Two new mineral species of the mayenite group, fluormayenite Ca12Al14O32[□4F2] (I4¯3d, a = 11.9894(2) Å, V = 1723.42(5) Å3, Z = 2) and fluorkyuygenite Ca12Al14O32[(H2O)4F2] (I4¯3d, a = 11.966(2) Å, V = 1713.4(1) Å3, Z = 2), are major constituents of larnite pyrometamorphic rocks of the Hatrurim Complex (Mottled Zone) distributed along the Dead Sea rift on the territory of Israel, Palestinian Autonomy and Jordan. Holotype specimens of fluormayenite and fluorkyuygenite were collected at the Jabel Harmun, Judean Mts., Palestinian Autonomy and in the Hatrurim Basin, Negev Desert, Israel, respectively. Mineral associations of holotype fluormayenite and fluorkyuygenite are similar and include larnite, shulamitite, Cr-containing spinel–magnesioferrite series, ye’elimite, fluorapatite–fluorellestadite, periclase, brownmillerite, oldhamite as well as the retrograde phases portlandite, hematite, hillebrandite, afwillite, foshagite, ettringite, katoite and hydrocalumite. Fluormayenite and fluorkyuygenite crystals, usually < 20 μm in size, are colourless, in places with greenish or yellowish tint, the streak is white. Both minerals are transparent with a vitreous lustre; they do not show fluorescence. Fluormayenite and fluorkyuygenite are isotropic and have similar refractive indices: n = 1.612(3) and n = 1.610(3) (589 nm), respectively. The hardness of fluormayenite and fluorkyuygenite is H (Mohs) 5½–6; VHN load 50 g, 771(38) kg mm−2; and 5–5½; VHN load 50 g, 712(83) kg m−2, respectively. Both minerals have the microporous tetrahedral framework structure characteristic of the mayenite supergroup. In fluormayenite 1/3 of the structural cages are occupied by fluorine. In fluorkyuygenite, in addition to fluorine and negligible amounts of OH, H2O molecules occupy about 2/3 of the cages. The holotype fluormayenite from Jabel Harmun has the crystal chemical formula (Ca11.951Na0.037)Σ11.987(Al13.675Fe3+0.270Mg0.040Si0.009P0.005S6+0.013)Σ14.013 O31.503(OH)1.492[□4.581F1.375Cl0.044]Σ6, fluorkyuygenite from the Hatrurim Basin has the composition Ca12.034(Al13.344Fe3+0.398 Si0.224)Σ13.966O32[(H2O)3.810F1.894 (OH)0.296]Σ6. Raman spectra of fluormayenite and fluorkyuygenite in the spectral region 200–1000 cm−1 are similar and are characterized by the four strong main bands at about 320 (ν2 AlO4), 520 (ν4 AlO4), 700, 770 (ν1 AlO4) cm−1. In the O-H vibration region fluorkyuygenite shows a broad band between 2600–3500 cm−1 (νH2O). The molecular water is completely released from the fluorkyuygenite structure at about 400°C. Fluorkyuygenite crystallized initially as fluormayenite, which later was altered under influence of water vapour-enriched gases during a combustion process. Fluormayenite has been synthesized and fluorkyuygenite is an analogue of the recently discovered chlorkyuygenite, Ca12Al14O32[(H2O)4Cl2], from the Northern Caucasus, Russia.

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