The Megacryst Suite

Abstract

Single crystals of large size (commonly > 2.0 cm) are widespread and sometimes abundant as inclusions in kimberlite; they have been referred to both as megacrysts (the terminology adopted here) and discrete nodules (Nixon and Boyd 1973b). In considering this mineral suite, the assumption is that the minerals are so large that they could not have been derived by fragmentation of upper-mantle xenolith rock types, the grain-size of which is usually in the range 2–4 mm. Two series of megacrysts are known, those which are rich in chromium and those which are chrome-poor; the chrome-rich ones are by far the rarer and have so far been described from the Excelsior Mine, South Africa, and from some Colorado kimberlite pipes. A chrome-rich (8.5 wt% Cr₂O₃) and low-lime (2.7% CaO) garnet, 2 cm in diameter, has been described from the Excelsior Mine, S. Africa (Boyd and Dawson 1972); it is compositionally similar than the high-Cr, low-Ca pyropes found as diamond inclusions, and occurring as small isolated crystals in the kimberlite of the Finsch Mine (Gurney and Switzer 1973). The garnet contains small altered crystals of ?olivine or ?orthopyroxene.