Shlykovite KCa[Si4O9(OH)] · 3H2O and cryptophyllite K2Ca[Si4O10] · 5H2O, new mineral species from the Khibiny alkaline pluton, Kola Peninsula, Russia

New minerals, shlykovite and cryptophyllite, hydrous Ca and K phyllosilicates, have been identified in hyperalkaline pegmatite at Mount Rasvumchorr, Khibiny alkaline pluton, Kola Peninsula, Russia. They are the products of low-temperature hydrothermal activity and are associated with aegirine, potassium feldspar, nepheline, lamprophyllite, eudialyte, lomonosovite, lovozerite, tisinalite, shcherbakovite, shafranovskite, 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 parallel-columnar veinlets. Cryptophyllite occurs as lamellae up to 0.02 × 0.1 × 0.2 mm in size intergrown with shlykovite being oriented parallel to {001} or chaotically arranged. Separate crystals of the new minerals are transparent and colorless; the aggregates are beige, brownish, light cream, and pale yellowish-grayish. The cleavage is parallel to (001) perfect. The Mohs hardness of shlykovite is 2.5-3. The calculated densities of shlykovite and cryptophyllite 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); cryptophyllite: α = 1.520(2), β = 1.523(2), γ = 1.527(2). The chemical composition of shlykovite determined by an electron microprobe (H2O determined from total deficiency) is as follows, wt %: 0.68 Na2O, 11.03 K2O, 13.70 CaO, 59.86 SiO2, 14.73 H2O; the total is 100.00. The empirical formula calculated on the basis of 13 O atoms (OH/H2O calculated from the charge balance) is (K0.96Na0.09)Σ1.05Ca1.00Si4.07O9.32(OH)0.68 · 3H2O.The idealized formula is KCa[Si4O9(OH)] · 3H2O. The chemical composition of cryptophyllite determined by an electron microprobe (H2O determined from the total deficiency) is as follows, wt %: 1.12 Na2O, 17.73 K2O, 11.59 CaO, 0.08 Al2O3, 50.24 SiO2, 19.24 H2O, the total is 100.00. The empirical formula calculated on the basis of (Si,Al)4(O,OH)10 (OH/H2O calculated from the charge balance) is (K1.80Na0.17)Σ1.97Ca0.99Al0.01Si3.99O9.94(OH)0.06 · 5.07H2O. The idealized formula is K2Ca[Si4O10] · 5H2O. The crystal structures of both minerals were solved on single crystals using synchrotron radiation. Shlykovite is monoclinic; the space group is P21/n; a = 6.4897(4), b = 6.9969(5), c = 26.714(2)Å, β = 94.597(8)°, V = 1209.12(15)Å3, Z = 4. Cryptophyllite is monoclinic; the space group is P21/n; a = 6.4934(14), b = 6.9919(5), c = 32.087(3)Å, β = 94.680(12)°, V= 1451.9(4)Å, Z = 4. The strongest lines of the X-ray powder patterns (d, Å-I, [hkl] are: shlykovite 13.33-100[002], 6.67-76[004], 6.47-55[100], 3.469-45[021], 3.068-57[121], 3.042-45[121], 2.945-62[ 23], 2.912-90[025, 12, 211]; cryptophyllite 16.01-100[002], 7.98-24[004], 6.24-48[101], 3.228-22[109], 3.197-27[0.0.10], 2.995-47[122], 2.903-84[123, 204, 124, 211], 2.623-20[028, 08, 126]. Shlykovite and cryptophyllite are members of new related structural types. Their structures are based on a two-layer packet consisting of tetrahedral Si layers linked with octahedral Ca chains. Mountainite, shlykovite and cryptophyllite could be combined into the mountainite structural family. Shlykovite is named in memory of Russian geologist V. G. Shlykov (1941-2007); the name cryptophyllite is from the Greek words meaning concealed and leaf that allude to its layered structure (phyllosilicate) in combination with a lamellar habit and intimate intergrowths with visually indistinguishable shlykovite. Type specimens of the minerals are deposited at the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow. © 2010 Pleiades Publishing, Ltd.