Diopside

Diopside is a monoclinic pyroxene mineral with composition MgCaSi2O6. It forms complete solid solution series with hedenbergite (FeCaSi2O6) 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°.

Diopside

General

Category Inosilicate mineral

Formula MgCaSi2O6

IMA symbol Di

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

Color Commonly 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 hardness 5.5–6.5

Luster Vitreous to dull

Streak white

Specific gravity 3.278

Optical properties Biaxial (+)

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

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.

Formation

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.

Classification

In the structural classification of the International Mineralogical Association (IMA), diopside belongs to the calcium pyroxenes (Ca-pyroxenes) in the pyroxene group, along with augite, burnettite, davisite, esseneite, grossmanite, hedenbergite, johannsenite, kushiroite, petedunnite, and tissintite. 

In the outdated 8th edition of the mineral classification according to Strunz, diopside belonged to the mineral class of “silicates” and there to the section “chain silicates and band silicates (inosilicates)”, where it formed together with hedenbergite and johannsenite the “diopside series” with the system number VIII/D.01b within the group of “clinopyroxenes (monoclinic-prismatic)”.

In the Lapis mineral catalog by Stefan Weiß, last revised and updated in 2018, which is still based on this old form of the systematics by Karl Hugo Strunz, the mineral was given the system and mineral number VIII/F.01-050. In the Lapis systematics, this also corresponds to the section "Chain and band silicates", where diopside, together with aegirine, aegirine-augite, augite, davisite, esseneite, grossmanite, hedenbergite, jadeite, jervisite, johannsenite, kanoite, clinoenstatite, clinoferrosilite, kosmochlor, kushiroite, namansilite, natalyite, omphacite, petedunnite, pigeonite, spodumene and tissintite, forms the group of "clinopyroxenes" with the system number VIII/F.01. 

The 9th edition of Strunz's mineral classification, last updated in 2009 by the International Mineralogical Association (IMA), classifies diopside in the expanded class of "silicates and germanates," but also in the division of "chain and band silicates (inosilicates)." This division is further subdivided according to the structure of the silicate chains or bands, so that the mineral, according to its structure, can be found in the subdivision "chain and band silicates with 2-periodic single chains Si 2 O 6; pyroxene family," where it forms the group of "Ca-clinopyroxenes, diopside group" with the system number 9.DA.15, together with augite, davisite, esseneite, hedenbergite, johannsenite, kushiroite, and petedunnite. 

In the Dana classification of minerals, which is predominantly used in English-speaking countries, diopside has the system and mineral number 65.01.03a.01. This also corresponds to the class of "silicates" and the section "chain silicate minerals," where the mineral, together with augite, davisite, esseneite, hedenbergite, johannsenite, and petedunnite, is found in an unnamed group/ " C 2/ c clinopyroxenes (Ca-clinopyroxenes)" with the system number 65.01.03a within the subsection "chain silicates: simple unbranched chains, W=1 with chains P=2."

Crystal structure

Diopside crystallizes monoclinic in the space group C 2/ c (space group no. 15) with the lattice parameters a = 9.75 Å; b = 8.92 Å; c = 5.25 Å and β = 106.0° and 4 formula units per unit cell. 

Modifications and variations

Baikalite: a dark-colored variety of iron-rich diopside. The name comes from Lake Baikal (Slyudyanka mine, Lake Baikal, Irkutskaya Oblast, Transbaikalia, Siberia in Russia) topotype of this variety.

Fedorovite or Fedorowite (Carlo Fernando Maria Viola, 1899): a variety of diopside containing net quantities of iron and sodium. The etymology is linked to the surname of the dedicatee, the geologist Evgraf Stepanovich Fedorov. It is known from a single deposit: Monti Ernici, Province of Rome, Lazio, Italy.

Chromo-diopside: chrome-bearing, green variety, often gem-like with the formula (Ca,Cr)MgSi 2 O 6This variety is very widespread throughout the world. It is found in France at the Etang de Lherz in Ariège,,.

Lavrovite: apple-green, vanadium-bearing variety of diopside, known from Malo-Bystrinskoe, Lake Baikal, Irkutskaya Oblast, Siberia, Russia.

Schefferite (JCA Michaelson): manganese variety of diopside, with the formula (Ca,Mn)(MgFeMn)Si 2 O 6originally described in Långban, Filipstad, Värmland, Sweden and dedicated to the Swedish physician, botanist and chemist Henrik Teofilus Scheffer ,,.

Wallerite (Breithaupt): obsolete synonym for schefferite initially dedicated to the mineralogist Johan Gottschalk Wallerius.

Traversellite: a variety of diopside in pseudomorphosis of amphibole described in the Traversella Mine, Chiusella Valley, Province of Turin in Italy.

Violane (Breithaupt) : mananesiferous variety of diopside of violet to pale blue color, found at Miniera di Prabornaz, Saint-Marcel d'Aoste, Valle d'Aosta Italy.

Mineralogy and occurrence

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

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)2O6) 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.

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 gemological level completely unrelated, emerald being a precious stone and diopside being a semi-precious stone.

Green diopside crystals included within a white feldspar matrix are also sold as gemstones, usually as beads or cabochons. This stone is often marketed as 'green spot jasper' or green spot stone'.

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

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.

Sourced from Wikipedia