Kimberlitic chlorites from Sierra Leone, West Africa: unusual chemistries and structural polytypes

Chlorites occur as ovoid discrete nodules (1–4 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 (8 wt.%) varieties with a subgroup consisting of low Fe and high K(0.8–2.3 wt.% K₂O). The chlorites have high SiO₂ (46.6 wt.% max) and MgO (28 wt.% max), and low A1₂O₃ (8–12 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 Ia 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 1M and 2M₁ phlogopites, respectively. Chloritization 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.