TUZLAITE, NACA[B5O8(OH)2].3H2O, A NEW MINERAL WITH A PENTABORATE SHEET STRUCTURE FROM THE TUZLA SALT MINE, BOSNIA AND HERCEGOVINA

Tuzlaite, NaCa[B5O8(OH)2] . 3H2O, is a monoclinic sheet borate [P2(1)/ c, a = 6.506(1), b = 13.280(3), c = 11.462(3) angstrom, beta = 92.97(2 )-degrees, V = 989(1) angstrom3, Z = 4], occurring in nearly monominer alic veinlets formed in the course of dolomitic marl diagenesis at the Tuzla salt mine, located in the northeast part of Bosnia and Hercegov ina. Intergrowths with halite have been observed where halite covers t uzlaite in later stages of diagenesis. The crystal structure, includin g H positions, was solved and refined from X-my single-crystal data to R = 2.8, R(w) = 3.3%. Tuzlaite possesses a new type of pentaborate sh eet structure with fourfold- and threefold-coordinated B (5:2DELTA + 3 T). Borate triangles and tetrahedra form ten-membered rings. The sheet s are connected by Na and Ca, which are coordinated to additional H2O. The new mineral is colorless to white. It is biaxial positive, with t he following refractive indices: n(x) = 1.532(2), n(y) = 1.544(2), and n(z) = 1.561(2). The optical orientation is Y = b, Z:a = 14-degrees ( in acute angle beta). Tuzlaite has one perfect cleavage, parallel to { 001}, and hardness 2-3; D(meas) = 2.2 1, D(calc) = 2.2 3 g/CM3. The ch emical composition is CaO 14.64%, Na2O 10.25%, SrO 0.21%, B2O, 52.19%, Al2O3 0.26%, and H2O 21.66%. Tuzlaite is chemically related to the pe ntaborates probertite, NaCaB5O7(OH)4.3H2O, and ulexite, NaCaB5O6(OH)6. 5H2O. The new mineral is named after the town Tuzla, where it was foun d.