Mayenite supergroup, part II: Chlorkyuygenite from Upper Chegem, Northern Caucasus, Kabardino-Balkaria, Russia, a new microporous mineral with “zeolitic” H2O

Abstract

The new mineral chlorkyuygenite, Ca 12 Al 14 O 32 [(H 2 O) 4 Cl 2 ] ( I 4 ¯ 3 d , a =12.0285(1)A, V =1740.34(3)A 3 ), was discovered as an accessory mineral in Ca-humite zones of calcareous skarn xenoliths in ignimbrites of the Upper Chegem Caldera, Northern Caucasus, Kabardino-Balkaria, Russia. Rounded grains and crystals with tris-tetrahedral form of chlorkyuygenite up to 50μm and aggregates up to 100–150μm in size are enclosed in chegemite, reinhardbraunsite and srebrodolskite. Chlorkyuygenite also forms rims on wadalite crystals. Chegemite–fluorchegemite, reinhardbraunsite–kumtyubeite, rondorfite, hydroxylellestadite, lakargiite, perovskite, kerimasite, elbrusite, ettringite-group minerals, hydrocalumite, bultfonteinite, and minerals of the katoite–grossular series are associated with chlorkyuygenite. Larnite, spurrite and galuskinite are noted as relics in Ca-humites. Chlorkyuygenite is colourless, occasionally with a greenish or yellowish tint, and the streak is white. The mineral is transparent with strong vitreous lustre, it is isotropic, n =1.672(1) (589 nm). The microhardness VHN load 50 g is 632(37) kg mm −2 , corresponding to 5–5½ hardness according to the Mohs scale; the calculated density is 2.941 g cm −3 . The calculated Gladstone-Dale’s compatibility factor 1–(K p /K c )=−0.016 is superior. The holotype specimen of chlorkyuygenite from the chegemite zone is characterized by relatively constant composition corresponding to the crystal-chemical formula Ca 11.979 (Al 12.986 Fe 3+ 0.823 Si 0.179 Ti 4+ 0.033 ) ∑14.021 O 32 [(H 2 O) 3.767 Cl 2.234 ] ∑6 . In the Raman spectra of chlorkyuygenite the following characteristic main bands are distinguished: 202, 321, 511, 705, 776 and 881 cm −1 . A broad band with two maxima near 3400 and 3200 cm −1 is observed in the OH region and it is related to H 2 O in the structural cages of chlorkyuygenite. The molecular water is completely released from the mineral structure at about 550°C. Chlorkyuygenite crystallized initially as chlormayenite, which later was altered under influence of volcanic gases containing water vapour.