ON THE CRYSTAL-CHEMISTRY OF A NEAR-ENDMEMBER TRIPLITE, Mn2+2(PO4)F, FROM THE CODERA VALLEY (SONDRIO PROVINCE, CENTRAL ALPS, ITALY)

Abstract

The crystal-chemistry of a near-endmember triplite, a manganese fluoro-phosphate [ideal chemical formula Mn2(PO4)F; a 12.109(2) A, b 6.5162(8) A, c 10.117(1) A, and β 106.16(2)°; space group I 2/ a ], from a granitic pegmatite exposed in the Codera Valley (Sondrio Province, Central Alps), has been investigated by means of wavelength dispersive electron-microprobe, infrared and Raman spectroscopies, and X-ray powder and single-crystal diffraction. Single-crystal anisotropic structure refinement gave a final agreement index R 1 = 0.0318 for 76 refined parameters and 1128 unique reflections with F o > 4σ( F o). The structure refinement of triplite from the Codera Valley confirms the topological features previously described both for a natural sample from Mica Lode (Colorado) and for the synthetic compound Mn2(PO4)F. The structure consists of isolated PO4 tetrahedra joined with distorted MO4F2 octahedra. The M(1) and M(2) octahedra share their edges to form chains; the chains of M(1) octahedra are parallel to [010], and those of M(2) are parallel to [100]. The analysis of the difference-Fourier maps of the electron density of triplite from the Codera Valley reveals a unique position for F, as reported in the literature for the OH-free synthetic Mn2(PO4)F. In contrast, two mutually exclusive F sites, with partial site occupancy, were reported in a previous structural study of OH-bearing triplite from Colorado (Mica Lode).