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

Abstract

The crystal structure of tschermakite from Fiskenaesset, Greenland, (K 0.05 Na 0.38 ) ∑0.43 (Ca 1.80 Na 0.05 Fe 2+ 0.15 ) ∑2.00 (Mg 3.65 Fe 2+ 0.18 Mn 0.03 Fe 3+ 0.09 Al 0.81 Cr 0.19 Ti 0.05 ) ∑5.00 (Si 6.43 Al 1.57 ) ∑8.00 O 22 [(OH) 1.95 F 0.04 Cl 0.01 ] ∑2.00 , a 9.8059(5), b 17.9721(8), c 5.3012(2) A, β 105.063(1)°, V 902.15 A 3 , space group C 2/ m , Z = 2, has been refined to an R 1 index of 3.2% using Mo K α single-crystal X-ray diffraction. The ratio Fe 3+ /Fe tot was determined by Mossbauer spectroscopy. The unit formula (calculated from the results of electron-microprobe analysis), the refined site-scattering values, observed average bond-lengths, and Mossbauer 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, Fe 3+ and Cr 3+ ) 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 Fe 2+ at M (4) indicated by Mossbauer 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.