Veatchite

Abstract: OME years ago a few small grains of an unknown mineral were separated by F. H. Stewart from well cuttings from the Permian lower evaporite bed of the Eskdale no. 2 borehole, sunk by the D'Arcy Exploration Company near Aislaby, north Yorkshire. The mineral (not then identified) was mentioned in an account of the mineralogy of this evaporite bed (Stewart, 1949, p. 626). Flame tests suggested that boron and strontium were present, and this was confirmed by Dr. R. L. Mitchell of the Macaulay Institute for Soil Research in Aberdeen, who, by spectrographic work, found that these were the main constituents, and that very much smaller quantities of Ca, Mg, Ba, Na, and K were present. The mineral appeared to be a strontium borate, and was found to be optically similar to veatehite, which was then considered to be a calcium borate (Switzer, 1938). When Switzer and Brannoek (1950) showed that veatchite was actually a strontium borate (the only natural one known) it became apparent that the Yorkshire mineral was identical with veatehite. The identification has been confirmed by a microchemieal analysis by R. A. Chalmers and X-ray work by R. Phillips. PHYSICAL AND OPTICAL CHARACTERS. The mineral occurs in Yorkshire as co[ourless transparent crystals of acicular and platy habits. The needles occasionally reach 3 ram. in length and 0"5 ram. in thickness. The plates (flattened on (010)) reach 2 ram. in maximum width and 0.5 mm. in thickness. Some of the crystals show parallel growth. Several prism, dome, and ill-developed pyramid faces can be seen, but all these faces are striated and dull, and no reasonable reflections were obtained on the goniometer. Several of the crystals are doubly terminated. There are three cleavages -perfect (010), good (001), fair (100). The (100) cleavage has not previously been recorded. The angle from the

Abstract: Volkovskite [KCa 4 B 22 O 32 (OH) 10 Cl·4H 2 O] 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); 2 V Meas = 14.6°(7.2°), 2 V Calc . = 14.4°. There is no dispersion and no pleochroism. Volkovskite is triclinic, space group P 1 with cell parameters refined from the X-ray powder-diffraction pattern: a 6.539(3), b 24.194(5), c 6.576(3) A, α 92.2(4.4), β 119.1(1.3), γ 97.4(5.4)°, V 894.7(6) A 3 . The six strongest lines of the X-ray powder diffraction pattern [ d in A( 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.