SHELDRICKITE, A NEW SODIUM-CALCIUM-FLUOROCARBONATE MINERAL SPECIES FROM MONT SAINT-HILAIRE, QUEBEC

Sheldrickite, ideally NaCa3(CO3)(2)F3H2O, is a newly identified minera l species from Mont Saint-Hilaire, Quebec. It occurs as an aggregate o f blocky, colorless to white crystals up to 2 mm wide, with individual crystals up to 0.1 mm, and as aggregates of silky white flakes to 2 m m across. Associated minerals include pectolite, shortite, microcline, polylithionite, arfvedsonite and minor molybdenite. The mineral has a vitreous to silky luster and a white streak It is soft (Mobs hardness 3) and brittle, with an uneven fracture and good {001} parting. Sheld rickite is uniaxial positive, omega = 1.538(2) and epsilon = 1.563(4). It is trigonal, space group P3(2), a 6.718(3), c 15.050(4) Angstrom, V 588.3(3) Angstrom(3), and Z = 3. The strongest seven X-ray powder-di ffraction lines [d in Angstrom (I)(hkl)] are: 5.809(30)(100), 5.010(30 )(003), 3.358(30)(110), 2.791(50)(113), 2.508(40)(006), 2.010(100)(116 ) and 1.939(40)(300). The infrared spectrum is given. Three electron-m icroprobe analyses and a thermogravimetric analysis gave Na2O 9.16 (9. 11-9.24), CaO 48.84 (48.62-49.15), SrO 0.36 (0.33-0.40), F 16.17 (15.9 1-16.52), CO2 25.81 and H2O 5.61, O = F -6.81, total 99.14 wt.%. CO2 w as calculated by stoichiometry from the results of the crystal-structu re analysis. The empirical formula based on 10 anions is Na-1.01(Ca2.9 7Sr0.01)(Sigma 2.98)(CO3)(2)[F-2.90(OH)(0.07)](Sigma 2.97). H2O or, id eally, NaCa3(CO3)(2)F-3 . H2O. Dcalc. = 2.86 g/cm(3) and Dmeas. = 2.86 (4) g/cm(3). The structure has been refined to R = 4.6% using a twinne d crystal. The structure is layered on (001). One layer consists of (C O3)(2-) groups oriented perpendicular to (001), the Na phi(8) polyhedr a and the H2O groups. A second layer consists of Ca phi(9) polyhedra a nd F- anions. The structure of sheldrickite bears little resemblance t o the chemically similar rouvilleite, but it seems to be a modificatio n of the structures of bastnasite and cebaite. The name honors George M. Sheldrick creator of the SHELX software, widely used for the refine ment of crystal structures.