ON THE CRYSTAL-CHEMISTRY OF A NEAR-ENDMEMBER TRIPLITE, Mn2+2(PO4)F, FROM THE CODERA VALLEY (SONDRIO PROVINCE, CENTRAL ALPS, ITALY)
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Abstract:
The crystal-chemistry of a near-endmember triplite, a manganese fluoro-phosphate [ideal chemical formula Mn 2 (PO 4 )F; a 12.109(2) Å, b 6.5162(8) Å, c 10.117(1) Å, and b 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 Mn 2 (PO 4 )F.The structure consists of isolated PO 4 tetrahedra joined with distorted MO 4 F 2 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 Mn 2 (PO 4 )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).Keywords:
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Transmission electron microscopy; wonesite from the Post Pond Volcanics, Vermont. A new method of deriving the chemical compositions of the phases in fine lamellar intergrowths from electron microscope X-ray analyses is described. The method, which utilizes the bulk chemical analysis of the intergrowth as a standard, is used to derive the chemical compositions of the intergrown talc and mica. The exsolution of wonesite provides a structural analogy for amphibole exsolution phenomena that involve the amphibole A-site.--Modified journal abstract.
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Crystal-structure refinements done on six samples of cosalite with different types and degrees of copper and silver substitutions, as well as electron-microprobe studies of 33 different samples of cosalite, both enlarged and clarified the spectrum of cation substitutions and the crystal chemistry of this species. The Ag + Bi ↔ 2Pb scheme of substitution acts at the structural site Me 1. The 2(Cu + Ag) ↔ Pb scheme is a result of two combined mechanisms: creation of vacancies in the Bi-containing octahedral site Me2 accompanied by a progressive occupancy of two triangular faces of this octahedron by Cu + Ag [ i.e. , Bi ↔ 2(Cu + Ag)], which is combined with progressive replacement of Pb in the adjacent Me 1 octahedron by Bi; their combination gives the resulting closely followed chemical relationship. The structural mechanism of the 3Cu ↔ Bi substitution observed in thin zones in cosalite from Felbertal remains unknown. Cosalite offers a unique case of solid solution based on a combined omission – interstitial insertion substitution.
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The crystal chemistry of macfallite from Keweenaw County, Michigan was studied using electron microprobe, thermogravimetry (TG), differential thermal analysis (DTA), powder Fourier transform infrared (FTIR) spectroscopy, and single-crystal X-ray diffraction methods. The chemical formula derived from the electron-microprobe measurements is
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