Woodallite, a new chromium analogue of iowaite from the Mount Keith nickel deposit, Western Australia

Woodallite is a new Cr-rich member of the hydrotalcite group from the large, low-grade Mount Keith nickel deposit, in the northeastern Goldfields district of Western Australia. Woodallite occurs as whorls and clusters of minute platelets up to 6 mm across in lizardite+brucite-altered dunite. Individual platelets are typically 10-100 μm in maximum dimension and are often curved. Associated minerals include chromite, lizardite, iowaite, pentlandite, magnetite, tochilinite and brucite. Electron microprobe analysis gave: Mg 25.90 wt.%; Cr 10.81; Fe 4.86; Al 0.68; Cl 9.89; S 0.03; Si 0.01; Ni 0.01; Na 0.01, yielding (after correction for loss of volatiles) an empirical formula of the hydrotalcite group. The simplified formula is Mg₆Cr₂(OH)₁₆Cl₂·4H₂O. Combined thermogravimetric analysis and mass spectroscopy showed a two-stage weight loss of 12.7% and 27.3% occurring over the ranges 25-300°C and 300-660°C, respectively. The first weight loss is attributed to loss of interlayer water, chlorine-bearing species (e.g. HCl) and some CO₂, the second to loss of hydroxide water, remaining CO₂ and Cl species. The mineral is deep magenta to purple in colour, transparent, with a resinous to waxy lustre, and a perfect basal {0001} cleavage. Woodallite has a Mohs hardness of 1.5-2, and a palepink to white streak. The strongest lines in the X-ray powder pattern are [d_obs (I_obs) (hkl)] 8.037 (100) (003); 4.021 (48) (006); 2.679 (1) (009); 2.624 (3) (012); 2.349 (5) (015); 2.007 (6) (0,0,12); 1.698 (2) (0,1,11); 1.524 (2) (213). These lines were indexed on a hexagonal cell with a = 3.103(2), c = 24.111(24) Å, V = 201.14 ų and Z = 3/8. The new mineral is isostructural with the hydrotalcite group and has space group R3m. The measured density is 2.062 gm/cm³. Woodallite is uniaxial negative with ω = 1.555 and ε = 1.535 (white light); pleochroism is distinct from violet to pinkish lilac. Woodallite forms as a result of hydrothermal alteration of primary magmatic chromite by Cl-rich solutions at temperatures <320°C. Relict chromite fragments are frequently present in the whorls, and associated magnetite is altered extensively to iowaite. The mineral is named after Roy Woodall, eminent Australian industry geologist.