WIDGIEMOOLTHALITE - THE NEW NI ANALOG OF HYDROMAGNESITE FROM WESTERN-AUSTRALIA

Widgiemoolthalite is a new hydrated nickel carbonate mineral from the 132 North nickel mine near Widgiemooltha in Western Australia. It is f ound together with a number of other secondary nickel minerals in the weathered zone overlying a nickel sulfide body. Widgiemoolthalite is b luish green with a silky luster and occurs mainly as spheroids of radi ating fibers. The fibers are biaxial positive and length fast with n(m in) = 1.630(5) and n(max) = 1.640(5). The measured specific gravity is 3.13; calculated, 3.24. It has a composition corresponding to Ni4.62M g0.38C4.15H11.94O19.28, or (Ni,Mg)5.00(CO3)4.15(OH)1.70.5.12H2O, on th e basis of Ni + Mg = 5. The ideal formula, based on its crystallograph ic similarity to hydromagnesite, is Ni5 (CO3)4 (OH)2.4-5H2O. The X-ray powder diffraction pattern can be indexed on a monoclinic unit cell s imilar to that of hydromagnesite, with a = 10.06(17), b = 8.75(5), c = 8.32(4) angstrom, and beta = 1 14.3(8)-degrees. Strongest reflections [d in angstroms, (I/I(o)), (hkl)] are 5.75(10)(011), 6.30(5)(110), 4. 36(4)(020), 2.871(4)(222BAR, 022), 4.14(3)(102BAR), and 2.120(3)(132).