"Rock of the Month #125, posted for November 2011" ---

Chrome diopside is an attractive green semi-precious stone, a mineral of the pyroxene family. The ideal formula of diopside is CaMgSi₂O₆. A few tenths of one percent chromium can be a powerful chromophore, and one consequence of this is beautiful green colour in silicates both chrome-rich (such as uvarovite and andradite-demantoid garnets) and others with only minor Cr content (such as Cr diopside, emerald and some Cr micas). Thus 1-5 weight percent Cr₂O₃ - as found in some kimberlitic pyroxene - is ample to colour chrome diopside.

Diopside is a member of the clinopyroxene family, a monoclinic phase with characteristic pyroxene cleavage, Mohs hardness 5-6, specific gravity 3.2-3.38. Diopside commonly occurs in contact-metamorphic marbles, and with andradite-rich garnets in skarns, but chrome diopside is more typical of ultramafic rocks, and kimberlites in particular.

For this reason, research on chrome diopside in the past 40 years has been dominated by its potential use in mineral exploration for diamond. It was studied in South Africa and Russia as a constituent in mantle nodules found in kimberlites (e.g., Dawson and Smith, 1977). Soon the chemistry of diopside, along with garnets, ilmenites and chrome spinels, was examined as a pointer toward a) kimberlite in general and more importantly, b) diamondiferous kimberlite. Stephens and Dawson (1977) analysed diopsides from many kimberlites, their xenoliths, and diopsides found as inclusions within diamonds. Udachnaya and other kimberlites of Yakutia in Siberia are part of this story (e.g., Sobolev et al., 1997; Ilupin, 1997; Nimis, 1998, 2002). Diopside (of higher and lower Cr contents) has become a staple heavy indicator mineral for geochemical exploration in glaciated terrains, for both kimberlite and potentially Ni-Cu-PGE mineralized mafic-ultramafic rocks (Averill, 2009).

References

Averill,SA (2009) Useful Ni-Cu-PGE versus kimberlite indicator minerals in surficial sediments: similarities and differences. In `Application of Till and Stream Sediment Heavy Mineral and Geochemical Methods to Mineral Exploration in Western and Northern Canada' (Paulen,RC and McMartin,I editors), GAC Short Course Notes 18, 222pp., 125-139.

Dawson,JB and Smith,JV (1977) The MARID (mica- amphibole- rutile- ilmenite- diopside) suite of xenoliths in kimberlite. Geochim.Cosmochim.Acta 41, 309-323.

Ilupin,IP (1997) Chemical composition of chrome spinellid and the heterogeneity of the Daldyn kimberlite field. Geoc.Int. 35, 527-531.

Nimis,P (1998) Evaluation of diamond potential from the composition of peridotitic chromian diopside. Eur.J.Mineral. 10, 505-519.

Nimis,P (2002) The pressures and temperatures of formation of diamond based on thermobarometry of chromium diopside inclusions. Can.Mineral. 40, 871-884.

Sobolev,NV, Kaminsky,FV, Griffin,WL, Yefimova,ES, Win,TT, Ryan,CG and Botkunov,AI (1997) Mineral inclusions in diamonds from the Sputnik kimberlite pipe, Yakutia. Lithos 39, 135-157.

Stephens,WE and Dawson,JB (1977) Statistical comparison between pyroxenes from kimberlites and their associated xenoliths. J.Geol. 85, 433-449.