• journal_title：The Canadian Mineralogist
• Contributor：Alexander P. Khomyakov ; Fernando Cámara ; Elena Sokolova
• Publisher：Mineralogical Association of Canada
• Date：2010-
• Format：text/html
• Language：en
• Identifier：10.3749/canmin.48.2.291
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：48
• issue：2
• firstpage：291
• section：Articles

Carbobystrite, ideally Na₈[Al₆Si₆O₂₄](CO₃)·4H₂O, trigonal, a 12.6678(5), c 10.3401(4) Å, V 1437.0(2) ų, 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/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)(1̅32, 122), 6.378(80) (1̅20), 2.661(80)(041, 123, 133), 3.867(70)(1̅31, 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 (Na_7.40K_0.38)_∑7.78(Al_6.04Si_6.02)_∑12.06O₂₄(CO₃)·3.5H₂O; the simplified formula is Na₈[Al₆Si₆O₂₄](CO₃)·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 ε (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 cancrinitegroup 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.