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

Phosinaite-(Ce), ideally Na(13)Ca(2)REE[Si4O12](PO4)(4), has been foun d in sodalite syenite xenoliths at the Poudrette Quarry, Mont Saint-Hi laire, Quebec. The mineral occurs as colorless to pale brown anhedral, elongate grains up to 2 mm set in the groundmass, and euhedral prisma tic crystals up to 2 x 1 mm in vugs in sodalite. Crystals commonly are coated in a brown film of an amorphous Na-Ca-K silicate, previously b elieved to be a hydrocarbon. Important associated phases include micro cline, analcime, albite, villiaumite, astrophyllite, thermonatrite, vi tusite, revdite and vuonnemite. Average results of electron-microprobe analyses (n = 4) gave Na2O 31.68, CaO 8.92, MnO 0.89, La2O3 4.18, Ce2 O3 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)(Sigma 12.86)(Ca1.84 Mn0.16)(Sigma 2.00)(Ce0.49La0.32Th0.09Nd0.07Pr0.02)(Sigma 0.99)[(Si3.7 7P0.15Al0.04)(Sigma 3.96)O-12](PO4)(4), based on 28 oxygen atoms. Phos inaite-(Ce) from Mont Saint-Hilaire is found to have a distinctive con centration of Th compared to that from the Khibina and Lovozero massif s, Russia. Single-crystal study showed it to be orthorhombic, P22(1)2( 1), with a 12.297(2), b 14.660(3) c 7.245(1) Angstrom, V 1306.1(1) Ang strom(3) and Z = 2. The observed powder-pattern is consistent with tha t previously published; the reflection reported at 6.92 Angstrom, 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%. I n addition to confirmation of the basic features of the structure orig inally proposed for the mineral, site-occupancy refinement for one oct ahedrally coordinated site gave (Na0.66Ca0.34). A high degree of order of Si and P was established on the basis of results from electron-mic roprobe analyses, site-occupancy refinement, mean T-O bond distances a nd calculated bond-valence sums, suggesting that the mineral be classi fied as a silicophosphate.