Clinohumite
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Clinohumite is an uncommon member of the humite group, a magnesium
silicate In chemistry, a silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is al ...
according to the chemical formula ( Mg, Fe)9( Si O4)4( F,O H)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
gemstone A gemstone (also called a fine gem, jewel, precious stone, or semiprecious stone) is a piece of mineral crystal which, in cut and polished form, is used to make jewelry or other adornments. However, certain rocks (such as lapis lazuli, opal, ...
s. Only two sources of gem-quality material are known: the
Pamir Mountains The Pamir Mountains are a mountain range between Central Asia and Pakistan. It is located at a junction with other notable mountains, namely the Tian Shan, Karakoram, Kunlun, Hindu Kush and the Himalaya mountain ranges. They are among the world ...
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 monoclinic mineral, clinohumite is typically a dark to light brownish or orangy yellow, somewhat resembling the hessonite variety of grossular. Clinohumite's
crystal habit In mineralogy, crystal habit is the characteristic external shape of an individual crystal or crystal group. The habit of a crystal is dependent on its crystallographic form and growth conditions, which generally creates irregularities due to l ...
is usually granular, but may also be
prismatic An optical prism is a transparent optical element with flat, polished surfaces that are designed to refract light. At least one surface must be angled — elements with two parallel surfaces are ''not'' prisms. The most familiar type of optical ...
; crystals are almost always small. Simple and multiple crystal twinning (on ) is common, resulting in a highly variable habit. Clinohumite is brittle with a hardness of 6 and a poor basal cleavage. Its specific gravity is 3.2–3.4, and its
fracture Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displa ...
is
conchoidal Conchoidal fracture describes the way that brittle materials break or fracture when they do not follow any natural planes of separation. Mindat.org defines conchoidal fracture as follows: "a fracture with smooth, curved surfaces, typically sli ...
to uneven; its streak is white. Clinohumite's transparency ranges from transparent to translucent; its luster ranges from a dull vitreous to resinous. Its refractive index (as measured via
sodium light A sodium-vapor lamp is a gas-discharge lamp that uses sodium in an excited state to produce light at a characteristic wavelength near 589  nm. Two varieties of such lamps exist: low pressure and high pressure. Low-pressure sodium lamps are ...
, 589.3 nm) is as follows: α 1.631; β 1.638–1.647; γ 1.668;, with a maximum
birefringence Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive). The birefring ...
of 0.028 (biaxial positive). Under shortwave ultraviolet light, some clinohumite may 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). Phillip Youngman, master faceter of
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, 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 Fluorine is a chemical element with the symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale yellow diatomic gas. As the most electronegative reactive element, it is extremely reacti ...
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

Clinohumite is a product of contact metamorphism and is commonly found as indistinct grains embedded in limestone. Its type occurrence is within the limestone ejecta of the Mount Vesuvius 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 Sør Rondane and Balchen Mountains of Antarctica; Mount Bischoff,
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, Utah, and Washington, US. Clinohumite also occurs as a minor component of some masses of
peridotite Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high prop ...
from the Earth's mantle emplaced into the
Earth's crust Earth's crust is Earth's thin outer shell of rock, referring to less than 1% of Earth's radius and volume. It is the top component of the lithosphere, a division of Earth's layers that includes the crust and the upper part of the mantle. The ...
and as a very rare component of peridotite xenoliths. These occurrences and implications have been summarized by Luth (2003) in a discussion of the possible importance of the mineral as a significant reservoir of water in the Earth's mantle. Titanium is a minor constituent of clinohumite in most such occurrences. Clinohumite is stable throughout the upper mantle to depths of at least and is a potential host phase for H (water) in this region of the Earth's interior. Minerals associated with humite include grossular, wollastonite,
forsterite Forsterite (Mg2SiO4; commonly abbreviated as Fo; also known as white olivine) is the magnesium-rich end-member of the olivine solid solution series. It is isomorphous with the iron-rich end-member, fayalite. Forsterite crystallizes in the orthor ...
, monticellite,
cuspidine Cuspidine is a fluorine bearing calcium silicate mineral (sorosilicate) with formula: Ca4(Si2O7)(F,OH)2. Cuspidine crystallizes in the monoclinic crystal system and occurs as acicular to spear shaped pale red to light brown crystals. It is a membe ...
,
fluoborite Fluoborite has a chemical formula of Mg3(BO3)(F,OH)3. Its name comes from its main chemical components, FLUOrine and BORon. It was first described in 1926. Fluoborite's crystal system is hexagonal, meaning it has one six-fold axis of rotation. It ...
, ludwigite,
dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...
,
calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
, talc,
biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumino ...
,
spinel Spinel () is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula in the cubic crystal system. Its name comes from the Latin word , which means ''spine'' in reference to its pointed crystals. Properties S ...
, vesuvianite, sanidine, meionite and
nepheline Nepheline, also called nephelite (), is a rock-forming mineral in the feldspathoid groupa silica-undersaturated aluminosilicate, Na3 K Al4 Si4 O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatites ...
.


Crystal structure

The structure is monoclinic with space group ''P''21/''b'' (''a''-unique). The unit cell has ''a'' = 4.7488 Å; ''b'' = 10.2875 Å; ''c'' = 13.6967 Å; and alpha = 100.63°; V = 667.65 Å3; Z = 2 for pure Mg hydroxyl-clinohumite. The odd setting of space group ''P''21/''c'' is chosen to preserve the ''a'' and ''b'' axes of olivine. The structure is closely related to that of olivine as well as the other humite minerals. Mg and Fe are in octahedral coordination with oxygen and silicon (Si) is in tetrahedral coordination. There are five distinct octahedral sites and two different tetrahedral sites. One of the octahedral sites is bonded to two OH,F atoms and is the site where Ti is partitioned.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. Clinohumite is a nesosilicate with no oxygen atoms shared between two silicons.


See also

* Alleghanyite * Chondrodite * Humite * Jerrygibbsite


References

{{Commons category Magnesium minerals Iron(II) minerals Gemstones Humite group Monoclinic minerals Minerals in space group 14