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THE CRYSTAL CHEMISTRY OF Al-RICH AMPHIBOLES: SADANAGAITE AND POTASSIC-FERRISADANAGAITE

Frank C. Hawthorne^1, and George E. Harlow²

¹ Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
² Department of Earth and Planetary Sciences, American Museum of Natural History, New York, N.Y. 10024-5192, USA

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

The crystal structures of magnesiosadanagaite (MS) from Mogok, Myanmar, monoclinic, a 9.857(2), b 17.899(4), c 5.318(1) Å, β 105.36(1)°, V 904.74(8) ų, C2/m, Z = 2, and potassic-ferrisadanagaite (FS) from the Ilmen alkaline massif, South Urals, Russia, a 9.9257(4), b 18.0917(7), c 5.3709(2) Å, β 105.19(1)°, V 930.75(2) ų, C2/m, Z = 2, have been refined to R values of ~3% using single-crystal MoK X-ray data. The crystals used in the collection of the intensity data were subsequently analyzed by electron-microprobe techniques, leading to the following compositions: MS: (Na_0.82 K_0.17) (Ca_1.95 Na_0.05) (Mg_3.36 Fe²⁺_0.23 Al_1.20 Cr³⁺_0.07 Ti⁴⁺_0.16) (Si_5.47 Al_2.53) O₂₂ [(OH)_1.58 F_0.42]; FS: (Na_0.31 K_0.62) (Ca_1.72 Na_0.28) (Mg_0.56 Fe²⁺_2.12 Mn²⁺_0.26 Zn_0.02 Al_0.72 Fe³⁺_1.07 Ti⁴⁺_0.21) (Si_5.24 Al_2.76) O₂₂ [(OH)_1.70 F_0.30]. Site populations were assigned from the results of site-scattering refinement and stereochemical analysis, taking into account the unit formula determined for each crystal. The <T–O> distances, 1.678 and 1.684 Å, indicate that ^[4]Al is strongly ordered at the T(1) site in each crystal, but the <T(2)–O> distances, 1.648 and 1.655 Å, indicate significant Al at the T(2) site. The Fe²⁺ and Fe³⁺ contents in each crystal were assigned from the <M–O> distances. The A(2) and A(m) sites are occupied by Na and (Na,K), respectively, in crystal MS, and by Na and K, respectively, in crystal FS. Both amphiboles show SRO (short-range order) of species at the M(4), O(3), A(m) and A(2) sites, and the relative abundances of these arrangements were assigned.

Keywords: amphibole, magnesiosadanagaite, potassic-ferrisadanagaite, crystal-structure refinement, electron-microprobe analysis, Mogok, Myanmar, Ilmen, South Urals, Russia.