Abstract

Volkovskite [KCa4B22O32(OH)10Cl·4H2O] has been identified among numerous borate minerals obtained from potash deposits in New Brunswick. The crystals are vitreous, translucent, and colorless, varying from pale to deep orange.

The crystals possess perfect {010} and {100} cleavages occurring as thinly layered pseudohexagonal/triangular, elongated platy masses. Volkovskite is biaxial positive, with indices of refraction α 1.539(2), β 1.540(2), and γ 1.605(2); 2VMeas = 14.6°(7.2°), 2VCalc. = 14.4°. There is no dispersion and no pleochroism. Volkovskite is triclinic, space group P1 with cell parameters refined from the X-ray powder-diffraction pattern: a 6.539(3), b 24.194(5), c 6.576(3) Å, α 92.2(4.4), β 119.1(1.3), γ 97.4(5.4)°, V 894.7(6) Å3. The six strongest lines of the X-ray powder diffraction pattern [d in Å(I)(hkl)] are: 11.937(28.8)(020), 7.938(100)(030), 5.951(31.4)(040), 3.400(73.6)(070), 2.644(74.3)(090), and 1.983(13.0)(0120).

A crystal structure refinement confirms the basic model of Rastsvetaeva et al. (1992). The crystal structure determination refined to R = 2.1% for 5266 unique reflections. The layered borate structure consists of a double layer of borate sheets parallel to (010). Between the double layers there are two types of cross linkage: Ca polyhedra bond two double layer slabs and K polyhedra bond the double slabs. Many borate minerals found in potash deposits form layered structures, such as biringuccite, nasinite, gowerite, veatchite polytypes, and volkovskite. The volkovskite structure obtained in this study is compared to related layered borates.

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