PHOSINAITE-(Ce) FROM MONT SAINT-HILAIRE, QUEBEC: NEW DATA AND STRUCTURE REFINEMENT

ANDREW M. McDONALD* and GEORGE Y. CHAO
Ottawa-Carleton Geoscience Centre, Department of Earth Sciences, Carleton University, Ottawa, Ontario K1S 5B6

JOEL D. GRICE
Mineral Sciences Section, Earth Sciences Division, Canadian Museum of Nature, Ottawa, Ontario K1P 6P4

* Present address: Department of Earth Sciences, Laurentian University, Sudbury, Ontario P3A 2E6.

Abstract

Phosinaite-(Ce), ideally Na13Ca2REE[Si4O12](PO4)4, has been found in sodalite syenite xenoliths at the Poudrette Quarry, Mont Saint-Hilaire, Québec. 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 2 × 1 mm in vugs in sodalite. Crystals are often 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-microprobe analyses (n=4) gave Na2O 31.68, CaO 8.92, MnO 0.89, La2O3 4.18, Ce2O3 6.52, Nd2O3 0.99, Pr2O3 0.26, ThO2 1.84, Al2O3 0.11, SiO2 18.36 and P2O5 23.69 wt.%, corresponding to (Na12.72Ca0.14) 12.86(Ca1.84Mn0.16) 2.00(Ce0.49La0.32Th0.09Nd0.07Pr0.02) 0.99 [(Si3.77P0.15Al0.04) 3.96O12](PO4)4, based on 28 oxygen atoms. Phosinaite-(Ce) from Mont Saint-Hilaire, Quebec, is found to have a distinctive Th concentration compared to that from the Khibina and Lovozero massifs, Russia. Single crystal study showed it to be orthorhombic, P22121 with a 12.297(2), b 14.660(3), c 7.245(1) Å, with V 1306.1(1) Å3 and Z = 2. The observed powder pattern is consistent with that previously published; the reflection reported at 6.92 Å was not observed in our material. The crystal structure was refined using single crystal X-ray diffraction data to R = 3.1% and wR2 = 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 (Na0.66Ca0.34). Essential ordering of Si and P was established based on results from electron-microprobe analyses, site-occupancy refinement, mean T-O bond distances and calculated bond-valence sums, suggesting the mineral be classified as a silicophosphate.