ABSTRACT

     Horváthite-(Y), ideally NaY(CO₃)F₂, is a new mineral species from Mont 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, with a white streak and a vitreous luster.  Associated minerals include microcline, aegirine, dawsonite, rhodochrosite, sodalite, natrolite, albite, sérandite, siderite, taeniolite, catapleiite, astrophyllite, genthelvite, and a franconite-group mineral.  It is soft (Mohs hardness ~4), brittle, with an uneven fracture and with very good {100} cleavage.  Horváthite-(Y) is biaxial negative, a 1.457 (1), b 1.543 (1), g 1.622 (1), 2V_meas. = 82 (1)° and 2V_calc. = 83°. It is orthorhombic, space group Pmcn, a 6.964 (3), b 9.173 (6), c 6.302 (2) ų and Z = 4.  The stongest eight X-ray powder-diffraction lines [d in Å (I)(hkl)] are:

     An average electron-microprobe analysis gave:

     Total 97.90 wt.%.  The proportion of CO₂ was calculated by stoichiometry from the results of the crystal-structure analysis.  The empirical formula based on three atoms of oxygen is:


(Na_0.90Ca_0.07)_S0.97 (Y_0.89Gd_0.01Dy_0.03Er_0.04Yb_0.02Ho_0.01)_S1.00


(CO₃)F_2.01 or, ideally   NaY(CO₃)F₂.

     D_calc. is equal to 3.613 g/cm³, and D_meas. exceeds 3.3 g/cm³.  The structure has 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 structure of horvathite-(Y) seems unique among mineral structures, but ressembles that of synthetic BaCu(CO₃)F₂.

Keywords:  Horváthite-(Y), new mineral species, carbonate, crystal structure, Mont Saint-Hilaire, Quebec.

     ¹E-mail address:  jgrice@mus-nature.ca