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

B. A. Grguric, I. C. Madsen, A. Pring

Research output: Contribution to journalArticle

24 Citations (Scopus)

Abstract

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 [dobs (Iobs) (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 Å3 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.

Original languageEnglish
Pages (from-to)427-435
Number of pages9
JournalMineralogical Magazine
Volume65
Issue number3
DOIs
Publication statusPublished - 1 Jun 2001
Externally publishedYes

Keywords

  • Hydrotalcite group
  • Mount Keith deposit
  • New mineral
  • Western Australia
  • Woodallite

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