Petrologic and crystal-chemical significance of some deep-seated phlogopitesl

Phlogopites with reverse pleochroism (a> y: B), but with rims of phlogopite with normal pleochroism (a < y: F), from South African kimberlites and associated ultramafic xenoliths were studied to assess their geochemical and petrologic significance. Reverse-phlogopite megacrysts and reverse phlogopites in peridotites are poorer in TiOz, AlzO:, and Cr2O3 and richer in SiO2, MgO, and FeO * than their rims of normal phlogopite, a difference we ascribe to as metasomatism and/or mantling of the reverse phlogopite by kimberlite. All reverse-core phlogopites have similar compositions and represent grains that crystallized in situ in mantle peridotite, the megacrysts being derived from dissaggre-gated reverse-phlogopite-bearing peridotite. These reverse micas are higher in FeO * and lower in Al2O3 and Cr2O3 than any previously described "primary " mantle phlogopites. Optical-absorption spectra suggest that tetrahedrally coordinated Fe3+, IV(Fe3*), controls the phlogopite pleochroism, with reverse cores containing IV(Fe3*), whereas the normal rims do not. If a preference of Si> Al> Fe3+ is assumed for tetrahedral sites, then the type of pleochroism is best correlated with overall diferences in the availability of Si +