K2Ca[Si4O10] ⋅ 5H2O, New Mineral Species from the Khibiny Alkaline Pluton, Kola Peninsula, Russia1

Abstract—New minerals, shlykovite and cryptophyllite, hydrous Ca and K phyllosilicates, have been identi fied in hyperalkaline pegmatite at Mount Rasvumchorr, Khibiny alkaline pluton, Kola Peninsula, Russia. They are the products of lowtemperature hydrothermal activity and are associated with aegirine, potassium feldspar, nepheline, lamprophyllite, eudialyte, lomonosovite, lovozerite, tisinalite, shcherbakovite, shafra novskite, ershovite, and megacyclite. Shlykovite occurs as lamellae up to 0.02 × 0.02 × 0.5 mm in size or fibers up to 0.5 mm in length usually combined in aggregates up to 3 mm in size, crusts, and parallelcolumnar vein lets. Cryptophyllite occurs as lamellae up to 0.02 × 0.1 × 0.2 mm in size intergrown with shlykovite being ori ented parallel to {001} or chaotically arranged. Separate crystals of the new minerals are transparent and col orless; the aggregates are beige, brownish, light cream, and pale yellowishgrayish. The cleavage is parallel to (001) perfect. The Mohs hardness of shlykovite is 2.5–3. The calculated densities of shlykovite and crypto phyllite are 2.444 and 2.185 g/cm3, respectively. Both minerals are biaxial; shlykovite: 2Vmeas = –60(20)°; cryptophyllite: 2Vmeas> 70°. The refractive indices are: shlykovite: α = 1.500(3), β = 1.509(2), γ = 1.515(2)