Geominerals/Sulfides

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Molybdenite on quartz is from the Moly Hill mine, La Motte, Abitibi RCM, Abitibi-Témiscamingue, Québec, Canada. Credit: Didier Descouens.{{free media}}

Sulfide also sulphide[1] is an inorganic anion of sulfur with the chemical formula S2−
or a compound containing one or more S2−
ions. Solutions of sulfide salts are corrosive. Sulfide also refers to chemical compounds: large families of inorganic and organosulfur compounds, e.g. lead sulfide and dimethyl sulfide. Hydrogen sulfide (H
2
S
) and bisulfide (SH
) are the conjugate acids of sulfide.

The sulfide ion, S2−
, does not exist in aqueous alkaline solutions of Na
2
S
.[2][3] Instead sulfide converts to hydrosulfide:

S2−
+ H
2
O
→ SH + OH

Upon treatment with an acid, sulfide salts convert to hydrogen sulfide:

S2− + H+SH
SH
+ H+
H
2
S

Oxidation of sulfide is dependent on the conditions, producing

  1. elemental sulfur,
  2. polysulfides,
  3. polythionates,
  4. sulfites, or
  5. sulfates.

Metal sulfides react with halogens, forming sulfur and metal salts.

8 MgS + 8 I2 → S8 + 8 MgI2

Aqueous solutions of transition metals cations react with sulfide sources (H
2
S
, NaHS, Na
2
S
) to precipitate solid sulfides. Such inorganic sulfides typically have very low solubility in water, and many are related to minerals with the same composition (see below). One famous example is the bright yellow species CdS or "cadmium yellow". The black tarnish formed on sterling silver is Ag2S. Such species are sometimes referred to as salts. In fact, the bonding in transition metal sulfides is highly covalent, which gives rise to their semiconductor properties, which in turn is related to the deep colors. Several have practical applications as pigments, in solar cells, and as catalysts. The fungus Aspergillus niger plays a role in the solubilization of heavy metal sulfides.[4]

Many important metal ores are sulfides.[5] Significant examples include: argentite (silver sulfide), cinnabar (mercury sulfide), galena (lead sulfide), molybdenite (molybdenum sulfide), pentlandite (nickel sulfide), realgar (arsenic sulfide), and stibnite (antimony), sphalerite (zinc sulfide), and pyrite (iron disulfide), and chalcopyrite (iron-copper sulfide).

Dissolved free sulfides (H2S, HS and S2−) are very aggressive species for the corrosion of many metals such as steel, stainless steel, and copper. Sulfides present in aqueous solution are responsible for stress corrosion cracking (SCC) of steel, and is also known as sulfide stress cracking. Corrosion is a major concern in many industrial installations processing sulfides: sulfide ore mills, deep oil wells, pipelines transporting soured oil, Kraft paper factories.

Microbially-induced corrosion (MIC) or biogenic sulfide corrosion are also caused by sulfate reducing bacteria producing sulfide that is emitted in the air and oxidized in sulfuric acid by sulfur oxidizing bacteria. Biogenic sulfuric acid reacts with sewerage materials and most generally causes mass loss, cracking of the sewer pipes and ultimately, structural collapse. This kind of deterioration is a major process affecting sewer systems worldwide and leading to very high rehabilitation costs.

Oxidation of sulfide can also form thiosulfate (S
2
O2−
3
) an intermediate species responsible for severe problems of pitting corrosion of steel and stainless steel while the medium is also acidified by the production of sulfuric acid when oxidation is more advanced.

In organic chemistry, "sulfide" usually refers to the linkage C–S–C, although the term thioether is less ambiguous. For example, the thioether dimethyl sulfide is CH3–S–CH3. Polyphenylene sulfide (see below) has the empirical formula C6H4S. Occasionally, the term sulfide refers to molecules containing the –SH functional group. For example, methyl sulfide can mean CH3–SH. The preferred descriptor for such SH-containing compounds is thiol or mercaptan, i.e. methanethiol, or methyl mercaptan.

Confusion arises from the different meanings of the term "disulfide". Molybdenum disulfide (MoS2) consists of separated sulfide centers, in association with molybdenum in the formal +4 oxidation state (that is, Mo4+ and two S2−). Iron disulfide (pyrite, FeS2) on the other hand consists of S2−
2
, or S–S dianion, in association with divalent iron in the formal +2 oxidation state (ferrous ion: Fe2+). Dimethyldisulfide has the chemical binding CH3–S–S–CH3, whereas carbon disulfide has no S–S bond, being S=C=S (linear molecule analog to CO2). Most often in sulfur chemistry and in biochemistry, the disulfide term is commonly ascribed to the sulfur analogue of the peroxide –O–O– bond. The disulfide bond (–S–S–) plays a major role in the conformation of proteins and in the catalytic activity of enzymes.

Chemical sulfides[edit | edit source]

Formula Melting point (°C) Boiling point (°C) CAS number
H2S Hydrogen sulfide is a very toxic and corrosive gas characterised by a typical odour of "rotten egg". −85.7 −60.20 CAS# 7783-06-4
CdS Cadmium sulfide can be used in photocells. 1750 CAS# 1306-23-6
Calcium polysulfide ("lime sulfur") is a traditional fungicide in gardening.
CS2 Carbon disulfide is a precursor to organosulfur compounds. −111.6 46 CAS# 75-15-0
PbS Lead sulfide is used in infra-red sensors. 1114 CAS# 1314-87-0
MoS2 Molybdenum disulfide, the mineral molybdenite, is used as a catalyst to remove sulfur from fossil fuels; also as lubricant for high-temperature and high-pressure applications. CAS# 1317-33-5
Cl–CH2CH2–S–CH2CH2–Cl Sulfur mustard (mustard gas) is an organosulfur compound (thioether) that was used as a chemical weapon in the First World War. 13–14 217 CAS# 505-60-2
Ag2S Silver sulfide is a component of silver tarnish. CAS# 21548-73-2
Na2S Sodium sulfide, as the hydrate, is used in manufacture of Kraft paper and as a precursor to organosulfur compounds. 920 1180 CAS# 1313-82-2
ZnS Zinc sulfide is used for lenses and other optical devices in the infrared part of the spectrum. ZnS-doped with silver is used in alpha detectors while zinc sulfide with traces of copper has applications in photoluminescent strips for emergency lighting and luminous watch dials. 1185 CAS# 1314-98-3
C6H4S Polyphenylene sulfide is a polymer commonly called "Sulfar". Its repeating units are bonded together by sulfide (thioether) linkages. CAS# 26125-40-6
CAS# 25212-74-2
SeS2 Selenium disulfide is an Antifungal medication used in anti-dandruff preparations, such as Selsun Blue. The presence of the highly toxic selenium in healthcare and cosmetics products represents a general health and environmental concern. <100 CAS# 7488-56-4
FeS2 Known as "fool's gold", pyrite, is a common mineral. 600 CAS# 1317-66-4

Sulfide compounds can be prepared in several different ways:[6]

  1. Direct combination of elements:
    Example: Fesolid + Ssolid → FeSsolid
  2. Reduction of a sulfate:
    Example: MgSO4solid + 4Csolid → MgSsolid + 4COgaseous
  3. Precipitation of an insoluble sulfide:
    Example: M2+ + H2Sgaseous → MSsolid + 2H+aqueous solution

Abramovites[edit | edit source]

Abramovite is considered a sulfosalt like SnS and PbS. Credit: D. Nishio-Hamane.{{free media}}

Abramovite (IMA symbol: Abm[7]) is a very rare mineral of the sulfides and sulfosalt categories, with the chemical formula Pb
2
SnInBiS
7
that occurs as tiny elongated lamellar-shaped crystals, up 1 mm × 0.2 mm in size, and characterized by its non-commensurate structure.[8]

Abramovite is named after the mineralogist Dmitry Vadimovich Abramov (born 1963) of the A.E. Fersman Mineralogical Museum, Russia[9]

It was discovered as fumarole crust on the Kudriavy (Kudryavyi) volcano, Iturup Island, Kuril Islands, Sakhalin Oblast, Far East Region, Russia.[8]

Abramovite is a product of precipitation from fumarolic gases (600 °C [1,112 °F]) in an active stratovolcano.[9]

Abramovite together with Kylindrite and Lévyclaudite form the Kylindrite group of minerals.[10]

Acanthites[edit | edit source]

Crystallized acanthite (4.0 × 2.5 × 1.5 cm) is from Imiter mine, Jbel Saghro mountain range, Morocco. Credit: .{{free media}}

Acanthites crystallize in the Monoclinic system.[11]

Acanthite (IMA symbol: Aca[7] is a form of silver sulfide with the chemical formula Ag
2
S
, is the stable form of silver sulfide below 173 °C (343 °F). Below 173 °C acanthite forms directly.[12][13] Acanthite is the only stable form in normal air temperature.

Acanthite is a common silver mineral in moderately low-temperature hydrothermal veins and in zones of supergene enrichment. It occurs in association with native silver, pyrargyrite, proustite, polybasite, stephanite, aguilarite, galena, chalcopyrite, sphalerite, calcite and quartz.[12]

Aguilarites[edit | edit source]

Aguilarite is from San Carlos Mine, La Luz, Guanajuato municipality, Mexico. Credit: .{{free media}}

Aguilarite (International Mineralogical Association (IMA) symbol Agu[7] is an uncommon sulfosalt mineral with formula Ag
4
SeS
. It was described in 1891 and named for discoverer Ponciano Aguilar.

Aguilarite is bright lead-gray on fresh surfaces but becomes dull iron black when exposed to air.[14] The mineral occurs with massive habit, as elongated pseudododecahedral crystals up to 3 cm (1.2 in), or as intergrowths with acanthite or naumannite.[15]

In the late 19th century, Ponciano Aguilar, superintendent of the San Carlos mine in Guanajuato, Mexico, found several specimens of a mineral thought to be naumannite.[15][16] The samples were given to F. A. Genth for identification, who, along with S. L. Penfield, discovered that it was a new mineral. The mineral was described in the American Journal of Science in 1891 and named aguilarite in honor of Ponciano Aguilar.[16] When the International Mineralogical Association was founded, aguilarite was grandfathered as a valid mineral species.[17]

Aguilarite is uncommon, and forms at relatively low temperatures in hydrothermal deposits rich in silver and selenium but deficient in sulfur.[15] The mineral is known from a number of countries in North and South America, Europe, Asia, and Australasia.[14][15] Aguilarite occurs in association with acanthite, calcite, naumannite, pearceite, proustite, silver, stephanite, and quartz.[15]

In 2013, aguilarite's chemistry and crystal structure were reexamined.[18] The significant reevaluation of aguilarite did not discredit its status as a valid mineral, but it was established as the selenium analogue of acanthite instead of sulfur-rich naumannite.[18] The sample primarily studied came from the Gem and Mineral Collection of the Department of Geosciences at Princeton University.[18]

The work of Petruk et al. in 1974 formed the basis of knowledge regarding the silver–sulfur–selenium system for about forty years. They indexed their x-ray diffraction patterns of aguilarite on an orthorhombic cell similar to naumannite.[18] Aguilarite is, in fact, monoclinic and is isostructural to acanthite and not naumannite.[18] Petruk et al. may not have been able to resolve closely spaced peaks due to low resolution equipment, making aguilarite appear similar to naumannite.[18] Additionally, a number of inconsistencies in unit cell dimensions in the 1974 work show that aguilarite does not have the same structure as naumannite.[18]

The crystal structure of aguilarite consists of planes nearly parallel to miller index (010) composed of tetrahedrally coordinated nonmetal atoms and AgX
3
triangles (where X is a nonmetal). The planes are joined by twofold-coordinated silver atoms.[18]

Aguilarite is part of the acanthite-like solid solution series Ag
2
S–Ag
2
S
0.4
Se
0.6
. The mineral comprises the range from 50 atomic percent selenium up to the transition from monoclinic to orthorhombic.[18]

Aikinites[edit | edit source]

Aikinite is from Beresowsk, Ural - exhibited in the Mineralogical Museum, Bonn, Germany. Credit: Ra'ike.{{free media}}

Aikinite (International Mineralogical Association (IMA) symbol: Aik[7] is a sulfide mineral of lead, copper and bismuth with formula PbCuBiS
3
, forms black to grey or reddish brown acicular orthorhombic crystals with a Mohs hardness of 2 to 2.5 and a specific gravity of 6.1 to 6.8, was originally found in 1843 in the Beryozovskoye deposit, Ural Mountains, is named after Arthur Aikin (1773–1854), an English geologist.

It has been found in Western Tasmania, in mines located near Dundas, Tasmania.

Argentites[edit | edit source]

Argentite is the stable form of Ag
2
S
above 173 °C (343 °F). As argentite cools below that temperature its cubic form is distorted to the monoclinic form of acanthite.

Berryites[edit | edit source]

Black acicular crystals of the rare Pb-Ag sulphide from a Colorado locality: Mike Mine, San Juan Co., Colorado, United States of America. Credit: David Hospital.{{free media}}

Berryites have the chemical formula Cu
3
Ag
2
Pb
3
Bi
7
S
16
.[19]

Environment: "In quartz veins with other sulfides and sulfosalts, and in siderite-rich cryolite."[19]

Geological Setting: "Hydrothermal veins"[20]

Association: Emplectite, aikinite, cuprobismutite, cupropavonite (Colorado, USA); galena, chalcopyrite, sphalerite, quartz (Nordmark, Sweden); galena, cosalite, ourayite, matildite, aikinite (Ivigtut, Greenland); aikinite, matildite, benjaminite, quartz, barite (Tary Ekan deposit, Kazakhstan).[21]

"Structurally related to litochlebite and watkinsonite."[20]

Cinnabars[edit | edit source]

Cinnabar is a naturally occurring cochineal-red, towards brownish red and lead-gray, mercury-sulfide mineral. Credit: H. Zell.

Cinnabar or cinnabarite (red mercury(II) sulfide (HgS), native vermilion), is the common ore of mercury. Its color is cochineal-red, towards brownish red and lead-gray. Cinnabar [may be] found in a massive, granular or earthy form and is bright scarlet to brick-red in color.[22] Generally cinnabar occurs as a vein-filling mineral associated with recent volcanic activity and alkaline hot springs. Cinnabar is deposited by epithermal ascending aqueous solutions (those near surface and not too hot) far removed from their igneous source.

Covellites[edit | edit source]

This covellite specimen is from the Black Forest of Germany. Credit: Ra'ike.{{free media}}
Covellite specimen is from the Leonard Mine, Butte, Butte District, Silver Bow County, Montana, USA. Credit: Didier Descouens.

Covellite [CuS] has been found in veins at depths of 1,150 meters, as the primary mineral. Covellite formed as clusters in these veins reaching one meter across. Covellite is a hexagonal form of CuS.[23] Covellite is a chalcogen.

Cubanites[edit | edit source]

These are bronze to brass-yellow, striated, cyclically-twinned cubanite crystals from the Chibougamau mines of Quebec. Credit: Rob Lavinsky.

Cubanite is a yellow mineral of copper, iron, and sulfur, CuFe2S3.[24] Cubanite occurs in high temperature hydrothermal deposits with pyrrhotite and pentlandite as intergrowths with chalcopyrite. It results from exsolution from chalcopyrite at temperatures below 200 to 210 °C.[25] It has also been reported from carbonaceous chondrite meteorites.[25]

Galenas[edit | edit source]

This piece features a pristine, 3-dimensional, superb galena crystal sitting perfectly atop matrix. Credit: Rob Lavinsky.{{free media}}

Galena in the image on the right is the metallic cuboidal crystal atop a matrix. Galena is PbS, 50 atomic % lead and 50 atomic % sulfur. Each cubic unit cell contains four PbS molecules in a face-centered cubic lattice.

Gallites[edit | edit source]

This is an example of gallite from Namibia. Credit: Hudson Institute of Mineralogy.

Gallite (CuGaS2) is 25 at % gallium.

Germanites[edit | edit source]

This sample of germanite is displayed in the Smithsonian Museum of Natural History. Credit: R Nave.

The sample of germanite on the right has a composition of Cu26Fe4Ge4S32. Generally, germanite has a composition closer to Cu3(Ge, Ga, Fe, Zn) (S,As)4.[23] "This sample also contains tennantite."[26]

Heazlewoodites[edit | edit source]

Zaratite is an emerald-green coating, hellyerite (powder-blue) and heazlewoodite (light bronze). Credit: Robert M. Lavinsky.{{free media}}

Heazlewoodite, Ni3S2, is a rare sulfur-poor nickel sulfide mineral found in serpentinitized dunite.[27][28][29] It occurs as disseminations and masses of opaque, metallic light bronze to brassy yellow grains which crystallize in the trigonal crystal system. It has a hardness of 4, a specific gravity of 5.82. Heazlewoodite was first described in 1896 from Heazlewood, Tasmania, Australia.[29]

Heazlewoodite is formed within terrestrial rocks by metamorphism of peridotite and dunite via a process of nucleation. Heazlewoodite is the least sulfur saturated of nickel sulfide minerals and is only formed via metamorphic exsolution of sulfur from the lattice of metamorphic olivine.

Heazlewoodite forms from sulfur and nickel which exist in pristine olivine in trace amounts, and which are driven out of the olivine during metamorphic processes. Magmatic olivine generally has up to ~4000 ppm Ni and up to 2500 ppm S within the crystal lattice, as contaminants and substituting for other transition metals with similar ionic radii (Fe2+ and Mg2+).

During metamorphism, sulfur and nickel within the olivine lattice are reconstituted into metamorphic sulfide minerals, chiefly millerite, during serpentinization and talc carbonate alteration.

When metamorphic olivine is produced, the propensity for this mineral to resorb sulfur, and for the sulfur to be removed via the concomitant loss of volatiles from the serpentinite, tends to lower sulfur fugacity.

In this environment, nickel sulfide mineralogy converts to the lowest-sulfur state available, which is heazlewoodite.

Heazlewoodite is known from few ultramafic intrusions within terrestrial rocks. The Honeymoon Well ultramafic intrusive, Western Australia is known to contain heazlewoodite-millerite sulfide assemblages within serpentinized olivine adcumulate dunite, formed from the metamorphic process.

The mineral is also reported, again in association with millerite, from the ultramafic rocks of New Caledonia.

Lorándite[edit | edit source]

A slender, gemmy, lorandite crystal is perched atop contrasting calcite. Credit: Robert M. Lavinsky.{{free media}}

Lorándite can have the formula TlAsS2.

Millerites[edit | edit source]

Millerite is an odd, scarce nickel sulfide mineral (NiS) that tends to form radiating clusters or tufts of long, hairlike needles. Credit: James St. John.{{free media}}

Millerite is a nickel sulfide mineral, NiS. Millerite is a common metamorphic mineral replacing pentlandite within serpentinite ultramafics. It is formed in this way by removal of sulfur from pentlandite or other nickeliferous sulfide minerals during metamorphism or metasomatism.

Millerite is also formed from sulfur poor olivine cumulates by nucleation. Millerite is thought to form from sulfur and nickel which exist in pristine olivine in trace amounts, and which are driven out of the olivine during metamorphic processes. Magmatic olivine generally has up to ~4000 ppm Ni and up to 2500 ppm S within the crystal lattice, as contaminants and substituting for other transition metals with similar ionic radii (Fe2+
and Mn2+
).

During metamorphism, sulfur and nickel within the olivine lattice are reconstituted into metamorphic sulfide minerals, chiefly millerite, during serpentinization and talc carbonate alteration. When metamorphic olivine is produced, the propensity for this mineral to resorb sulfur, and for the sulfur to be removed via the concomitant loss of volatiles from the serpentinite, tends to lower sulfur fugacity.

This forms disseminated needle like millerite crystals dispersed throughout the rock mass. Millerite may be associated with heazlewoodite and is considered a transitional stage in the metamorphic production of heazlewoodite via the above process.

"Millerite, NiS, fractured under high vacuum and reacted with air and water has been analyzed by X-ray photoelectron spectroscopy (XPS). The pristine millerite surface gives rise to photoelectron peaks at binding energies of 853.1 eV (Ni 2p3/2) and 161.7 eV (S 2p), thus resolving ambiguities concerning binding energies quoted in the literature. Air-reacted samples show the presence of NiSO
4
and Ni(OH)
2
species. There is evidence for polysulfide species (S2-􏰄
n
, where 2 􏰀≤ n 􏰀≤ 8) on air-oxidized surfaces. These may occur in a sub-surface layer or may be intermixed with the Ni(OH)
2
in the oxidized layer. The NiSO
4
species at the millerite surface occur as discrete crystallites whereas the Ni(OH)
2
forms a thin veneer covering the entire millerite surface. The NiSO
4
crystallites form on the surface of millerite but not on surfaces of adjacent minerals. Surface diffusion of Ni2+
􏰃 and SO2−
4
across the millerite surface [may] be responsible for the transport and subsequent growth of NiSO
4
crystallites developed on millerite surfaces. [It] is clear that Ni and SO2−
4
does not diffuse onto surfaces of adjacent minerals in sufficient quantity to form crystallites [...]. XPS results for water-reacted surfaces show little difference from the vacuum fractured surfaces with the exception that minor amounts of polysulfide and hydroxy nickel species are present. Similar reaction products to those formed in air [NiSO
4
and Ni(OH)
2
] are believed to be produced, but these are removed from the millerite surface by dissolution, leaving behind a sulfur-enriched surface (polysulfide) and hydroxyl groups chemisorbed to nickel ions at the millerite surface.”[30]

"The presence of NiSO
4
can be explained through oxidation of the sulfide ion in millerite to sulfate by molecular oxygen according to the following scheme:

NiS +􏰃 2O
2
NiSO
4

In fact, it is most likely that the salt is hydrated. The presence of water in the O 1s spectrum supports the suggestion. The free energy of formation of hydrated NiSO
4
species is about 300 to 400 kcal/mol more negative than anhydrous NiSO
4
, the difference being largest for the greatest degree of hydration. Even without hydration, the oxidation of NiS to NiSO
4
by molecular oxygen has a [reaction (rxn)] 􏰉∆G
rxn
􏰅= -􏰄162.6 kcal/mol. Therefore, the oxidation of NiS to NiSO
4
is thermodynamically favored and should occur provided it is kinetically favored.”[30]

"Coincident with formation of the hydroxy nickel surface complex is the formation of polysulfides. The nickel that reacts with the water and oxygen of ambient air is no longer bonded to sulfide. This sulfide is therefore available to react with other near-surface species, including other sulfide ions, which may lead to the formation of polysulfides (including disulfide) according to the following reaction scheme:”[30]

nNiS 􏰃+ (n-􏰄1)H
2
O
􏰃+ (n-􏰄1)/2O
2
Ni2􏰃􏰄+
0
- S2+
n
+ S2−
n
+􏰃 (n􏰄-1)Ni(OH)
2
,

"where 2 ≤􏰀 n ≤􏰀 8. The designation Ni2􏰃􏰄+
0
- S2+
n
is used to denote polysulfide bonded to nickel in the lattice at the millerite surface. The Ni(OH)
2
and polysulfide may exist as separate, thin layers on the millerite surface with the Ni(OH)
2
presumably forming the overlayer. Alternatively, the polysulfides may be intermixed with the Ni(OH)
2
in the oxidized overlayer.”[30]

Orpiments[edit | edit source]

The mineral orpiment from an arsenic mine in southern Russia is a source of yellow and orange pigments and is highly toxic. Credit: USGS.{{free media}}

The mineral orpiment at right is a source of yellow and orange pigments and has the chemical formula As
2
S
3
.

Patrónites[edit | edit source]

Patronite is from the Minas Ragra, Peru. Credit: Ra'ike.{{free media}}

Patronite is the vanadium sulfide mineral with the chemical formula VS
4
. The material is usually described as V4+
(S2−
2
)
2
.[31] Structurally, it is a "linear-chain" compound with alternating bonding and nonbonding contacts between the vanadium centers. The vanadium is octa-coordinated, which is an uncommon geometry for this metal.[32]

The mineral was first described in 1906 for an occurrence in the Minas Ragra vanadium mine near Junín, Cerro de Pasco, Peru, was named for Peruvian metallurgist Antenor Rizo-Patron (1866–1948) the discoverer of the deposit.[33][34] At the type locality in Peru it occurs in fissures within a red shale likely derived from an asphaltum deposit. Associated minerals include, native sulfur, bravoite, pyrite, minasragrite, stanleyite, dwornikite, quartz and vanadium bearing lignite.[34] It has also been reported from the Yushkinite gorge on the Middle Silova-Yakha River on the Paikhoi Range of the polar Urals of Russia and from the Tsumeb mine in Namibia.[33]

Pentlandites[edit | edit source]

Pyrrhotite with pentlandite from the Sudbury Impact Structure in Ontario, Canada. Credit: James St. John.{{free media}}

This massive sulfide specimen on the right consists of brassy gray-brown pyrrhotite (Fe
(1-x)
S
- imperfect iron monosulfide) with brighter brassy-colored patches of pentlandite ((Ni,Fe)
9
S
8
- nickel iron sulfide), plus a network of grayish to black patches of magnetite (Fe
3
O
4
- iron oxide).

Pentlandite is an iron–nickel sulfide with the chemical formula (Fe,Ni)
9
S
8
. Pentlandite has a narrow variation range in Ni:Fe but it is usually described as having a Ni:Fe of 1:1. It also contains minor cobalt, usually at low levels as a fraction of weight.

Pyrites[edit | edit source]

Pyrite cubic crystals are on marl from Navajún, Rioja, Spain. Credit: .

The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale brass-yellow hue have earned it the nickname fool's gold because of its superficial resemblance to gold. Pyrite is the most common of the sulfide minerals [on Earth]. Pyrite is usually found associated with other sulfides or oxides in quartz veins, sedimentary rock, and metamorphic rock, as well as in coal beds, and as a replacement mineral in fossils. Despite being nicknamed fool's gold, pyrite is sometimes found in association with small quantities of gold. Gold and arsenic occur as a coupled substitution in the pyrite structure. In the Carlin–type gold deposits, arsenian pyrite contains up to 0.37 wt% gold.[35]

Realgars[edit | edit source]

An aesthetic cluster of gemmy, bright, cherry-red realgar crystals nicely attached to a bit of matrix. Credit: Rob Lavinsky.
Realgar, an arsenic sulfide mineral 1.5-2.5 Mohs hardness, is highly toxic and is used to make red-orange pigment. Credit: .

Realgar an arsenic sulfide mineral of 1.5-2.5 Mohs hardness is used to make red-orange pigment.

Realgar is a sulfide mineral. But, with equal atomic numbers of sulfur and arsenic, it may act as a pnictide.

This piece on the right is from the less well-known Royal Reward Mine of Washington.

Violarites[edit | edit source]

Violarite specimen is from the Perseverance Mine, Leinster, Leonora Shire, Western Australia, Australia. Credit: JF Carpentier.{{fairuse}}

Violarite (Fe2+Ni23+S4) is a supergene sulfide mineral associated with the weathering and oxidation of primary pentlandite nickel sulfide ore minerals.

Violarite is formed by oxidisation of primary sulfide assemblages in nickel sulfide mineralisation. The process of formation involves oxidation of Ni2+ and Fe2+ which is contained within the primary pentlandite-pyrrhotite-pyrite assemblage.

Violarite is produced at the expense of both pentlandite and pyrrhotite, via the following basic reaction;

Pentlandite + Pyrrhotite --> Violarite + Acid

(Fe,Ni)9S8 + Fe(1-x)S + O2 → Fe2+Ni23+S4 + H2SO3

Violarite is also reported to be produced in low-temperature metamorphism of primary sulfides, though this is an unusual paragenetic indicator for the mineral.

Continued oxidation of violarite leads to replacement by goethite and formation of a gossanous boxwork, with nickel tending to remain as impurities within the goethite or haematite, or rarely as carbonate minerals.

Violarite is reported widely from the oxidised regolith above primary nickel sulfide ore systems worldwide. It is of particular note from the Mount Keith dunite body, Western Australia, where it forms an important ore mineral.

It is also reported from open cast mines around the Kambalda Dome, and Widgiemooltha Dome, in association with polydymite, gaspeite, widgiemoolthalite and hellyerite, among other supergene nickel minerals.

Hypotheses[edit | edit source]

  1. Most minerals on Earth are oxides.

See also[edit | edit source]

References[edit | edit source]

  1. https://www.lexico.com/en/definition/sulphide
  2. May, P.M.; Batka, D.; Hefter, G.; Könignberger, E.; Rowland, D. (2018). "Goodbye to S2-". Chem. Comm. 54 (16): 1980–1983. doi:10.1039/c8cc00187a. PMID 29404555. 
  3. Meyer, B; Ward, K; Koshlap, K; Peter, L (1983). "Second dissociation constant of hydrogen sulfide". Inorganic Chemistry 22 (16): 2345. doi:10.1021/ic00158a027. 
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