Phosinaite-(Ce) from Mont Saint-Hilaire, Quebec; new data and structure refinement

Carleton University, Department of Earth Sciences, Ottawa, ON, Canada

Phosinaite-(Ce), ideally Na ₁₃ Ca ₂ REE[Si ₄ O ₁₂ ](PO ₄ ) ₄ , has been found in sodalite syenite xenoliths at the Poudrette Quarry, Mont Saint Hilaire, Quebec. The mineral occurs as colorless to pale brown anhedral, elongate grains up to 2 mm set in the groundmass, and euhedral prismatic crystals up to 2X1 mm in vugs in sodalite. Crystals commonly are coated in a brown film of an amorphous Na-Ca-K silicate, previously believed to be a hydrocarbon. Important associated phases include microcline, analcime, albite, villiaumite, astrophyllite, thermonatrite, vitusite, revdite and vuonnemite. Average results of electron-microproble analyses (n = 4) gave Na ₂ O 31.68, CaO 8.92, MnO 0.89, La ₂ O ₃ 4.18, Ce ₂ O ₃ 6.52, Nd ₂ O ₃ 0.99, Pr ₂ O ₃ 0.26, ThO ₂ 1.84, Al ₂ O ₃ 0.11, SiO ₂ 18.36 and P ₂ O ₅ 23.69 wt.%, corresponding to (Na (sub 12.72) Ca (sub 0.14) ) (sub Sigma 12.86) (Ca (sub 1.84) Mn (sub 0.16) ) (sub Sigma 2.00) (Ce (sub 0.49) La (sub 0.32) Th (sub 0.09) Nd (sub 0.07) Pr (sub 0.02) ) (sub Sigma 0.991 [(Si (sub 3.77) P (sub 0.15) Al (sub 0.04) ) (sub Sigma 3.96) O ₁₂ ] (PO ₄ ) ₄ , based on 28 oxygen atoms. Phosinaite-(Ce) from Mont Saint-Hilaire is found to have a distinctive concentration of Th compared to that from the Khibina and Lovozero massifs, Russia. Single-crystal study showed it to be orthorhombic, P22 ₁ 2 ₁ , with a 12.297(2), b 14.660(3), e 7.245(1) Aa, V 1306.1(1) Aa ³ and Z = 2. The observed powder-pattern is consistent with that previously published; the reflection reported at 6.92 Aa was not observed in our material. The crystal structure was refined using single-crystal X-ray-diffraction data to R = 3.1% and wR ² = 7.99%. In addition to confirmation of the basic features of the structure originally proposed for the mineral, site-occupancy refinement for one octahedrally coordinated site gave (Na (sub 0.66) Ca (sub 0.34) ). A high degree of order of Si and P was established on the basis of results from electron-microprobe analyses, site-occupancy refinement, mean T-O bond distances and calculated bond-valence sums, suggesting that the mineral be classified as a silicophosphate.

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				D. M.C. Huminicki and F. C. Hawthorne The Crystal Chemistry of the Phosphate Minerals Reviews in Mineralogy and Geochemistry, January 1, 2002; 48(1): 123 - 253. [Full Text] [PDF]