Kimberlites are a clan of volatile-rich
(dominantly carbon dioxide) potassic ultrabasic rocks. Commonly, they
exhibit a distinctive inequigranular texture resulting from the presence
of macrocrysts (and in some instances megacrysts) set in a fine grained
matrix. The megacryst/macrocryst assemblage consists of rounded anhedral
crystals of magnesian ilmenite, Cr-poor titanian pyrope, olivine, Cr-poor
clinopyroxene, phlogopite, enstatite and Ti-poor chromite. Olivine is
the dominant member of the macrocryst assemblage. The matrix minerals
may include: second generation euhedral primary olivine and/or phlogopite,
together with perovskite, spinel (titaniferous magnesian aluminous chromite,
titanian chromite, members of the magnesian ulvospinel-ulvospinel-magnetite
series), diopside (Al- and Ti- poor), monticellite, apatite, calcite,
and primary late-stage serpentine (commonly Fe rich). Some kimberlites
contain late-stage poikilitic eastonite phlogopites. Nickeliferous sulphides
and rutile are common accessory minerals. The replacement of early-formed
olivine, phlogopite, monticellite, and apatite by deuteric serpentine
and calcite is common. Evolved members of the clan may be devoid of,
or poor in, macrocrysts, and composed essentially of calcite, serpentine,
and magnetite together with minor phlogopite, apatite and perovskite.