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

The crystal structure of tschermakite from Fiskenaesset, Greenland, (K(0.05)Na(0.38))(Sigma 0.43) (Ca(1.80)Na(0.05)Fe(0.15)(2+))(Sigma 2.00) (Mg(3.65) Fe(0.18)(2+)Mn(0.03)Fe(0.09)(3+)Al(0.81)Cr(0.19)Ti(0.05))(Sigma 5.00) (Si(6.43)Al(1.57))(Sigma 8.00) O(22) [(OH)(1.95)F(0.04)Cl(0.01)](Sigma 2.00), a 9.8059(5), b 17.9721(8), c 5.3012(2) angstrom, beta 105.063(1)degrees, V 902.15 angstrom(3), space group C2/m, Z = 2, has been refined to an R(1) index of 3.2% using MoK alpha 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.