Clinohumite

Source: Wikipedia, the free encyclopedia.
Clinohumite
2V angle
Measured: 52° to 90°
References[2][3][4][5]
Major varieties
TitanclinohumiteTitanoan; (Mg,Fe2+,Ti)9
[(F,OH,O)2|(SiO4)4] [6][7]

Clinohumite is an uncommon member of the humite group, a magnesium silicate according to the chemical formula (Mg, Fe)9(SiO4)4(F,OH)2. The formula can be thought of as four olivine (Mg2SiO4), plus one brucite (Mg(OH)2). Indeed, the mineral is essentially a hydrated olivine and occurs in altered ultramafic rocks and carbonatites. Most commonly found as tiny indistinct grains, large euhedral clinohumite crystals are sought by collectors and occasionally fashioned into bright, yellow-orange gemstones. Only two sources of gem-quality material are known: the Pamir Mountains of Tajikistan, and the Taymyr region of northern Siberia. It is one of two humite group minerals that have been cut into gems, the other being the much more common chondrodite.

Properties

A

conchoidal to uneven; its streak
is white.

Clinohumite's transparency ranges from transparent to translucent; its

fluoresce
an orangy yellow; there is little to no response under longwave UV.

The Taymyr material is reported to be a dark reddish brown while the Pamir material is a bright yellow to orange or brownish orange. The Pamir material also has a hardness slightly greater than 6, a lower specific gravity (3.18), and higher maximum birefringence (0.036).[9] Phillip Youngman, master faceter of Los Osos, California, noticed not only that Pamir material is harder than expected, but also that it is less brittle than expected. Youngman observed that clinohumite reacted like beryl to cutting and polishing, and that it reminded him of polishing diopside.

Like other members of the humite group, the relative amounts of

hydroxyl and fluorine vary in clinohumite, and iron commonly substitutes for some of the magnesium, bringing about changes in physical and optical properties. Titanium substitution also causes pronounced changes in optical properties, producing the variety titanclinohumite. Consequently, it is relatively easy to determine that a stone is a humite group mineral, but difficult to determine exactly which member. Other common impurities of clinohumite include aluminium, manganese, and calcium
.

Formation and occurrence

Two intergrown crystals (1.5 x 1 x 0.5 cm) from Koksha Valley, Badakhshan Province, Afghanistan

Clinohumite is a product of

volcano complex near Naples, Italy
, where clinohumite was discovered in 1876. The aforementioned gem-quality occurrences of Pamir and Taymyr were discovered only recently: the former in the early 1980s, and the latter in 2000. These deposits are scarce and only sporadically mined, so clinohumite remains one of the rarest gemstones with only a few thousand carats known to exist in private collections.

Other (non-gem quality) occurrences of clinohumite include: the

Northern Cape Province, South Africa; Andalusia, Spain; Värmland and Västmanland, Sweden; Isle of Skye, Scotland; and the states of California, Colorado, Massachusetts, New Jersey, New Mexico, New York, Oklahoma, Utah, and Washington, US.[10]

Clinohumite also occurs as a minor component of some masses of

upper mantle to depths of at least 410 km (250 mi) and is a potential host phase for H (water) in this region of the Earth's interior.[12][13]

Minerals associated with humite include grossular, wollastonite, forsterite, monticellite, cuspidine, fluoborite, ludwigite, dolomite, calcite, talc, biotite, spinel, vesuvianite, sanidine, meionite and nepheline.[3]

Crystal structure

Crystal structure of clinohumite in polyhedral representation, a-axis projection, b-horizontal. H atom are blue spheres.

The structure is

nesosilicate
with no oxygen atoms shared between two silicons.

See also

References

  1. S2CID 235729616
    .
  2. ^ Mineralienatlas
  3. ^ a b Handbook of Mineralogy
  4. ^ Clinohumite on Mindat.org
  5. ^ Clinohumite on Webmineral
  6. ^ Deer, W.A., R.A. Howie, and J. Zussman (1966). An Introduction to the Rock Forming Minerals. London: Longsman, Green and Co., Ltd.{{cite book}}: CS1 maint: multiple names: authors list (link)
  7. ^ Roberts, W.L., G.R. Rapps, Jr., and J. Weber (1975). Encyclopedia of Minerals. New York: Van Nostrand Reinhyold Company.{{cite book}}: CS1 maint: multiple names: authors list (link)
  8. ^ Arem, Joel E. (1977). Color Encyclopedia of Gemstones. Van Nostrand Reinhold Company, New York, 149 pages.
  9. ^ Henn, U., Hyršl, J., and Milisenda, C. (2000). "Gem-quality clinohumite from Tajikistan and the Taymyr region, Northern Siberia." Journal of Gemmology, Vol. 27, No. 6, pp. 335–340.
  10. ISBN 0-7506-1674-1
    .
  11. ISBN 0-08-043751-6
  12. ^ J.R. Smyth, D.J. Frost, F. Nestola, C.M. Holl and G. Bromiley (2006), "Olivine hydration in the deep upper mantle: Effects of temperature and silica activity." Geophysical Research Letters 33, L15301.
  13. ^ Pradeepkumar, A P., Krishnanath, R. (2000). "A Pan-African 'Humite Epoch' in East Gondwana: implications for Neoproterozoic Gondwana geometry." Journal of Geodynamics, Vol. 29, No. 1-2, pp. 43–62 [1].
  14. ^ Berry, A.J. and James, M. (2001) "Refinement of hydrogen positions in synthetic hydroxyl-clinohumite by powder neutron diffraction." American Mineralogist, 86, pp. 181–184.
  15. ^ Friedrich, A., Lager, G.A., Kunz, M., Chakoumakos, B.C., Smyth, J.R., and Schultz, A.J. (2001) "Temperature-dependent single-crystal neutron diffraction study of natural chondrodite and clinohumites." American Mineralogist, 86, pp. 981–989.
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