HORVATHITE-(Y), RARE-EARTH FLUOROCARBONATE, A NEW MINERAL SPECIES FROM MONT SAINT-HILAIRE, QUEBEC

Horvathite-(Y), ideally, NaY(CO3)F-2, is a new mineral species from Mo nt Saint-Hilaire, Quebec. It occurs as tabular crystals up to 1 X 1 x 0.25 mm stacked in fan-shaped groups. It is colorless to pale beige, w ith a white streak and a vitreous luster. Associated minerals include microcline, aegirine, dawsonite, rhodochrosite, sodalite, natrolite, a lbite, serandite, siderite, taeniolite, catapleiite, astrophyllite, ge nthelvite, and a franconite-group mineral. It is soft (Mohs hardness s imilar to 4), brittle, with an uneven fracture and with very good {100 } cleavage. Horvathite-(Y) is biaxial negative, alpha 1.457(1), beta 1 .543(1), gamma 1.622(1), 2V(meas.) = 82(1)degrees and 2V(calc.) = 83 d egrees. It is orthorhombic, space group Pmcn, a 6.964(3), b 9.173;(6), c 6.302(2) Angstrom, V 402.6(4) Angstrom(3) and Z = 4. The strongest eight X-ray powder-diffraction lines [d in Angstrom (I)(hkl)] are 5.19 (90)(011), 3.477(100)(200), 2.800(50)(130), 2.087(50)(132,222), 2.057( 50)(141,013), 1.966(50)(321), 1.849(50)(330) and 1.763(50)(213,051). A n average electron-microprobe analysis gave Na2O 12.62, CaO 1.86, Y2O3 45.07, Gd2O3 0.45, Dy2O3 2.49, Er2O3 3.02, Yb2O3 1.64, Ho2O3 0.81, Tb 2O3 0.12, F 17.22 and CO2 19.83, O=F-7.23, total 97.90 wt.%. The propo rtion of CO2 was calculated by stoichiometry from the results of the c rystal-structure analysis. The empirical formula based on three atoms of oxygen is: (Na0.90Ca0.07)(Sigma 0.97)(Y0.89Gd0.01Dy0.03Er0.04Yb0.02 Ho0.01,)(Sigma 1.00)(CO3)F-2.01 or, ideally, NaY(CO3)F-2. D-calc. is e qual to 3.613 g/cm(3) and D-meas. exceeds 3.3 g/cm(3). The structure h as been refined to R=1.6%. The structure is layered perpendicular to [ 100], with a Na-Y-carbonate group layer and a layer of F atoms. The st ructure of horvathite-(Y) seems unique among mineral structures, but r esembles that of synthetic BaCu(CO3)F-2.