Minerals/Metals/Body-centered cubics

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A face-centered cubic close-packed unit cell structure is outlined in red. Credit: Owen Graham.
This is a stick and ball model of a fcc unit cell. Credit: Baszoetekouw.
These models compare a fcc unit cell with hexagonal close-packing (hcp). Credit: Twisp.

Metals in general fall into two groups of close-packing structures: face-centered cubic (fcc) and hexagonal close-packed (hcp). These two are the most efficient way to pack a bunch of hard marbles into the smallest space.

An fcc structure differs from a hcp in the number of distinct close-packed layers: hcp has two, symbolized with A below B, and fcc which has three with A below B, but C above B before A again.

Body-centered cubics[edit]

This is a body-centered cubic unit cell. Credit: Baszoetekouw.

The body-centered cubic (bcc) metals have a structure for their unit cells shown in the diagram on the left. This is not a close-packed structure. As such it is expected to occur in close-packed structures at higher temperatures.

Many pure element metals occur in a bcc structure:

  1. α-Cr,
  2. α-Fe and δ-Fe,
  3. β-Hf,
  4. α-Li,
  5. α-Mn and δ-Mn,
  6. α-Mo,
  7. α-Nb,
  8. α-Ta,
  9. β-Ti,
  10. α-V,
  11. α-W, and
  12. β-Zr.

The minerals that are or contain these metals that crystallize in a bcc lattice: titanium (Ti) through chromium (Cr), zirconium (Zr) through molybdenum (Mo), and hafnium (Hf) through tungsten (W) are studied here.

Titanium minerals[edit]

This is an iron-titanium phase diagram. Credit: Hirokai.

"Microbeam analysis of eclogites from the ultrahigh-pressure metamorphic belt in Dabieshan, China has revealed native titanium inclusions in garnets of coesite eclogite. The inclusions are about 10 μm in size, have a submetallic luster from the thin oxidation film on the surface, and are brown under reflected light."[1]

Titanium "undergoes a phase transformation (hcp to bcc) at 882 °C [5]."[2]

As the phase diagram on the left indicates, there is a miscibility gap between bcc iron (α-Fe) and hcp (α-Ti) up to about 800°C.

Vanadium minerals[edit]

In this backscattered electron micrograph on the left, the native vanadium crystals have been colorized in red. Credit: MikhailI Ostrooumov and Yuri Taran.
This Fe-V phase diagram shows which phases are to be expected at equilibrium for different combinations of vanadium content and temperature. Credit: Computational Thermodynamics Inc..

"[N]ative vanadium [occurs] in natural fumarolic incrustations and in the mineral assemblage precipitated in silica tubes inserted into high-temperature (750-830°C) fumaroles of Colima volcano – the most active volcano of Mexico, and one of the most active in the Americas. [...] The new mineral and its name (“vanadium”) have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (Williams et al., 2013; IMA # 2012- 021a). The holotype material has been deposited in the Geological Museum of National Mexican University (New mineral collection of Mexican Mineralogical Society with cataloged under FIM 12/01)."[3]

In the image on the right, the backscattered electron micrograph on the left side, has the native vanadium crystals colorized in red. The energy dispersive X-ray spectroscopy (EDS) spectrum on the right shows the vanadium peaks plus small amounts of Fe and S.[3]

As the phase diagram on the left indicates vanadium is bcc down to lower temperatures from its melting point.

Chromium minerals[edit]

This is a native chromium nugget. Credit: Neal Ekengren.
Fe-Cr phase diagram shows which phases are to be expected at equilibrium for different combinations of chromium content and temperature. Credit: Computational Thermodynamics Inc.

Native chromium such as the nugget in the image on the right is very rare. It is also a hard mineral, probably because of an oxide coating giving it a slight bluish cast.

"An unusual mineral association (diamond, SiC, graphite, native chromium, Ni-Fe alloy, Cr2+-bearing chromite), indicating a high-pressure, reducing environment, occurs in both the peridotites and chromitites."[4]

As the phase diagram for the Fe-Cr system on the left shows, chromium is bcc from 600°C on up to melting. Chromium is also bcc at room temperature and pressure.

Zirconium minerals[edit]

This is a binary phase diagram of the iron-zirconum system. Credit: D. Arias and J.P. Abriata.

As the Fe-Zr phase diagram on the left demonstrates, zirconium has a hcp structure (α-Zr) at lower temperatures, including room temperature, and a bcc structure (β-Zr) at higher temperatures up to melting.

Niobium minerals[edit]

This is an iron-niobium phase diagram. Credit: E. Paul and L.J. Swartendruber.

As can be seen in the iron-niobium phase diagram on the left, niobium is single phase (α-Nb) up to its melting temperature. This is a bcc structure.

It appears to be the case that native niobium does not occur in the surface rocks on Earth.

Molybdenum minerals[edit]

This is a scanning electron micrograph of native molybdenum particles in lunar regolith. Credit: A. V. Mokhov and P. M. Kartashov.
This is a calculated iron-molybdenum phase diagram. Credit: Computational Thermodynamics Inc.

The electron micrograph on the right shows a couple of pieces of native molybdenum found in lunar regolith at the Luna 24 landing site after transport back to Earth and analysis.

The phase diagram for the iron-molybdenum system demonstrates that molybdenum is bcc (α-Mo) for its intermediate and higher temperatures. It's also bcc at room temperature.

Hafnium minerals[edit]

This is an iron-hafnium phase diagram. Credit: H. Okamoto.

Note in the iron-hafnium phase diagram on the left that hafnium occurs in two phases: hcp (α-Hf) at lower temperatures and bcc (β-Hf) at higher temperatures up to melting.

Tantalum minerals[edit]

This is a piece of native tantalum from Kvanefjeld Mountain, Kuannersuit Plateau, Ilímaussaq complex, Narsaq, Kujalleq, Greenland. Credit: V.V. Seredin.
This is a National Bureau of Standards phase diagram for Fe-Ta. Credit: L.J. Swartzendruber and E. Paul.

On the right is a scanning electron micrograph of a piece of native tantalum from Kvanefjeld Mountain, Kuannersuit Plateau, Ilímaussaq complex, Narsaq, Kujalleq, Greenland.

The iron-tantalum phase diagram on the left shows the bcc (α-Ta) phase from lower temperatures through and up to melting.

Tungsten minerals[edit]

The small, bright crystalline mass on the right of this electron micrograph is native tungsten. Credit: Andrei V. Mokhov.
This is an iron-tungsten phase diagram. Credit: Satyendra.

In the scanning electron micrograph on the right is a bright grain, or crystalline mass, of native tungsten. The sample is a fragment of lunar silicate glass from the Luna 24 landing site, Mare Crisium, The Moon. The fragment is bright in backscattered electrons.

The iron-tungsten phase diagram on the left shows that the bcc phase of tungsten (α-W) occurs from lower temperatures on up to the melting temperature.

Hypotheses[edit]

Main source: Hypotheses
  1. Taking on a bcc structure to defer melting to a higher temperature came before a fcc or hcp structure.

See also[edit]

References[edit]

  1. Jing Chen, Jiliang Li, and Jun Wu (30 April 2000). "Native titanium inclusions in the coesite eclogites from Dabieshan, China". Earth and Planetary Science Letters 177 (3-4): 237-40. doi:10.1016/S0012-821X(00)00057-1. http://www.sciencedirect.com/science/article/pii/S0012821X00000571. Retrieved 2015-08-19. 
  2. B.B. Panigrahi, M.M. Godkhindi , K. Das, P.G. Mukunda, and P. Ramakrishnan (15 April 2005). "Sintering kinetics of micrometric titanium powder". Materials Science and Engineering: A 396 (1-2): 255-62. doi:10.1016/j.msea.2005.01.016. http://www.sciencedirect.com/science/article/pii/S0921509305000778. Retrieved 2015-08-19. 
  3. 3.0 3.1 MikhailI Ostrooumov and Yuri Taran (20 May 2015). "Discovery of Native Vanadium, a New Mineral from the Colima Volcano, State of Colima (Mexico)". Revista de la Sociedad Española de Mineralogía: 109-10. http://www.uhu.es/fexp/sem2015/arc/macla/macla_20_109-110.pdf. Retrieved 2015-08-19. 
  4. Wen-Ji Bai, Mei-Fu Zhou, and Paul T. Robinson (August 1993). "Possibly diamond-bearing mantle peridotites and podiform chromitites in the Luobusa and Donqiao ophiolites, Tibet". Canadian Journal of Earth Sciences 30 (8): 1650-9. doi:10.1139/e93-143. http://www.nrcresearchpress.com/doi/abs/10.1139/e93-143. Retrieved 2015-08-19. 

External links[edit]

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