The Canadian Mineralogist
Volume 34, pages 657-665 (1996)
PENOBSQUISITE: A NEW BORATE MINERAL WITH A COMPLEX FRAMEWORK STRUCTURE
JOEL D. GRICE, ROBERT A. GAULT and JERRY VAN VELTHUIZEN
Research Division, Canadian Museum of Nature, P.O. Box 3443, Station D, Ottawa, Ontario K1P 6P4
Abstract
Penobsquisite, a new mineral species from the Potash Corporation of Saskatchewan (New Brunswick Division) mine at
Penobsquis, Kings County, New Brunswick, occurs in the Upper Halite member of the evaporite sequence, associated with halite, boracite,
hilgardite, pringleite, trembathite, sellaite, fluorite, hematite and malachite. It occurs as euhedral, transparent, pale yellow, vitreous crystals ranging
in size from 0.5 to 1.5 mm. Penobsquisite is biaxial positive, with indices of refraction 1.550(2), 1.554(2) and 1.592(2); 2Vmeas. = 33(2) , 2Vcalc. = 36.6 .
There is no dispersion and no pleochroism. The optical orientation is Y = b, Z c = 16.6 (in obtuse angle ). Penobsquisite is monoclinic, space group
P21, a 11.63(4), b 9.38(1), c 8.735(9) Å, 98.40(7) , V 942.7(1) Å3 and Z = 2. The strongest lines of the X-ray powder-diffraction pattern [d in Å(I)(hkl)]
are: 8.65(3)(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 {101} and sphenoids {111}, {110}, {110}, {212} and {112}. 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 stoichiometry from the crystal-structure analysis. The
empirical formula based on 24 anions is: Ca1.99(Fe0.67Mg0.29Mn0.02) 0.98[B9O12.95(OH)5.99]Cl1.05•4.01H2O, ideally Ca2Fe[B9O13(OH)6]Cl•4H2O;
Dmeas. = 2.26(3) g/cm3, Dcalc. = 2.27 g/cm3. The IR spectrum, with absorption bands at 3336, 1646, 1345, 1205, 995, 693 and 548 cm-1, indicates the
presence of H2O groups and complex borate groups. The crystal structure determination refined to indices R = 2.2% and Rw = 2.0% for 2756
unique reflections. The borate framework of penobsquisite has sets of intersecting channels that parallel [010] and [001]. These large channels
resemble the related borate structures of pringleite and ruitenbergite. The framework consists of sheets of 12-membered rings of alternating
triangular and tetrahedral borate polyhedra cross-linked by adjacent slabs of 3-membered rings having a crankshaft chain configuration.