ABSTRACT

     Sheldrickite, ideally NaCa₃(CO₃)₂F₃ · H₂O, is a newly identified mineral species from Mont Saint-Hilaire, Quebec.  It occurs as aggregate of 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 2mm across.  Associated minerals include pectolite, shortite, microcline, polylithionite, arfvedsonite and accessory molybdenite.  The mineral has vitreous to silky luster and a white streak.  It is soft (Mohs hardness 3) and brittle, with an uneven fracture and good {001} parting.  Sheldrickite is uniaxial positive,
w = 1.538(2) and e = 1.563(4).  It is trigonal, space group P3₂, a 6.718(3), c 15.050(4) Å, V 588.3(3) ų, Z = 3.  The strongest seven X-ray powder-diffraction lines [d in Å (I)(hkl)] are:

     The infrared spectrum is given.  Three electron-microprobe analyses and a thermogravimetric analysis gave:

     CO₂ was calculated by stoichiometry from the results of the crystal-structure analysis.  The empirical formula based on 10 anions is:


Na_1.01(Ca_2.97Sr_0.01)_S2.98CO₃)₂[F_2.90(OH)_0.07]_S2.97 · H₂O


or, ideally, NaCa₃(CO₃)₂F₃ · H₂O.

     D_calc. = 2.86 g/cm³, D_meas.2.86(4) g/cm³.  The structure has been refined to R = 4.6% using a twinned crystal.  The structure is layered on {001}.  One layer consists of (CO₃)^2- groups oriented perpendicular to {001}, the Naj₈ polyhedra and the H₂O groups.  A second layer consists of Caj₉ polyhedra and F- anions.  The structure of sheldrickite bears little ressemblance to the chemically similar rouvilleite, but it seems to be a modification of the structures of bastnäsite and cebaite.  The name honors George M. Sheldrick, author of the SHELX software, widely used for the refinement of crystal structures.

Keywords:  sheldrickite, new mineral species, carbonate, crystal structure, twinning, Mont-Hilaire, Quebec.