Clinohumite

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Clinohumite
Clinohumite.jpg
General
Category Nesosilicate
Formula
(repeating unit)
(Mg,Fe)9(SiO4)4(F,OH)2
IMA symbol Chu [1]
Crystal system Monoclinic
Crystal class Prismatic (2/m)
(same H-M symbol)
Space group P21/c
Unit cell a = 13.71 Å, b = 4.75 Å,
c = 10.29 Å; β = 100.83°; Z = 2
Identification
ColorBrownish to orange, yellow, red
Crystal habit Granular, prismatic, twinned
Twinning Simple, lamellar common on {100}
Cleavage Poor on {100}
Fracture Subconchoidal to uneven
Mohs scale hardness6
Luster Vitreous to resinous
Streak White
Diaphaneity Transparent to translucent
Specific gravity 3.17–3.35
Optical propertiesbiaxial (+)
Refractive index nα = 1.623 – 1.702 nβ = 1.636 – 1.709 nγ = 1.651 – 1.728
Birefringence +0.028
Pleochroism X = golden yellow, yellow-brown, deep reddish yellow; Y = pale yellow, yellow-orange, light yellow; Z = pale yellow, yellow-orange, colorless
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(Si O 4)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.

Contents

Properties

A monoclinic mineral, clinohumite is typically a dark to light brownish or orangy yellow, somewhat resembling the hessonite variety of grossular. [8] Clinohumite's crystal habit is usually granular, but may also be prismatic; crystals are almost always small. Simple and multiple crystal twinning (on {001}) 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 is conchoidal 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, 589.3 nm) is as follows: α 1.631; β 1.638–1.647; γ 1.668;, with a maximum birefringence 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). [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 Hydroxylclinohumite-21345.jpg
Two intergrown crystals (1.5 x 1 x 0.5 cm) from Koksha Valley, Badakhshan Province, Afghanistan

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, Waratah, Tasmania; the Saualpe Mountains of Carinthia, the Koralpe mountains of Styria, and the Vals, Virgen, and Ziller valleys of the Tyrol, Austria; the Jacupiranga mine of Cajati, São Paulo State, Southeast Region, Brazil; the Pirin Mountains of Bulgaria; Bancroft, Ontario, Notre Dame du Laus, Wakefield, and Villedieu Township, Quebec, Canada; Southern and Western Finland; Bavaria and Saxony, Germany; eastern Greenland; Ambasamudram in Tamil Nadu, India; Honshū, Japan; Suan, North Korea; Nordland, Norway; KwaZulu-Natal and 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 peridotite from the Earth's mantle emplaced into the Earth's crust and as a very rare component of peridotite xenoliths. These occurrences and implications have been summarized by Luth (2003) [11] 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 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. Clinohumite-structure.jpg
Crystal structure of clinohumite in polyhedral representation, a-axis projection, b-horizontal. H atom are blue spheres.

The structure is monoclinic with space group P21/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. [14] The odd setting of space group P21/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. [15] Clinohumite is a nesosilicate with no oxygen atoms shared between two silicons.

See also

Related Research Articles

<span class="mw-page-title-main">Biotite</span> Group of phyllosilicate minerals within the mica group

Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula K(Mg,Fe)3AlSi3O10(F,OH)2. It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous end-members include siderophyllite and eastonite. Biotite was regarded as a mineral species by the International Mineralogical Association until 1998, when its status was changed to a mineral group. The term biotite is still used to describe unanalysed dark micas in the field. Biotite was named by J.F.L. Hausmann in 1847 in honor of the French physicist Jean-Baptiste Biot, who performed early research into the many optical properties of mica.

<span class="mw-page-title-main">Spinel</span> Mineral or gemstone

Spinel is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula MgAl
2
O
4
in the cubic crystal system. Its name comes from the Latin word spinella, a diminutive form of spine, in reference to its pointed crystals.

<span class="mw-page-title-main">Garnet</span> Mineral, semi-precious stone

Garnets are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.

<span class="mw-page-title-main">Olivine</span> Magnesium iron silicate solid solution series mineral

The mineral olivine is a magnesium iron silicate with the chemical formula (Mg,Fe)2SiO4. It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickly on the surface. For this reason, olivine has been proposed as a good candidate for accelerated weathering to sequester carbon dioxide from the Earth's oceans and atmosphere, as part of climate change mitigation. Olivine also has many other historical uses, such as the gemstone peridot, as well as industrial applications like metalworking processes.

<span class="mw-page-title-main">Peridot</span> Green gem-quality mineral

Peridot, sometimes called chrysolite, is a yellowish-green transparent variety of olivine. Peridot is one of the few gemstones that occur in only one color.

<span class="mw-page-title-main">Kimberlite</span> Igneous rock which sometimes contains diamonds

Kimberlite is an igneous rock and a rare variant of peridotite. It is most commonly known to be the main host matrix for diamonds. It is named after the town of Kimberley in South Africa, where the discovery of an 83.5-carat (16.70 g) diamond called the Star of South Africa in 1869 spawned a diamond rush and the digging of the open-pit mine called the Big Hole. Previously, the term kimberlite has been applied to olivine lamproites as Kimberlite II, however this has been in error.

<span class="mw-page-title-main">Dunite</span> Ultramafic and ultrabasic rock from Earths mantle which is made of the mineral olivine

Dunite, also known as olivinite, is an intrusive igneous rock of ultramafic composition and with phaneritic (coarse-grained) texture. The mineral assemblage is greater than 90% olivine, with minor amounts of other minerals such as pyroxene, chromite, magnetite, and pyrope. Dunite is the olivine-rich endmember of the peridotite group of mantle-derived rocks.

<span class="mw-page-title-main">Serpentine subgroup</span> Group of phyllosilicate minerals

Serpentine subgroup are greenish, brownish, or spotted minerals commonly found in serpentinite. They are used as a source of magnesium and asbestos, and as decorative stone. The name comes from the greenish colour and smooth or scaly appearance from the Latin serpentinus, meaning "serpent rock".

<span class="mw-page-title-main">Peridotite</span> Coarse-grained ultramafic igneous rock type

Peridotite ( PERR-ih-doh-tyte, pə-RID-ə-) 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 proportions of magnesium-rich olivine, with appreciable iron. Peridotite is derived from Earth's mantle, either as solid blocks and fragments, or as crystals accumulated from magmas that formed in the mantle. The compositions of peridotites from these layered igneous complexes vary widely, reflecting the relative proportions of pyroxenes, chromite, plagioclase, and amphibole.

<span class="mw-page-title-main">Forsterite</span> Magnesium end-member of olivine, a nesosilicate mineral

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 orthorhombic system (space group Pbnm) with cell parameters a 4.75 Å (0.475 nm), b 10.20 Å (1.020 nm) and c 5.98 Å (0.598 nm).

<span class="mw-page-title-main">Diopside</span> Pyroxene mineral

Diopside is a monoclinic pyroxene mineral with composition MgCaSi
2
O
6
. It forms complete solid solution series with hedenbergite and augite, and partial solid solutions with orthopyroxene and pigeonite. It forms variably colored, but typically dull green crystals in the monoclinic prismatic class. It has two distinct prismatic cleavages at 87 and 93° typical of the pyroxene series. It has a Mohs hardness of six, a Vickers hardness of 7.7 GPa at a load of 0.98 N, and a specific gravity of 3.25 to 3.55. It is transparent to translucent with indices of refraction of nα=1.663–1.699, nβ=1.671–1.705, and nγ=1.693–1.728. The optic angle is 58° to 63°.

<span class="mw-page-title-main">Brazilianite</span>

Brazilianite, whose name derives from its country of origin, Brazil, is a typically yellow-green phosphate mineral, most commonly found in phosphate-rich pegmatites.

<span class="mw-page-title-main">Enstatite</span> Pyroxene: magnesium-iron silicate with MgSiO3 and FeSiO3 end-members

Enstatite is a mineral; the magnesium endmember of the pyroxene silicate mineral series enstatite (MgSiO3) – ferrosilite (FeSiO3). The magnesium rich members of the solid solution series are common rock-forming minerals found in igneous and metamorphic rocks. The intermediate composition, (Mg,Fe)SiO
3
, has historically been known as hypersthene, although this name has been formally abandoned and replaced by orthopyroxene. When determined petrographically or chemically the composition is given as relative proportions of enstatite (En) and ferrosilite (Fs) (e.g., En80Fs20).

<span class="mw-page-title-main">Tsavorite</span> Variety of grossular

Tsavorite or tsavolite is a variety of the garnet group species grossular, a calcium-aluminium garnet with the formula Ca3Al2Si3O12. Trace amounts of vanadium or chromium provide the green color.

<span class="mw-page-title-main">Grossular</span> Garnet, nesosilicate mineral

Grossular is a calcium-aluminium species of the garnet group of minerals. It has the chemical formula of Ca3Al2(SiO4)3 but the calcium may, in part, be replaced by ferrous iron and the aluminium by ferric iron. The name grossular is derived from the botanical name for the gooseberry, grossularia, in reference to the green garnet of this composition that is found in Siberia. Other shades include cinnamon brown (cinnamon stone variety), red, and yellow. Grossular is a gemstone.

<span class="mw-page-title-main">Hauyne</span> Silicate mineral

Hauyne or haüyne, also called hauynite or haüynite, is a tectosilicate sulfate mineral with endmember formula Na3Ca(Si3Al3)O12(SO4). As much as 5 wt % K2O may be present, and also H2O and Cl. It is a feldspathoid and a member of the sodalite group. Hauyne was first described in 1807 from samples discovered in Vesuvian lavas in Monte Somma, Italy, and was named in 1807 by Brunn-Neergard for the French crystallographer René Just Haüy (1743–1822). It is sometimes used as a gemstone.

<span class="mw-page-title-main">Chondrodite</span>

Chondrodite is a nesosilicate mineral with formula (Mg,Fe)
5
(SiO
4
)
2
(F,OH,O)
2
. Although it is a fairly rare mineral, it is the most frequently encountered member of the humite group of minerals. It is formed in hydrothermal deposits from locally metamorphosed dolomite. It is also found associated with skarn and serpentinite. It was discovered in 1817 at Pargas in Finland, and named from the Greek for "granule", which is a common habit for this mineral.

<span class="mw-page-title-main">Ringwoodite</span> High-pressure phase of magnesium silicate

Ringwoodite is a high-pressure phase of Mg2SiO4 (magnesium silicate) formed at high temperatures and pressures of the Earth's mantle between 525 and 660 km (326 and 410 mi) depth. It may also contain iron and hydrogen. It is polymorphous with the olivine phase forsterite (a magnesium iron silicate).

Antigorite Monoclinic mineral

Antigorite is a lamellated, monoclinic mineral in the phyllosilicate serpentine subgroup with the ideal chemical formula of (Mg,Fe2+)3Si2O5(OH)4. It is the high-pressure polymorph of serpentine and is commonly found in metamorphosed serpentinites. Antigorite, and its serpentine polymorphs, play an important role in subduction zone dynamics due to their relative weakness and high weight percent of water (up to 13 weight % H2O). It is named after its type locality, the Geisspfad serpentinite, Valle Antigorio in the border region of Italy/Switzerland and is commonly used as a gemstone in jewelry and carvings.

References

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  2. Mineralienatlas
  3. 1 2 Handbook of Mineralogy
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  5. Clinohumite on Webmineral
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  11. Luth, R. W. (2003) Mantle Volatiles – Distribution and Consequences. In The Mantle and Core (ed. R. W. Carlson) Vol. 2 Treatise on Geochemistry (eds. H. D. Holland and K. K. Turekian), Elsevier-Pergamon, Oxford. ISBN   0-08-043751-6
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  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 .
  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.