ARISITE-(Ce), A NEW RARE-EARTH FLUORCARBONATE FROM THE ARIS PHONOLITE, NAMIBIA, MONT SAINT-HILAIRE AND THE SAINT-AMABLE SILL, QUEBEC, CANADA

Arisite-(Ce), ideally NaCe2(CO3)2[(CO3)1– x F2 x ]F, is a new layered, rare-earth fluorcarbonate mineral from the Aris phonolite (Namibia), Mont Saint-Hilaire (Quebec), and the Saint-Amable sill (Quebec). At the Aris phonolite, arisite-(Ce) occurs as vitreous, transparent, beige, beige-yellow, light lemon yellow to pinkish, tabular, hexagonal plates and prisms up to 1.5 mm in miarolitic cavities. At Mont Saint-Hilaire, arisite-(Ce) occurs in alkaline pegmatite veins, sodalite syenite and sodalite syenite xenoliths as pale pink, silvery or very pale brown, micaceous plates with a rough hexagonal outline, as thin fibers (5 μm thick), as isolated crystals, as irregular clusters, rosettes or spherical aggregates 0.1 to 2 mm in diameter. In the Saint-Amable phonolite sill, arisite-(Ce) occurs as thin, flexible, pearly to silvery white, rounded to irregular micaceous plates (0.3 to 0.5 mm) in spherical aggregates or rosettes. Arisite-(Ce) is brittle, has a conchoidal fracture, poor cleavage perpendicular to (001), and a Mohs hardness of approximately 3–3½; it is non-fluorescent under either long- or short-wave UV radiation, dissolves slowly in room-temperature dilute HCl, and sinks in methylene iodide. Its density D calc is 4.126 g/cm3 for Z = 1 (Aris). Arisite-(Ce) is uniaxial negative, has a sharp extinction, with both ω and e exhibiting a range of values within each grain: 1.696 < ω < 1.717(4) and 1.594 < e < 1.611(3), a result of chemical zoning attributed to both Ce-for-La and Na-for-Ca substitutions. The average empirical formula for arisite-(Ce) from the Aris phonolite is (Na0.97Ca0.03)∑1.00(Ce0.92La0.80Nd0.11Pr0.04Sm0.01Ca0.09)∑1.97(CO3)2[(CO3)0.71F0.59] F. The mineral is hexagonal, P 6 m 2, a 5.1109(2), c 8.6713(4) A, V 196.16(6) A3. The strongest 11 lines of the powder X-ray-diffraction patterns [ d hkl NAM/ d hkl MSH in A( I obsNAM/ I obsMSH, hkl )] are: 4.439/4.428(100/100,100), 4.352/4.317(52/60,002), 3.103/3.097(87/80,102), 2.561/2.558(38/50,110), 2.424/2.415(21/40,103), 2.171/2.162(12/30,004), 1.9748/1.969(42/60,202), 1.9501/1.941(16/30,104), 1.9169/1.910(12/20,113), 1.6547/1.650(11/30,114) and 1.5640/1.560(13/20,212). Arisite-(Ce) is a late-stage, postmagmatic to hydrothermal mineral at all three localities. It is a member of the flat-lying layered REE fluorcarbonate group that includes lukechangite-(Ce), cordylite-(Ce), huanghoite-(Ce), cebaite-(Ce), kukharenkoite-(Ce) and kukharenkoite-(La). All these fluorcarbonates have crystal structures characterized by separate layers of carbonate, F, REE and alkali or alkaline-earth elements. The mineral’s name recalls the type locality, the Aris phonolite, Namibia.