Crystal structure of near-endmember arrojadite-(BaNa) from Big Fish River, Yukon, Canada

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Marco E. Ciriotti
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Crystal structure of near-endmember arrojadite-(BaNa) from Big Fish River, Yukon, Canada

Messaggio da Marco E. Ciriotti » ven 07 dic, 2018 11:38

Prossima pubblicazione.

▪ Tomes, H.E., Di Cecco, V.E., Tait, K.T., Cámara, F. (2018): Crystal Structure of Near-endmember Arrojadite-(BaNa) from Big Fish River, Yukon, Canada. Canadian Mineralogist, 56, (in press).

Arrojadite-(BaNa), BaNa3(NaCa)Fe2+13Al(PO4)11(PO3OH)(OH)2, has been identified from Big Fish River, Yukon Territory, Canada. Arrojadite-(BaNa) was first discovered in the mine dumps of the Luna albite pegmatite, Dorio municipality, Lecco province, Italy (Vignola et al. 2016). At Big Fish River it is associated with apatite group minerals, gormanite, siderite, quartz, and other minerals in thin veins, phosphate nodules, and within the surrounding ironstone shales and mudstones of the Rapid Creek Formation. Arrojadite-(BaNa) occurs here as clear and colorless to slightly yellow or light–medium grey, ≤0.5 mm thick by 1 mm wide, tabular and platy crystals, as well as polycrystalline material. Zoning is visible in some tabular crystals, where the center of the crystal is medium grey in color and the edges appear clear and colorless. Differences exist in chemical composition and crystal structure between crystals from Big Fish River and the type specimen. In this paper, near-endmember arrojadite-(BaNa) from Big Fish River is presented with the empirical formula Ba1.01Na1.98Ca0.82Na2.0(Fe2+10.37Mg2.06Mn0.59)Σ13.03(Al1.10)(PO4)11(PO3OH)(OH)2.00 [calculated using 12 P, 50 (O,OH,F) per formula unit]. According to the crystal structure model which refined to R1 = 5.4%, arrojadite-(BaNa) from Big Fish River is monoclinic, space group Cc, with unit cell dimensions a 16.5163(8) Å, b 10.0067(5) Å, c 24.5435(12) Å, β 106.121(2)°, V 3896.9(3) Å3, for Z = 4. Raman data showing clusters of bands in the OH-stretching region (3500–3600 cm–1) suggest that the non-centrosymmetric space group Cc is correct for arrojadite-(BaNa) from Big Fish River. However, the arrojadite-(BaNa) specimen from the type locality, the Luna albite pegmatite, shows a single merged peak consistent with the centrosymmetric space group C2/c reported by Vignola et al. (2016).
Marco E. Ciriotti

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