| Diopside | |
|---|---|
 Diopside – Bellecombe, Châtillon, Aosta Valley, Italy | |
| General | |
| Category | Inosilicatemineral |
| Formula | MgCaSi2O6 |
| IMA symbol | Di[1] |
| Strunz classification | 9.DA.15 |
| Crystal system | Monoclinic |
| Crystal class | Prismatic (2/m) (sameH-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 |
| References | [2][3][4] |
Diopside is a monoclinic pyroxenemineral with compositionMgCaSi
2O
6. It forms complete solid solution series withhedenbergite (FeCaSi
2O
6) andaugite, and partial solid solutions withorthopyroxene andpigeonite. It forms variably colored, but typically dull green crystals in themonoclinic prismatic class. It has two distinct prismatic cleavages at 87 and 93° typical of thepyroxene series. It has aMohs hardness of six, aVickers hardness of 7.7 GPa at a load of 0.98 N,[5] and aspecific gravity of 3.25 to 3.55. It is transparent to translucent withindices 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 is found inultramafic (kimberlite andperidotite)igneousrocks, and diopside-rich augite is common inmafic rocks, such as olivinebasalt andandesite. Diopside is also found in a variety ofmetamorphic rocks, such as in contact metamorphosedskarns developed from high silicadolomites. It is an important mineral in theEarth'smantle and is common in peridotitexenoliths erupted in kimberlite and alkali basalt.
Diopside is a precursor ofchrysotile (whiteasbestos) byhydrothermal alteration andmagmatic differentiation;[6] it can react with hydrous solutions ofmagnesium andchlorine to yield chrysotile by heating.[7] Somevermiculite deposits, most notably those inLibby, Montana, are contaminated with chrysotile (as well as other forms of asbestos) that formed from diopside.[8]
At relatively high temperatures, there is amiscibility gap between diopside andpigeonite, and at lower temperatures, between diopside andorthopyroxene. Thecalcium/(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 inperidotite xenoliths have been important in reconstructions of temperatures in theEarth's mantle.
Chrome diopside ((Ca,Na,Mg,Fe,Cr)
2(Si,Al)
2O
6) is a common constituent ofperidotite xenoliths, and dispersed grains are found nearkimberlite pipes, and as such are a prospecting indicator fordiamonds. Occurrences are reported inCanada,South Africa,Russia,Brazil, and a wide variety of other locations. In the US, chromian diopside localities are described in theserpentinite belt in northern California, in kimberlite in the Colorado-Wyoming State Line district, in kimberlite in the Iron Mountain district, Wyoming, inlamprophyre at Cedar Mountain in Wyoming, and in numerous anthills and outcrops of the Tertiary Bishop Conglomerate in theGreen 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.[9][citation needed]
Gemstone quality diopside is found in two forms: black star diopside and chrome diopside (which includeschromium, giving it a rich green color). At 5.5–6.5 on theMohs 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 aprecious stone and diopside being asemi-precious stone.[10]
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 amanganese-rich variety of diopside, violet to light blue in color.[11]
Diopside derives its name from theGreekdis, "twice", andòpsè, "face" in reference to the two ways of orienting the verticalprism.
Diopside was discovered and first described about 1800, by Brazilian naturalistJose Bonifacio de Andrada e Silva.
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.
