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

Abstract

Carbobystrite, ideally Na 8 [Al 6 Si 6 O 24 ](CO 3 )·4H 2 O, trigonal, a 12.6678(5), c 10.3401(4) A, V 1437.0(2) A 3 , c:a = 0.8162, space group P 31 c , 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, ω 1.500(2), e 1.488 (2). The strongest six reflections in the X-ray powder-diffraction pattern [ d in A(I)( hkl )] are: 4.689(100)(012), 3.249(100)(132, 122), 6.378(80) (120), 2.661(80)(041, 123, 133), 3.867(70)(131, 121), and 3.664(70)(030). An electron-microprobe analysis gives Na 2 O 22.66, K 2 O 1.75, SiO 2 35.74, Al 2 O 3 30.42, CO 2 4.37, H 2 O 6.26, sum 101.20 wt.%, with CO 2 and H 2 O calculated from structural data. The empirical formula (based on O = 30.5 atoms per formula unit) is (Na 7.40 K 0.38 ) ∑7.78 (Al 6.04 Si 6.02 ) ∑12.06 O 24 (CO 3 )·3.5H 2 O; the simplified formula is Na 8 [Al 6 Si 6 O 24 ](CO 3 )·4H 2 O. The name carbobystrite denotes the compositional relation between the new mineral and bystrite, (Na,K) 7 Ca[Al 6 Si 6 O 24 ](S) 1.5 ·H 2 O; they are related by the substitution Na + + (CO 3 ) 2− + 2.5H 2 O → Ca 2+ + (S 2− ) 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 (CO 3 ) groups and six H 2 O groups for the LOSOD-type cages and two H 2 O groups for the cancrinite cages, respectively. This is the first complex-sequence cancrinitegroup mineral with (CO 3 ) 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.