Carbobystrite, Na8[AI6Si6O 24](CO3)·4H2O, a new Cancrinite-group mineral species from the Khibina alkaline massif, Kola peninsula, Russia : description and crystal structure

Carbobystrite, ideally Na8[Al6Si6O 24](CO3)·4H2O, trigonal, a 12.6678(5), c 10.3401(4) Å, V 1437.0(2) Å3, 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. rbobystrite has a Mohs hardness of 6, is brittle with a conchoidal fracture, and has no cleavage. The calculated density is 2.366 g/cm3. Carbobystrite is uniaxial negative, non-pleochroic, ω 1.500(2), ε 1.488 (2). The strongest six reflections in the X-ray powder-diffraction pattern [d in Å(I)(hkl)] are: 4.689(100)(012), 3.249(100)(1̄32, 122), 6.378(80) (1̄20), 2.661(80)(041, 123, 1̄33), 3.867(70)(1̄31, 121), and 3.664(70)(030). An electron-microprobe analysis gives Na2O 22.66, K2O 1.75, SiO2 35.74, A12O3 30.42, CO24.37, 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)Σ 7.78(Al6.04Si6.02) Σ12.06(CO3)·3.5H2O; the simplified formula is Nag[Al6 i6O24](CO 3)·4H2O. The name carbobystrite denotes the compositional relation between the new mineral and bystrite, (Na,K) 7Ca[Al6Si6O24](S) 1.5·H2O they are related by the substitution Na+ + (CO3)2-+ 2.5H2O -→ 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 e (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.