ABSTRACT

     Calcioburbankite, ideally Na₃(Ca,REE,Sr)₃(CO₃)₅, is a new mineral species found at Mont Saint-Hilaire, Quebec.  It is the Ca-analogue of burbankite, (Na,Ca)₃(Sr,Ba,Ce)₃(CO₃)₅, and of khanneshite, (Na,Ca)₃(Ba,Sr,Ba,Ce)₃(CO₃)₅.

     All data were collected from a single, subhedral, deep orange, prismatic crystal 2 X 1.8 X 2 cm in size.  Associated minerals include ancylite-(Ce), calcite, a chlorite-group mineral, donnayite-(Y), fluorapatite, natrolite, pyrite, rhodochrosite and rutile.  It is elongated parallel to [001], with {100} dominant.  It has a conchoidal fracture, a vitreous luster and indistinct {100} cleavage.  The mineral is soft, Mohs hardness 3 to 4, gives a white streak and is brittle.  Optically, the mineral is uniaxial negative, w 1.636(1) and e 1.631(1) (l 589nm).  It is translucent, nonpleochroic and non-fluorescent.  Combined X-ray powder-diffraction and precession studies show that the mineral is hexagonal, possible space-groups P6₃mc, P6₃mmc or P62c. 

     The unit-cell data, refined from the powder data [d in Å (I)(hkl)] are:

     Electron-microprobe results gave:

CO₂ (calculated according to stoichiometry) 35.13, total 99.40 wt.%.  The empirical formula (based on 15 atoms of oxygen) is:


Na_3.07(Ca_1.32Ce_0.55Sr_0.46La_0.36Nd_0.14Pr_0.05Sm_0.02Ba_0.02)_S2.92 (CO₃)₅,


for Z = 2, D_calc 3.46 and D_meas 3.45(3) g/cm³.  The mineral formed late in the paragenesis, possibly in a vent.  The name designates its calcium-dominant position within the hexagonal members of the burbankite group of minerals.  The physical data of calcioburbankite are compared to those of other members of the burbankite group.

Keywords:  calcioburbankite, Ca-analogue, burbankite group, new mineral species, carbonate, Mont Saint-Hilaire, Quebec.