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The Canadian Mineralogist; April 2008; v. 46; no. 2; p. 455-466; DOI: 10.3749/canmin.46.2.455
© 2008 Mineralogical Association of Canada
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Articles

THE CRYSTAL CHEMISTRY OF ALKALI AMPHIBOLES FROM THE KAJLIDONGRI MANGANESE MINE, INDIA

Frank C. Hawthorne1, Roberta Oberti1, Aberto Zanetti1 and V.K. Nayak2

1 CNR Istituto di Geoscienze e Georisorse, Università di Pavia, via Ferrata 1, I–27100 Pavia, Italy
2 Department of Applied Geology, Indian School of Mines, Dhanbad 826–004, India

The crystal structures of eight sodic–calcic and sodic amphiboles from the Kajlidongri manganese mine, India, have been refined to R values of 1–2% using single-crystal MoK{alpha} X-ray data. The crystals used in the collection of the intensity data were subsequently analyzed by electron- and ion-microprobe techniques. Site populations were assigned from the results of site-scattering refinement and stereochemical analysis, taking into account the unit formula determined for each crystal. These amphiboles range in composition from richterite to magnesioriebeckite to magnesio-arfvedsonite. All amphiboles contain Li, which ranges from 0.03 to 0.30 apfu and is completely ordered at the M(3) site. The amount of F in each crystal is equal to twice the amount of Li, suggesting that Li, which is ordered at the M(3) site, is locally associated with F at both the adjacent O(3) sites. The patterns of electron density in the A cavity and the populations at the A, O(3) and M(4) sites are in accord with previously proposed patterns of short-range order in monoclinic amphiboles. The compositions of these amphiboles lie along the join richterite–magnesioriebeckite and show a surprising degree of coherence in their major-element compositions considering the heterogeneity of the host rocks.

Keywords: amphibole, crystal-structure refinement, electron-microprobe analysis, ion-microprobe analysis, Kajlidongri, India.






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