CARBOBYSTRITE, Na8[Al6Si6O24](CO3)•4H2O, A NEW CANCRINITE-GROUP MINERAL SPECIES FROM THE KHIBINA ALKALINE MASSIF, KOLA PENINSULA, RUSSIA: DESCRIPTION AND CRYSTAL STRUCTURE

Carbobystrite ideally Na-8[Al6Si6O24](CO3)center dot 4H(2)O, trigonal, a 12.6678(5), c 10.3401(4) angstrom, V 1437.0(2) angstrom(3), c: a = 0.8162, space group P31c, Z = 2, R = 0.0271 (for 4861 observed reflections collected with a four-circle X-ray diffractometer), is a new cancrinite-group mineral species from Mount Koashva, Khibina alkaline massif, Kola Peninsula, Russia. It formed in a pegmatitic environment. Associated minerals are: microcline, sodalite, megakalsilite, natrolite, pectolite, aegirine, natrite, nacaphite, vitusite-(Ce), fluorcaphite, belovite-(Ce), umbite, lemmleinite-K, lomonosovite, lovozerite, phlogopite, sphalerite and galena. The mineral was found in only one hand specimen of pegmatite as a corroded irregularly shaped grain about 3 mm across, intergrown with sodalite, megakalsilite and natrite. It is transparent, colorless with a white streak, a vitreous luster, and fluoresces pale yellow under ultraviolet light. Carbobystrite has a Mohs hardness of 6, is brittle with a conchoidal fracture, and has no cleavage. The calculated density is 2.366 g/cm(3). Carbobystrite is uniaxial negative, non- pleochroic, omega 1.500(2), epsilon 1.488 (2). The strongest six reflections in the X-ray powder-diffraction pattern [d in angstrom(I)(hkl)] are: 4.689(100)(012), 3.249(100)((1) over bar 32, 122), 6.378(80) ((1) over bar 20), 2.661(80)(041, 123, (1) over bar 33), 3.867(70)((1) over bar 31, 121), and 3.664(70)(030). An electron-microprobe analysis gives Na2O 22.66, K2O 1.75, SiO2 35.74, Al2O3 30.42, CO2 4.37, H2O 6.26, sum 101.20 wt.%, with CO2 and H2O calculated from structural data. The empirical formula (based on O = 30.5 atoms per formula unit) is (Na7.40K0.38)(Sigma 7.78)(Al6.04Si6.02)(Sigma 12.06)O-24(CO3)center dot 3.5H(2)O; the simplified formula is Na-8[Al6Si6O24](CO3)center dot 4H(2)O. The name carbobystrite denotes the compositional relation between the new mineral and bystrite, (Na, K)(7)Ca[ Al6Si6O24](S)(1.5)center dot H2O; they are related by the substitution Na+ + (CO3)(2-) + 2.5H(2)O -> Ca2+ + (S2-)(1.5). The framework of Si, Al-O tetrahedra can be described as an ABAC stacking sequence. In the crystal structure, there are two LOSOD cages, and two. (cancrinite) cages per unit cell, giving a maximum of two (CO3) groups and six H2O groups for the LOSOD-type cages and two H2O groups for the cancrinite cages, respectively. This is the first complex-sequence cancrinite-group mineral with (CO3) groups in cages larger than the cancrinite cages. Carbobystrite suggests a new series of carbonate equivalents of all known complex sequences in the cancrinite group.