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

Abstract

<jats:title>Abstract</jats:title><jats:p>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 Mg<jats:sub>6.19</jats:sub>(Cr<jats:sub>1.21</jats:sub>Fe<jats:sub>0.51</jats:sub>Al<jats:sub>0.15</jats:sub>)<jats:sub>∑1.87</jats:sub>(OH)<jats:sub>16</jats:sub>[Cl<jats:sub>1.62</jats:sub>(CO<jats:sub>3</jats:sub>)<jats:sub>0.17</jats:sub>(SO<jats:sub>4</jats:sub>)<jats:sub>0.01</jats:sub>]·4H<jats:sub>2</jats:sub>O, by analogy with the hydrotalcite group. The simplified formula is Mg<jats:sub>6</jats:sub>Cr<jats:sub>2</jats:sub>(OH)<jats:sub>16</jats:sub>C<jats:sub>l2</jats:sub>·4H<jats:sub>2</jats:sub>O. 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 CO<jats:sub>2</jats:sub>, the second to loss of hydroxide water, remaining CO<jats:sub>2</jats:sub> 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 pale-pink to white streak. The strongest lines in the X-ray powder pattern are [<jats:italic>d</jats:italic><jats:sub>obs</jats:sub> (<jats:italic>I</jats:italic><jats:sub>obs</jats:sub>) (<jats:italic>hkl</jats:italic>)] 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) (2<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S0026461X00029881_inline01.png" />3). These lines were indexed on a hexagonal cell with <jats:italic>a</jats:italic> = 3.103(2), <jats:italic>c</jats:italic> = 24.111(24)Å, <jats:italic>V</jats:italic> = 201.14 Å<jats:sup>3</jats:sup> and <jats:italic>Z</jats:italic> = 3/8. The new mineral is isostructural with the hydrotalcite group and has space group <jats:italic>R</jats:italic><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S0026461X00029881_inline02.png" /><jats:italic>m</jats:italic>. The measured density is 2.062 gm/cm<jats:sup>3</jats:sup>. 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 Clrich solutions at temperatures &lt;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.</jats:p>