WALKERITE, A NEW BORATE MINERAL SPECIES IN AN EVAPORITIC SEQUENCE FROM SUSSEX, NEW BRUNSWICK, CANADA

Walkerite, ideally Ca 16 (Mg,Li,□) 2 [B 13 O 17 (OH) 12 ] 4 Cl 6 ·28H 2 O, is a new mineral species from the Potash Corporation of Saskatchewan (New Brunswick Division) mine at Penobsquis, near Sussex, Kings County, New Brunswick. It occurs in the Upper Halite Member of the Windsor Group evaporites, associated with halite, hydroboracite, hilgardite, volkovskite, boracite, szaibelyite, a mica-group mineral and anhydrite. The crystals of walkerite are colorless to white, fibrous to acicular, commonly forming bundles of fibers to 7 mm in length. The crystals are transparent to translucent, with a vitreous luster and a white streak. It is soft (Mohs hardness of 3), brittle with no apparent cleavage, but with a splintery fracture along [001]. Walkerite is biaxial positive, α 1.516 ± 0.002, β 1.532 ± 0.002 and γ 1.554 ± 0.002 (for λ = 590 nm); 2 V meas. = 82(3)°, 2 V calc. = 82°. There is no dispersion and no pleochroism. Optical orientation: X = a , Y = b and Z = c . It is orthorhombic, space group Pba 2, with unit-cell parameters refined from powder data: a 15.52(1), b 22.74(1), c 8.761(4) A, V 3091(2) A 3 and Z = 1. The strongest six powder-diffraction lines [ d in A( I )( hkl )] are: 12.820(100)(110), 7.785(80)(200), 6.319(40)(121), 5.649(30)(211), 3.176(30)(170) and 2.570(30)(461,550). Electron-microprobe analyses, a thermogravimetric analysis and induced coupled plasma – mass spectrometry results, supported by crystal-structure analysis, infrared-absorption spectrometry and secondary-ion mass spectrometry, yielded Li 2 O 0.12, Na 2 O 0.13, K 2 O 0.07, CaO 23.05, FeO 0.32, MgO 0.58, Cl 4.91, B 2 O 3 47.17 and H 2 O 25.48, O = Cl −1.11, total 100.72 wt.%. The empirical formula based on 150 anions, is (Ca 15.60 Na 0.16 K 0.06 ) ∑15.82 (Mg 0.55 Li 0.31 Fe 0.17 ) ∑1.03 B 51.43 O 68 (OH) 48 [Cl 5.26 (OH) 0.74 ] ∑6.00 H 2.53 ·28H 2 O; the idealized formula is presented above. The calculated density (from the empirical formula) is 2.05 g/cm 3 , and the measured density is 2.07 g/cm 3 . The structure has been refined to R = 0.040. The highly corrugated sheets of borate polyhedra are cross-linked by Ca and Mg polyhedra. The Cl atoms and H 2 O groups are located within the ovoid channels. The structure is compared to that of other complex borates, such as penobsquisite and pringleite.