Group I Kimberlites
Group-I kimberlites are of CO2-rich ultramafic potassic igneous rocks dominated by a primary mineral assemblage of forsteritic olivine, magnesian ilmenite, chromium pyrope, almandine-pyrope, chromium diopside (in some cases subcalcic), phlogopite, enstatite and of Ti-poor chromite. Group I kimberlites exhibit a distinctive inequigranular texture caused by macrocrystic (0.5–10 mm, 0.020–0.39 in) to megacrystic (10–200 mm, 0.39–7.9 in) phenocrysts of olivine, pyrope, chromian diopside, magnesian ilmenite and phlogopite, in a fine to medium grained groundmass.
The groundmass mineralogy, which more closely resembles a true composition of the igneous rock, contains forsteritic olivine, pyrope garnet, Cr-diopside, magnesian ilmenite and spinel.
Read more about this topic: Kimberlite, Petrology
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