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.
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:
- α-Fe and δ-Fe,
- α-Mn and δ-Mn,
- α-W, and
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.
"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."
Titanium "undergoes a phase transformation (hcp to bcc) at 882 °C ."
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.
"[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)."
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.
As the phase diagram on the left indicates vanadium is bcc down to lower temperatures from its melting point.
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."
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.
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.
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.
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.
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.
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.
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.
- Taking on a bcc structure to defer melting to a higher temperature came before a fcc or hcp structure.
- 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.
- 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.
- 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.
- 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.