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CRYSTAL STRUCTURE AND MÖSSBAUER SPECTROSCOPY OF TSCHERMAKITE FROM THE RUBY LOCALITY AT FISKENAESSET, GREENLAND

Yassir A. Abdu and Frank C. Hawthorne

Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada

E-mail address: frank_hawthorne{at}umanitoba.ca

The crystal structure of tschermakite from Fiskenaesset, Greenland, (K_0.05Na_0.38)_0.43 (Ca_1.80Na_0.05Fe²⁺_0.15)_2.00 (Mg_3.65 Fe²⁺_0.18Mn_0.03Fe³⁺_0.09Al_0.81Cr_0.19Ti_0.05)_5.00 (Si_6.43Al_1.57)_8.00 O₂₂ [(OH)_1.95F_0.04Cl_0.01]_2.00, a 9.8059(5), b 17.9721(8), c 5.3012(2) Å, β 105.063(1)°, V 902.15 ų, space group C2/m, Z = 2, has been refined to an R₁ index of 3.2% using MoK single-crystal X-ray diffraction. The ratio Fe³⁺/Fe_tot was determined by Mössbauer spectroscopy. The unit formula (calculated from the results of electron-microprobe analysis), the refined site-scattering values, observed average bond-lengths, and Mössbauer spectroscopy were used to assign site populations. Tetrahedrally coordinated Al occurs at both the T(1) and T(2) sites, but is strongly ordered at T(1), and ^[6]Al occurs at both the M(2) and M(3) sites, but is strongly ordered at M(2). Magnesium shows the site preference M(1) = M(2) > M(3), and higher-valence transition metals (Ti, Fe³⁺ and Cr³⁺) are ordered at M(2). The formula calculated from the electron-microprobe analysis showed an excess of C-group cations of 0.15 apfu, exactly in accord with the amount of Fe²⁺ at M(4) indicated by Mössbauer spectroscopy. At the A site, Na was split between the A(2) and A(m) sites, and K was assigned to the A(m) site. We derived the short-range arrangements involving the M(4), O(3) and A sites, and calculated their relative fractions. As with other complicated monoclinic amphiboles, short-range order is common and dictated by the valence-sum rule of localized bond-valence theory.

Keywords: tschermakite, amphibole, crystal structure, Mössbauer spectroscopy, short-range order, Fiskenaesset, Greenland.