PENOBSQUISITE - A NEW BORATE MINERAL WITH A COMPLEX FRAMEWORK STRUCTURE

Penobsquisite, a new mineral species from the Potash Corporation of Sa skatchewan (New Brunswick Division) mine at Penobsquis, Kings County, New Brunswick, occurs in the Upper Halite member of the evaporite sequ ence, associated with halite, boracite, hilgardite, pringleite, tremba thite, sellaite, fluorite, hematite and malachite. It occurs as euhedr al, transparent, pale yellow, vitreous crystals ranging in size from 0 .5 to 1.5 mm. Penobsquisite is biaxial positive, with indices of refra ction alpha 1.550(2), beta 1.554(2) and gamma 1.592(2); 2V(meas.) = 33 (2)degrees, 2V(calc.) = 36.6 degrees. There is no dispersion and no pl eochroism. The optical orientation is Y = b, Z (sic) c = 16.6 degrees (in obtuse angle beta). Penobsquisite is monoclinic, space group P2(1) , a 11.63(4), b 9.38(1), c 8.735(9) Angstrom, beta 98.40(7)degrees, V 942.7(1) Angstrom(3) and Z = 2. The strongest lines of the X-ray powde r-diffraction pattern [d in Angstrom(I)(hkl)] are: 8.65(2)(001), 7.29( 10)(110), 5.32(2)(111), 4.50(2)(211), 2.958(3)(320,312,031), 2.744(2)( 013,203) and 2.113(3)(114). Crystals are tabular on {100} and elongate on [010], with forms: pinacoids {100}, {101} and {10(1) over bar} and sphenoids {111}, {110}, {1(1) over bar0$}, {<(21)over bar>2} and {<(1 1)over bar>2}. Electron-microprobe analyses gave CaO 17.27(17.20-17.36 ). FeO 7.48(6.79-8.08), MgO 1.82(1.37-2.25), MnO 0.23(0.20-0.25), Cl 5 .77(5.62-5.86), B2O3 48.50(48.26-48.73) and H2O 19.52(19.42-19.61). O = Cl -1.30, total 99.29 wt.%. H2O and B2O3 were calculated by stoichio metry from the crystal-structure analysis. The empirical formula based on 24 anions is: Ca-1.99(Fe0.67Mg0.29Mn0.02)(Sigma 0.98)[B9O12.95(OH) (5.99)]Cl-1.05. 4.01H(2)O, ideally Ca2Fe[B9O13(OH)(6)]Cl . 4H(2)O; D-m eas. = 2.26(3) g/cm(3), D-calc. = 2.27 g/cm(3). The IR spectrum, with absorption bands at 3336, 1646, 1345, 1205, 995, 693 and 548 cm-(1), i ndicates the presence of H2O groups and complex berate groups. The cry stal-structure determination refined to indices R = 2.2% and R(w) = 2. 0% for 2756 unique reflections. The berate framework of penobsquisite has sets of intersecting channels that parallel [010] and [001]. These large channels resemble the related berate structures of pringleite a nd ruitenbergite. The framework consists of sheets of 12-membered ring s of alternating triangular and tetrahedral berate polyhedra cross-lin ked by adjacent slabs of 3-membered rings having a crankshaft chain co nfiguration.