Home > Mineral Abstracts > CanMin Vol. 35, pp. 181-187 (1997)




Vol. 35, pp. 181-187 (1997)



Sheldrickite, a new sodium-calcium-fluorocarbonate mineral species from Mont Saint-Hilaire, Quebec


Joel D. Grice, Robert A. Gault and Jerry van Velthuizen
Research Division, Canadian Museum of Nature,
P.O. 3443, Station D, Ottawa, Ontario K1P 6P4


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) Å3, Z = 3.  The strongest seven X-ray powder-diffraction lines [d in Å (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)
X.XXX (40)(300)


     The infrared spectrum is given.  Three electron-microprobe 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
  (15.91-16.52)

CO2 25.81
H2O 5.61
O = F -6.81
Sum 99.14 wt.%


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


Na1.01(Ca2.97Sr0.01)S2.98CO3)2[F2.90(OH)0.07]S2.97 · H2O


or, ideally, NaCa3(CO3)2F3 · H2O.


     Dcalc. = 2.86 g/cm3, Dmeas.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.


© 1997  The Canadian Mineralogist