Chemical and structural characterization of UICC crocidolite fibres from Koegas Mine, Northern Cape (South Africa)

In the present work we report the full structural and spectroscopic characterization of an UICC crocidolite standard sample, that is a fibrous riebeckite from Koegas Mine, Northern Cape (South Africa). The chemical composition was obtained by SEM-EDS and cation site partition was retrieved by complementing chemical, Mössbauer and X-ray powder diffraction data. Cell parameters, fractional coordinates, and site scattering for M(1), M(2), M(3), M(4) were refined using the Rietveld method. The UICC crocidolite standard sample shows the chemical formula ANa0.026BNa2.000C (Fe2+2.212Fe3+2.041Mg0.747)Σ=5.000T(Si7.950Al0.022)Σ=7.972O22O(3)(OH)2, very close to that of the end-member riebeckite A□BNa2C(Fe2+3Fe3+2)Σ=5TSi8O22O(3)(OH)2, the major difference being the Fe3+/Fetot ratio of 0.48, slightly higher than the ideal ratio of 0.40. Cation site distribution of the UICC crocidolite standard sample is: [Mg0.50Fe3+0.31Fe2+1.19]Σ=2.00 at M(1); [Mg0.10Fe3+1.60Fe2+0.30]Σ=2.00 at M(2); [Mg0.15Fe3+0.13Fe2+0.72]Σ=1.00 at M(3). Refined cell parameters are: a=9.73516(19) Å, b=18.0453(3) Å, c=5.32895(9) Å, β =103.5159(12) °, V=910.23(3) Å3. Quantitative Phase Analysis indicates that 93.7(3) wt% of the sample consists of crocidolite, the remaining being distributed among five different phases including magnetite and quartz.