Diopside

Last updated
Diopside
Diopside Aoste.jpg
Diopside - Bellecombe, Châtillon, Aosta Valley, Italy
General
Category Inosilicate mineral
Formula
(repeating unit)
MgCaSi2O6
Strunz classification 9.DA.15
Crystal system Monoclinic
Crystal class Prismatic (2/m)
(same H-M symbol)
Space group C2/c
Unit cell a = 9.746  Å, b = 8.899 Å
c = 5.251 Å; β = 105.79°; Z = 4
Identification
ColorCommonly light to dark green; may be blue, brown, colorless, white to snow white, grey, pale violet
Crystal habit Short prismatic crystals common, may be granular, columnar, massive
Twinning Simple and multiple twins common on {100} and {001}
Cleavage Distinct/good on {110}
Fracture Irregular/uneven, conchoidal
Tenacity Brittle
Mohs scale hardness5.5 - 6.5
Luster Vitreous to dull
Streak white
Specific gravity 3.278
Optical propertiesBiaxial (+)
Refractive index nα= 1.663 - 1.699, nβ= 1.671 - 1.705, nγ= 1.693 - 1.728
Birefringence δ = 0.030
2V angle Measured: 58° to 63°
Dispersion Weak to distinct, r>v
Melting point 1391 °C
References [1] [2] [3]

Diopside is a monoclinic pyroxene mineral with composition MgCaSi
2
O
6
. It forms complete solid solution series with hedenbergite (FeCaSi
2
O
6
) 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, [4] 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°.

Contents

Formation

Diopside crystal from De Kalb, New York (size: 4.3 x 3.3 x 1.9 cm) Diopside-225169.jpg
Diopside crystal from De Kalb, New York (size: 4.3 x 3.3 x 1.9 cm)

Diopside is found in ultramafic (kimberlite and peridotite) igneous rocks, and diopside-rich augite is common in mafic rocks, such as olivine basalt and andesite. Diopside is also found in a variety of metamorphic rocks, such as in contact metamorphosed skarns developed from high silica dolomites. It is an important mineral in the Earth's mantle and is common in peridotite xenoliths erupted in kimberlite and alkali basalt.

Mineralogy and occurrence

A green diopside found in Outokumpu, Finland Diopside-177506.jpg
A green diopside found in Outokumpu, Finland

Diopside is a precursor of chrysotile (white asbestos) by hydrothermal alteration and magmatic differentiation; [5] it can react with hydrous solutions of magnesium and chlorine to yield chrysotile by heating at 600 °C for three days. [6] Some vermiculite deposits, most notably those in Libby, Montana, are contaminated with chrysotile (as well as other forms of asbestos) that formed from diopside. [7]

At relatively high temperatures, there is a miscibility gap between diopside and pigeonite, and at lower temperatures, between diopside and orthopyroxene. The calcium/(calcium+magnesium+iron) ratio in diopside that formed with one of these other two pyroxenes is particularly sensitive to temperature above 900 °C, and compositions of diopside in peridotite xenoliths have been important in reconstructions of temperatures in the Earth's mantle.

Chrome diopside ((Ca,Na,Mg,Fe,Cr)
2
(Si,Al)
2
O
6
) is a common constituent of peridotite xenoliths, and dispersed grains are found near kimberlite pipes, and as such are a prospecting indicator for diamonds. Occurrences are reported in Canada, South Africa, Russia, Brazil, and a wide variety of other locations. In the US, chromian diopside localities are described in the serpentinite belt in northern California, in kimberlite in the Colorado-Wyoming State Line district, in kimberlite in the Iron Mountain district, Wyoming, in lamprophyre at Cedar Mountain in Wyoming, and in numerous anthills and outcrops of the Tertiary Bishop Conglomerate in the Green River Basin of Wyoming. Much chromian diopside from the Green River Basin localities and several of the State Line Kimberlites have been gem in character. [8] [ citation needed ]

As a gem

Gemstone quality diopside is found in two forms: black star diopside and chrome diopside (which includes chromium, giving it a rich green color). At 5.5–6.5 on the Mohs scale, chrome diopside is relatively soft to scratch. Due to the deep green color of the gem, they are sometimes referred to as Siberian emeralds, although they are on a mineralogical level completely unrelated, emerald being a precious stone and diopside being a semi-precious stone. [9]

Violane is a manganese-rich variety of diopside, violet to light blue in color. [10]

Etymology and history

Diopside derives its name from the Greek dis, "twice", and òpsè, "face" in reference to the two ways of orienting the vertical prism.

Diopside was discovered and first described about 1800, by Brazilian naturalist Jose Bonifacio de Andrada e Silva.

Potential uses

Diopside based ceramics and glass-ceramics have potential applications in various technological areas. A diopside based glass-ceramic named 'silceram' was produced by scientists from Imperial College, UK during the 1980s from blast furnace slag and other waste products. They also produced glass-ceramic is a potential structural material. Similarly, diopside based ceramics and glass-ceramics have potential applications in the field of biomaterials, nuclear waste immobilization and sealing materials in solid oxide fuel cells.

Related Research Articles

Gabbro Coarse-grained mafic intrusive rock

Gabbro is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is chemically equivalent to rapid-cooling, fine-grained basalt. Much of the Earth's oceanic crust is made of gabbro, formed at mid-ocean ridges. Gabbro is also found as plutons associated with continental volcanism. Due to its variant nature, the term gabbro may be applied loosely to a wide range of intrusive rocks, many of which are merely "gabbroic". By rough analogy, gabbro is to basalt as granite is to rhyolite.

Kimberlite Igneous rock which sometimes contains diamonds

Kimberlite is an igneous rock, which sometimes contains 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.

Pyroxene Group of inosilicate minerals with single chains of silica tetrahedra

The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula XY(Si,Al)
2
O
6
, where X represents calcium (Ca), sodium (Na), iron or magnesium (Mg) and more rarely zinc, manganese or lithium, and Y represents ions of smaller size, such as chromium (Cr), aluminium (Al), iron, magnesium (Mg), cobalt (Co), manganese (Mn), scandium (Sc), titanium (Ti), vanadium (V) or even iron. Although aluminium substitutes extensively for silicon in silicates such as feldspars and amphiboles, the substitution occurs only to a limited extent in most pyroxenes. They share a common structure consisting of single chains of silica tetrahedra. Pyroxenes that crystallize in the monoclinic system are known as clinopyroxenes and those that crystallize in the orthorhombic system are known as orthopyroxenes.

Augite

Augite is a common rock-forming pyroxene mineral with formula (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6. The crystals are monoclinic and prismatic. Augite has two prominent cleavages, meeting at angles near 90 degrees.

Serpentine subgroup Rock-forming hydrous magnesium iron phyllosilicate

The serpentine subgroup are greenish, brownish, or spotted minerals commonly found in serpentinite rocks. They are used as a source of magnesium and asbestos, and as a decorative stone. The name is thought to come from the greenish color being that of a serpent.

Peridotite A coarse-grained ultramafic igneous rock

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.

Phlogopite

Phlogopite is a yellow, greenish, or reddish-brown member of the mica family of phyllosilicates. It is also known as magnesium mica.

Wollastonite Single chain calcium inosilicate (CaSiO3)

Wollastonite is a calcium inosilicate mineral (CaSiO3) that may contain small amounts of iron, magnesium, and manganese substituting for calcium. It is usually white. It forms when impure limestone or dolomite is subjected to high temperature and pressure, which sometimes occurs in the presence of silica-bearing fluids as in skarns or in contact with metamorphic rocks. Associated minerals include garnets, vesuvianite, diopside, tremolite, epidote, plagioclase feldspar, pyroxene and calcite. It is named after the English chemist and mineralogist William Hyde Wollaston (1766–1828).

Jadeite Pyroxene mineral

Jadeite is a pyroxene mineral with composition NaAlSi2O6. It is monoclinic. It has a Mohs hardness of about 6.5 to 7.0 depending on the composition. The mineral is dense, with a specific gravity of about 3.4.

Tremolite Amphibole, double chain inosilicate mineral

Tremolite is a member of the amphibole group of silicate minerals with composition: Ca2(Mg5.0-4.5Fe2+0.0-0.5)Si8O22(OH)2. Tremolite forms by metamorphism of sediments rich in dolomite and quartz. Tremolite forms a series with actinolite and ferro-actinolite. Pure magnesium tremolite is creamy white, but the color grades to dark green with increasing iron content. It has a hardness on Mohs scale of 5 to 6. Nephrite, one of the two minerals of the gemstone jade, is a green variety of tremolite.

Enstatite 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).

Pyroxenite Igneous rock

Pyroxenite is an ultramafic igneous rock consisting essentially of minerals of the pyroxene group, such as augite, diopside, hypersthene, bronzite or enstatite. Pyroxenites are classified into clinopyroxenites, orthopyroxenites, and the websterites which contain both types of pyroxenes. Closely allied to this group are the hornblendites, consisting essentially of hornblende and other amphiboles.

Hornfels

Hornfels is the group name for a set of contact metamorphic rocks that have been baked and hardened by the heat of intrusive igneous masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. These properties are due to fine grained non-aligned crystals with platy or prismatic habits, characteristic of metamorphism at high temperature but without accompanying deformation. The term is derived from the German word Hornfels, meaning "hornstone", because of its exceptional toughness and texture both reminiscent of animal horns. These rocks were referred to by miners in northern England as whetstones.

Anthophyllite

Anthophyllite is an amphibole mineral: ☐Mg2Mg5Si8O22(OH)2 (☐ is for a vacancy, a point defect in the crystal structure), magnesium iron inosilicate hydroxide. Anthophyllite is polymorphic with cummingtonite. Some forms of anthophyllite are lamellar or fibrous and are classed as asbestos. The name is derived from the Latin word anthophyllum, meaning clove, an allusion to the most common color of the mineral.

Knorringite is a mineral species belonging to the garnet group, and forms a series with the species pyrope. It was discovered in 1968 in the Kao kimberlite pipe in the Butha-Buthe District of Lesotho and is named after Oleg Von Knorring, a professor of mineralogy at the University of Leeds in England.

Komatiite Ultramafic mantle-derived volcanic rock

Komatiite is a type of ultramafic mantle-derived volcanic rock defined as having crystallised from a lava of at least 18 wt% MgO. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Komatiite was named for its type locality along the Komati River in South Africa, and frequently displays spinifex texture composed of large dendritic plates of olivine and pyroxene.

Omphacite

Omphacite is a member of the clinopyroxene group of silicate minerals with formula: (Ca, Na)(Mg, Fe2+, Al)Si2O6. It is a variably deep to pale green or nearly colorless variety of clinopyroxene. It normally appears in eclogite, which is the high-pressure metamorphic rock of basalt. Omphacite is the solid solution of Fe-bearing diopside and jadeite. It crystallizes in the monoclinic system with prismatic, typically twinned forms, though usually anhedral. Its space group can be P2/n or C2/c depending on the thermal history. It exhibits the typical near 90° pyroxene cleavage. It is brittle with specific gravity of 3.29 to 3.39 and a Mohs hardness of 5 to 6.

Hellyerite

Hellyerite, NiCO3·6(H2O), is an hydrated nickel carbonate mineral. It is light blue to bright green in colour, has a hardness of 2.5, a vitreous luster, a white streak and crystallises in the monoclinic system. The crystal habit is as platy and mammillary encrustations on its matrix.

Pigeonite

Pigeonite is a mineral in the clinopyroxene subgroup of the pyroxene group. It has a general formula of (Ca,Mg,Fe)(Mg,Fe)Si
2
O
6
. The calcium cation fraction can vary from 5% to 25%, with iron and magnesium making up the rest of the cations.

Navajo volcanic field Volcanic field in southwestern United States

The Navajo volcanic field is a monogenetic volcanic field located in the Four Corners region of the United States, in the central part of the Colorado Plateau. The volcanic field consist of over 80 volcanoes and associated intrusions of unusual potassium-rich compositions, with an age range of 26.2 to 24.7 million years (Ma).

References

  1. C. D. Gribble, ed. (1988). "The Silicate Minerals". Rutley's Elements of Mineralogy (27th ed.). London: Unwin Hyman Ltd. p. 378. ISBN   0-04-549011-2.
  2. Mindat page for Diopside
  3. Handbook of Mineralogy
  4. M M Smedskjaer; M Jensen; Y-Z Yue (2008). "Theoretical calculation and measurement of the hardness of diopside". Journal of the American Ceramic Society. 91 (2): 514–518. doi:10.1111/j.1551-2916.2007.02166.x.
  5. A L Boettcher (1967). "The Rainy Creek alkaline-ultramafic igneous complex near Libby, Montana. I: Ultramafic rocks and fenite". Journal of Geology. 75: 536–553. doi:10.1086/627280.
  6. Eugenio Barrese; Elena Belluso; Francesco Abbona (1 February 1997). "On the transformation of synthetic diopside into chrysotile". European Journal of Mineralogy. 9 (1): 83–87. doi:10.1127/ejm/9/1/0083.
  7. "Asbestos in Your Home". United States Environmental Protection Agency. 2003. Archived from the original on October 8, 2006. Retrieved 2007-11-20.
  8. Hausel, W. Dan (2006). Geology and Geochemistry of the Leucite Hills Lamproitic field, Rocks Springs Uplift, Wyoming. laramie, Wyoming: Wyoming geological survey.
  9. Kalotay, Daphne (2010). Russian Winter (First ed.). New York, NY: Harper. pp.  184–185. ISBN   978-0-06-196216-5.
  10. Mindat page for Violane