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 (Mg
2SiO
4), 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
Los Osos
Los Osos (Spanish for "the bears") is an unincorporated town and census-designated place (CDP) in San Luis Obispo County, California. Located on the Central Coast of California, Los Osos had a population estimated to be 16,533 in 2019.
Histo ...
,
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,
Waratah, Tasmania
Waratah is a locality and town in North Western Tasmania adjacent to Savage River National Park. The town was constructed to support a tin mine
Mine, mines, miners or mining may refer to:
Extraction or digging
* Miner, a person engaged in m ...
; the
Saualpe Mountains of
Carinthia
Carinthia (german: Kärnten ; sl, Koroška ) is the southernmost States of Austria, Austrian state, in the Eastern Alps, and is noted for its mountains and lakes. The main language is German language, German. Its regional dialects belong to t ...
, the
Koralpe
The Koralpe ( en, Kor Alps, sl, Golica or ), also referred to as ''Koralm'', is a mountain range in southern Austria which separates eastern Carinthia (state), Carinthia from southern Styria. The southern parts of the range extend into Slovenia. ...
mountains of
Styria
Styria (german: Steiermark ; Serbo-Croatian and sl, ; hu, Stájerország) is a state (''Bundesland'') in the southeast of Austria. With an area of , Styria is the second largest state of Austria, after Lower Austria. Styria is bordered to ...
, 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
Southern may refer to:
Businesses
* China Southern Airlines, airline based in Guangzhou, China
* Southern Airways, defunct US airline
* Southern Air, air cargo transportation company based in Norwalk, Connecticut, US
* Southern Airways Express, M ...
and
Western Finland;
Bavaria and
Saxony,
Germany; eastern
Greenland;
Ambasamudram
Ambasamudram is the principal town of the Ambasamudram taluk in Tirunelveli district in the state of Tamil Nadu, India. The entire taluk had a population of 392,226 as of 2001, with 42.5% classified as rural. The town of Ambasamudram had a popu ...
in
Tamil Nadu,
India;
Honshū
, historically called , is the largest and most populous island of Japan. It is located south of Hokkaidō across the Tsugaru Strait, north of Shikoku across the Inland Sea, and northeast of Kyūshū across the Kanmon Straits. The island separa ...
,
Japan
Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
;
Suan, North Korea
Suan County is a county in North Hwanghae province, North Korea.
Administrative divisions
Suan county is divided into 1 ''ŭp
An ''eup'' or ''ŭp'' () is an administrative unit in both North Korea and South Korea similar to the unit of town.
...
;
Nordland
Nordland (; smj, Nordlánnda, sma, Nordlaante, sme, Nordlánda, en, Northland) is a county in Norway in the Northern Norway region, the least populous of all 11 counties, bordering Troms og Finnmark in the north, Trøndelag in the south, N ...
,
Norway;
KwaZulu-Natal
KwaZulu-Natal (, also referred to as KZN and known as "the garden province") is a province of South Africa that was created in 1994 when the Zulu bantustan of KwaZulu ("Place of the Zulu" in Zulu) and Natal Province were merged. It is locate ...
and
Northern Cape Province,
South Africa;
Andalusia,
Spain;
Värmland and
Västmanland,
Sweden
Sweden, formally the Kingdom of Sweden,The United Nations Group of Experts on Geographical Names states that the country's formal name is the Kingdom of SwedenUNGEGN World Geographical Names, Sweden./ref> is a Nordic country located on ...
;
Isle of Skye
The Isle of Skye, or simply Skye (; gd, An t-Eilean Sgitheanach or ; sco, Isle o Skye), is the largest and northernmost of the major islands in the Inner Hebrides of Scotland. The island's peninsulas radiate from a mountainous hub dominated ...
,
Scotland; and the states of
California,
Colorado,
Massachusetts,
New Jersey,
New Mexico,
New York
New York most commonly refers to:
* New York City, the most populous city in the United States, located in the state of New York
* New York (state), a state in the northeastern United States
New York may also refer to:
Film and television
* '' ...
,
Oklahoma
Oklahoma (; Choctaw language, Choctaw: ; chr, ᎣᎧᎳᎰᎹ, ''Okalahoma'' ) is a U.S. state, state in the South Central United States, South Central region of the United States, bordered by Texas on the south and west, Kansas on the nor ...
,
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