unusual chemistries and structural polytypes

Chlorites occur as ovoid discrete nodules (l-4 cm), as anhedral macrocrysts (0'20 mm) and euhedral crystals (0.05 mm) in kimberlite, and as cleavage replacement products of high Fe (reversely pleochroic), low Ti + cr phlogopites. The nodules are severely deformed and four textural groups are recognized (fibrous, platy, tectonized, multiple cleavage) that consist of intergrown chlorites and vermiculites of variable compositions. Major chemical types in the nodules are classified as high Fe (25 wt.%), intermediate Fe (15 wt.%),and low Fe (8wt.%) varieties with a subgroup consisting of low Fe and high K(0.8-2.3 wt.Vo KzO). The chlorites have high SiO2 (46.6 wt.Vo max) and MgO (28 wt. % max), and low Al2O3 g-12 wt.%) contents. High Fe3 * and later alteration result in vacancies (corroborated by electron densities) in the interlayer sheet. The chlorites and vermiculites are dominantly of the rare la structural type, intergrown with more typical IIb chlorite in some specimens. Two-layer "s " and "t " stacking sequences identified in Ia chlorites and vermiculites suggest alteration from parent lM andlMl phlogopites, respectively. Chloriti-zation of peridotitic phlogopite nodules and macrocrysts took place at relatively low P-T conditions concurrent with, or followed by crystallization of primary magmatic chlorite in the kimberlite groundmass