β-cristobalite at 1470 °C.[147][26] However, tridymite does usually not form from pure β-quartz, one needs to add trace amounts of certain compounds to achieve this.[148] Otherwise the β-quartz-tridymite transition is skipped and β-quartz transitions directly to cristobalite at 1050 °C without occurrence of the tridymite phase.

Crystal phases of tridymite[26]
Name Symmetry Space group T (°C)
HP (β) Hexagonal P63/mmc 460
LHP Hexagonal P6322 400
OC (α) Orthorhombic C2221 220
OS Orthorhombic 100–200
OP Orthorhombic P212121 155
MC Monoclinic Cc 22
MX Monoclinic C1 22

In the table, M, O, H, C, P, L and S stand for monoclinic, orthorhombic, hexagonal, centered, primitive, low (temperature) and superlattice. T indicates the temperature, at which the corresponding phase is relatively stable, though the interconversions between phases are complex and sample dependent, and all these forms can coexist at ambient conditions.[149] Mineralogy handbooks often arbitrarily assign tridymite to the triclinic crystal system, yet use hexagonal Miller indices because of the hexagonal crystal shape (see infobox image).[146]

Feldspars[edit | edit source]

This feldspar crystal is stark white showing excellent symmetry with appropriate faces. Credit: Robert M. Lavinsky.{{free media}}
Porphyritic rhyolite with euhedral feldspars indicates two phases of cooling: one slow, one fast, size: 12 cm. Credit: Stephen Hui.{{fairuse}}
This is a naturally occurring collection of intergrown feldspar crystals. Credit: Dave Dyet.{{fairuse}}

Def. a group of "aluminum silicates [aluminosilicates] of the alkali metals sodium, potassium, calcium and barium"[150] are called feldspars, or feldspar.

"Feldspar is by far the most abundant group of minerals in the earth's crust, forming about 60% of terrestrial rocks."[151]

"The mineralogical composition of most feldspars can be expressed in terms of the ternary system Orthoclase (KAlSi3O8), Albite (NaAlSi3O8) and Anorthite (CaAl2Si2O8)."[151]

"The minerals of which the composition is comprised between Albite and Anorthite are known as the plagioclase feldspars, while those comprised between Albite and Orthoclase are called the alkali feldspars due to the presence of alkali metals sodium and potassium."[151]

Albites[edit | edit source]

Albite crystals are from Crete. Credit: Rock Currier.{{free media}}

Albite (International Mineralogical Association (IMA) symbol: Ab[11]) is a plagioclase feldspar mineral, is the sodium endmember of the plagioclase solid solution series, represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi
3
O
8
, a tectosilicate, is usually pure white, hence its name from Latin, albus,[152] and is a common constituent in felsic rocks. The almost end member has the chemical formula Na
(1.0-0.9)
Ca
(0.0-0.1)
Al
(1.0-1.1)
Si
(3.0-2.9)
O
8
.[153]

There are two variants of albite, which are referred to as 'low albite' and 'high albite'; the latter is also known as 'analbite'. Although both variants are triclinic, they differ in the volume of their unit cell, which is slightly larger for the 'high' form. The 'high' form can be produced from the 'low' form by heating above 750 °C (1,380 °F)[154] High albite can be found in meteor impact craters such as in Winslow, Coconino Co., Arizona, United States.[155] Upon further heating to more than 1,050 °C (1,920 °F) the crystal symmetry changes from triclinic to monoclinic; this variant is also known as 'monalbite'.[156] Albite melts at 1,100–1,120 °C (2,010–2,050 °F).[157]

Albites occur in granitic and pegmatite masses (often as the variety Cleavelandite),[158] in some hydrothermal vein deposits, and forms part of the typical greenschist metamorphic facies for rocks of originally basaltic composition. Minerals that albite is often considered associated with in occurrence include biotite, hornblende, orthoclase, muscovite and quartz.[159]

Occurrence: "A major constituent of granites and granite pegmatites, alkalic diorites, basalts, and in hydrothermal and alpine veins. A product of potassium metasomatism and in low-temperature and low-pressure metamorphic facies and in some schists. Detrital and authigenic in sedimentary rocks."[153]

Polymorphs: Kumdykolite, Lingunite.[159]

Structural modifications: "Low- and high-temperature structural modifications exist ('low albite' and 'high albite'), with ordered and disordered Al-Si distribution, respectively."[159]

"The Na-rich end member of the Albite-Anorthite Series (= Plagioclase)."[159]

Empirical formula: Na
0.95
Ca
0.05
Al
1.05
Si
2.95
O
8
.[160]

Anorthites[edit | edit source]

Anorthite crystals are in a basalt vug from Vesuvius. Credit: Robert M. Lavinsky.{{free media}}

Anorthites have the chemical formula CaAl
2
Si
2
O
8
.[161]

Anorthite is rare on the Earth[45] but abundant on the Moon.[162]

Anorthite is a rare compositional variety of plagioclase that occurs in mafic igneous rock, also occurs in metamorphic rocks of granulite facies, in metamorphosed carbonate rocks, and corundum deposits,[163] type localities are Monte Somma and Valle di Fassa, Italy. It was first described in 1823,[164] more rare in surficial rocks than it normally would be due to its high weathering potential in the Goldich dissolution series.

It also makes up much of the lunar highlands; the Genesis Rock, collected during the 1971 Apollo 15 mission, is made of anorthosite, a rock composed largely of anorthite. Anorthite was discovered in samples from comet Wild 2, and the mineral is an important constituent of Ca-Al-rich inclusions in rare varieties of chondritic meteorites.

Anorthoclases[edit | edit source]

When potassium replaces the sodium characteristic in albite at amounts of up to 10%, the mineral is then considered to be anorthoclase.[165]

Oligoclases[edit | edit source]

Oligoclase is from Chihuahua, Mexico. Credit: Rob Lavinsky.{{free media}}

"The apical parts of large volcanoes along the East Pacific Rise (islands and seamounts) are encrusted with rocks of the alkali volcanic suite (alkali basalt, andesine- and oligoclase-andesite, and trachyte)."[166]

Orthoclases[edit | edit source]

This is a perfect, simple, tan orthoclase crystal from Penticton in British Columbia. Credit: Robert Lavinsky.{{free media}}

"The mineralogical composition of most feldspars can be expressed in terms of the ternary system Orthoclase (KAlSi3O8), Albite (NaAlSi3O8) and Anorthite (CaAl2Si2O8)."[151]

"The minerals of which the composition is comprised between Albite and Anorthite are known as the plagioclase feldspars, while those comprised between Albite and Orthoclase are called the alkali feldspars due to the presence of alkali metals sodium and potassium."[151]

"Volcanic rock fragments, feldspar (orthoclase, andesine, and rare sanidine), and small amounts of quartz, biotite, and hornblende make up the bulk of these rocks."[167]

Plagioclases[edit | edit source]

Brown and white, light to average, aphanitic porpheritic phenocrysts of sodic plagioclase 50-60 % with a ground mass of the same at 10-15 %. Credit: Kevin T.{{fairuse}}

Def. "[a]ny of a group of aluminum silicate feldspathic minerals ranging in their ratio of calcium to sodium"[168] is called plagioclase.

Feldspathoids[edit | edit source]

Def. any of a group of silicates "that did not contain enough silica to satisfy all the chemical bonds"[169] of the framework is called a feldspathoid.

Volcanic sources that have a low silica concentration are more likely to produce feldspathoid-containing rocks than feldspar-containing rocks.

Feldspathoid volcanic rocks occur in "a suite of basanites, olivine nephelinites, and olivine melilite nephelinites from the Raton-Clayton volcanic field, New Mexico."[170]

"Volcanism in the Raton-Clayton field commenced approximately 7.5 Ma ago with the eruption of alkali basalts and continued intermittently until at least 10,000 y.a. with the eruption of the Capulin Mountain silicic alkalic basalt (Stormer 1972a; Baldwin and Muehlberger 1959). The entire volcanic sequence was erupted onto the high plains east of the Sangre de Cristo Mountains, and as such, represents the eastern limit of late Cenozoic volcanism in the western U.S. Volcanic activity in the Raton-Clayton field was contemporaneous with volcanism in the Rio Grande rift, and the Raton-Clayton volcanic field is interpreted as part of the Rio Grande rift system."[170]

Kalsilites[edit | edit source]

Sharp pale tan prismatic hexagonal crystals of the mineral kalsilite, from: Löhley, Üdersdorf, Eifel, Rhineland-Palatinate, Germany. Credit: David Hospital.{{free media}}
Kaliophilite exhibited in the Southern Vermont Natural History Museum, Marlboro, Vermont, USA. Credit: Daderot.{{free media}}
Kaliophilite from Monte Somma, Somma-Vesuvius Complex, Naples Province, Campania, Italy. Credit: Leon Hupperichs.{{free media}}

Def. "a rare mineral, a form of KAlSiO4, found in volcanic rocks in parts of Italy"[171] is called a kaliophilite.

Kalsilite (KAlSiO
4
) is a vitreous white to grey feldspathoid that is found in some potassium-rich lavas, such as from Chamengo Crater in Uganda that has a relative Mohs hardness of 5.5.

"Kaliophilite [occurs in] blocks of biotite-pyroxenite volcanic ejecta from Mte. Somma, Vesuvius."[172]

Nephelines[edit | edit source]

Nepheline crystal is from Canaã alkaline massif, Rio de Janeiro. Credit: Zimbres.{{free media}}
This is a mineral specimen of nepheline (white) with small black inclusions. Credit: Andrew Silver, USGS.{{free media}}

Def. a (Na,K)AlSiO4 "feldspathoid mineral of silica-poor igneous, plutonic and volcanic rocks"[173] is called a nepheline.

Quadridavynes[edit | edit source]

The image shows light colored crystals of Quadridavyne from Mt Vesuvius, Somma-Vesuvius Complex, Naples Province, Campania, Italy. Credit: Hudson Institute of Mineralogy.{{fairuse}}

Def. a feldspathoid, tektosilicate "mineral found in volcanic ash"[174], chemical formula Na6Ca2Si6Al6O24Cl4, is called a quadridavyne.

Quadridavyne is a tektosilicate (feldspathoid), chemical formula Na6Ca2Si6Al6O24Cl4, with a type locality of Ottaviano, Monte Somma, Somma-Vesuvius Complex, Naples Province, Campania, Italy.[175]

Zeolites[edit | edit source]

A combination specimen of four zeolite species. The radiating natrolite crystals are protected in a pocket with associated stilbite. The matrix around and above the pocket is lined with small, pink-colored laumontite crystals. Heulandite is also present as a crystal cluster on the backside. Credit: Rob Lavinsky.{{free media}}

The zeolite structural group (Nickel-Strunz classification) includes:[176][177][178][179][180]

  • 09.GA. - Zeolites with T5O10 units (T = combined Si and Al) – the fibrous zeolites
    • Natrolite framework (NAT): gonnardite, natrolite, mesolite, paranatrolite, scolecite, tetranatrolite
    • Edingtonite framework (EDI): edingtonite, kalborsite
    • Thomsonite framework (THO): thomsonite-series
  • 09.GB. - Chains of single connected 4-membered rings
    • Analcime framework (ANA): analcime, leucite, pollucite, wairakite
    • Laumontite (LAU), yugawaralite (YUG), goosecreekite (GOO), montesommaite (MON)
  • 09.GC. - Chains of doubly connected 4-membered rings
    • Phillipsite framework (PHI): harmotome, phillipsite-series
    • Gismondine framework (GIS): amicite, gismondine, garronite, gobbinsite
    • Boggsite (BOG), merlinoite (MER), mazzite-series (MAZ), paulingite-series (PAU), perlialite (Linde type L framework, zeolite L, LTL)
  • 09.GD. - Chains of 6-membered rings – tabular zeolites
    • Chabazite framework (CHA): chabazite-series, herschelite, willhendersonite and SSZ-13
    • Faujasite framework (FAU): faujasite-series, Linde type X (zeolite X, X zeolites), Linde type Y (zeolite Y, Y zeolites)
    • Mordenite framework (MOR): maricopaite, mordenite
    • Offretite–wenkite subgroup 09.GD.25 (Nickel–Strunz, 10 ed): offretite (OFF), wenkite (WEN)
    • Bellbergite (TMA-E, Aiello and Barrer; framework type EAB), bikitaite (BIK), erionite-series (ERI), ferrierite (FER), gmelinite (GME), levyne-series (LEV), dachiardite-series (DAC), epistilbite (EPI)
  • 09.GE. - Chains of T10O20 tetrahedra (T = combined Si and Al)
    • Heulandite framework (HEU): clinoptilolite, heulandite-series
    • Stilbite framework (STI): barrerite, stellerite, stilbite-series
    • Brewsterite framework (BRE): brewsterite-series
  • Others
    • Cowlesite, pentasil (also known as ZSM-5, framework type MFI), tschernichite (beta polymorph A, disordered framework, BEA), Linde type A framework (zeolite A, LTA)

Analcites[edit | edit source]

Colorless sharply formed undamaged crystals of analcime to 25 mm in diameter on a 78 mm x 65 mm x 53 mm matrix. Credit: Carles Millan.

The image on the right contains analcime, or analcite, as colorless sharply formed undamaged crystals to 25 mm in diameter on a 78 mm x 65 mm x 53 mm matrix. They are associated with numerous black prismatic terminated crystals of aegirine, as well as smaller colorless prismatic terminated crystals of natrolite, these from 3 mm to 10 mm in length. Aegirine is a pyroxene. Natrolite is another feldspathoid like analcime of the zeolite group.

Def. a "mineral, a sodium aluminosilicate [with a chemical formula NaAlSi2O6·H2O,][181] having a zeolite structure, found in alkaline basalts"[182] is called an analcime.

Barrerites[edit | edit source]

Barrerite is a rare zeolite species related to stilbite, but found in just a few localities and not nearly as common. Credit: Rob Lavinsky.{{free media}}

Barrerite can have the chemical formula Na4Al4Si14O36•13(H2O).[183]

"Ca may be calcium and/or potassium."[184] Barrerite can have the chemical formula K4Al4Si14O36•13(H2O).[184]

Def. a "white to pinkish tectosilicate zeolite mineral"[185] is called a barrerite.

Leucites[edit | edit source]

Leucite, here a trapezohedral crystal, is a relatively rare feldspathoid. Credit: Rob Lavinsky.{{free media}}

Def. a feldspathoid "mineral of silica-poor igneous, plutonic and volcanic rocks"[186] is called a leucite.

Natrolites[edit | edit source]

Natrolite crystals in a cavity of a volcanic rock. Credit: G.dallorto.{{free media}}

Natrolite is another feldspathoid like analcime of the zeolite group.

"Occurs chiefly in cavities in basalt".[39]

Def. a "fibrous zeolite mineral, being a sodium aluminosilicate,[187] of the chemical formula Na2Al2Si3O10·2H2O"[188] is called a natrolite.

Stellerites[edit | edit source]

A very fine Australian zeolite mineral with lustrous stellerite bowties of deep salmon color. Credit: Rob Lavinsky.{{free media}}

Stellerite has the chemical formula CaAl2Si7O18•7(H2O).[189]

Def. a "hydrated calcium aluminosilicate zeolite, similar to stilbite"[190] is called a stellerite.

Stilbites[edit | edit source]

Photograph shows a stilbite specimen from Kiui Island, Alaska, USA. Credit: Dlloyd.{{fairuse}}

Stilbite (Desmine), a zeolite group, has the chemical formula NaCa2Al5Si13O36•16(H2O).[39]

Def. a "tectosilicate zeolite mineral consisting of hydrated calcium aluminium silicate, common in volcanic rocks"[191] is called a stilbite.

Stilbite-Na can have the chemical formula Na3Ca3Al8Si28O72•30(H2O).[192]

Stilbite-Ca can have the chemical formula NaCa4Al8Si28O72•30(H2O).[193]

Hypotheses[edit | edit source]

  1. Most minerals on Earth are oxides.

See also[edit | edit source]

References[edit | edit source]

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