IMA 2021-087a = fluorsigaiite

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Marco E. Ciriotti
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IMA 2021-087a = fluorsigaiite

Messaggio da Marco E. Ciriotti » gio 11 ago, 2022 10:53

Prossima pubblicazione.

Referenza:
▪ Wu, B., Gu, X., Rao, C., Wang, R., Xing, X., Zhong, F., Wan, J., Bonnetti, C. (2022): Fluorsigaiite, Ca2Sr3(PO4)3F, a new mineral of the apatite supergroup from the Saima alkaline complex, Liaoning Province, China. Mineralogical Magazine, 86, (in press).

Abstract:
Fluorsigaiite, ideally Ca2Sr3(PO4)3F, is a new Sr analogue of fluorphosphohedyphane from the apatite supergroup. It was discovered in lujavrite
from the Saima alkaline complex, Liaoning Province, China. Fluorsigaiite commonly occurs as individual prismatic, columnar and platy crystal of 10 to 50 μm in size, associated with microcline, nepheline, aegirine, natrolite, eudialyte, fluorapatite, a fluorstrophite-like mineral, stronadelphite, and calcite. Occasionally, crystals of fluorsigaiite form prismatic aggregates in interstices of lujavrite. Fluorsigaiite is translucent to transparent, colourless to yellowish white with a vitreous lustre and without fluorescence. The estimated Mohs hardness is 5, and the tenacity is brittle with uneven fractures. The calculated density is 3.842 g/cm3. Optically, fluorsigaiite is uniaxial (–) with ω = 1.64(1), ε = 1.63(1) in white light and without dispersion. Mean chemical composition (in wt.%) of fluorsigaiite is Na2O 0.75, CaO 15.17, SrO44.44, La2O3 3.64, Ce2O3 2.22, Pr2O3 0.19, Nd2O3 0.13, Sm2O3 0.05, Gd2O3 0.23, P2O5 31.87, F 1.91, H2O 0.46 and sum 100.26 with the empirical formula of (Sr2.82Ca1.79Na0.16La0.15Ce0.09Pr0.01Nd0.01Gd0.01)Σ5.04P2.97O12[F0.66(OH)0.34]Σ1, which is calculated on the basis of 13 total anions and F+(OH)=1. The strongest eight lines of its powder XRD pattern [d, Å (I, %) (hkl)] are: 3.563 (15) (002), 3.275 (15) (102), 3.144 (19) (120), 2.876 (100) (121), 2.861 (96) (112), 2.772 (27) (300), 1.991 (17) (222), and 1.895 (23) (213). Fluorsigaiite is hexagonal, in the space group P63/m and unit-cell parameters refined from single-crystal X-ray diffraction data are: a = 9.6101(2) Å, c = 7.1311(1) Å, V = 570.35(3) Å3 and Z = 2. It is isostructural with hedyphane group minerals, and contains different prevailing (species-defining) Ca and Sr cations at the Ca1 and Ca2 sites, respectively. Fluorsigaiite was likely formed from Sr-rich fluids at late-magmatic or hydrothermal stage of the Saima lujavrite.

NdR: Attualmente sono ben 47 le specie del supergruppo dell'apatite!
Marco E. Ciriotti

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