ABSTRACT

          Petersenite-(Ce) is a newly discovered rare-earth-element carbonate from the Poudrette quarry, Mont St-Hilaire, Quebec.  It is grey, with a slight pink tint, forming striated prisms up to 7mm in length.  It has a vitreous luster and is translucent to transparent.  Petersenite-(Ce) is relatively soft (Mohs hardness ~3) and very brittle, having a conchoidal fracture.  Optical properties are biaxial, with moderate dispersion, no absorption, and indices of refraction a 1.623(1), b 1.636(10, g 1.649(1), 2V_meas. = 89.7(5)° and 2V_calc. = 89.8°, and an optical orientation of X = b, Y & c = 30° and Z = a.  Electron-microprobe analyses gave:

          CO₂ (from crystal-structure analysis) , total 100.21 wt.%, which yields the empirical formula


(Na_3.75Ca_0.16)(Ce_0.96La_0.59Nd_0.23Sr_0.11Pr_0.08Sm_0.02Ba_0.01)(CO₃)₅ or,


ideally, Na₄(REE)₂(CO₃)₅ for Z = 4,


with D_calc. = 3.67 g/cm³  and D_meas. = 3.69 g/cm³.   Crystal-structure analysis shows the mineral to be monoclinic, space group P2₁, with a 20.872(4), b 6.367(1), c 10.601(2) Å and b 120.50(3)°.  The strongest lines in the X-ray powder pattern [d Å (I)(hkl)] are:

     The crystal structure has the subcell of burbankite and remondite-(Ce), but the supercell changes the ratio of [8]-coordinated to [10]-coordinated cations.  The layered structure is compared to that of other REE carbonates.

Keywords:  Petersenite-(Ce), new mineral species, rare-earth element, carbonate, crystal structure, Mont St-Hilaire, Quebec.