Kumtyubeite

kumtyubeite

reinhardbraunsite

lakargiite

kimseyite

Images

Formula: Ca5(SiO4)2F2
Nesosilicate (insular SiO4 groups), chegemite subgroup, humite group
Crystal System: Monoclinic
Specific gravity: 2.866 calculated
Hardness: 5 to 6
Streak: White
Colour: Light pink
Environments

Volcanic igneous environments
Metamorphic environments

Localities

At the type locality, Xenolith no. 1, Lakargi Mountain, Upper Chegem volcanic caldera, Chegemsky District, Kabardino-Balkaria, Russia, the central part of the xenolith is composed of bluish grey marble with massive dark grey spurrite - calcite rocks encasing the marble core. Some small xenolith outcrops (1 to 1.5 m across) occur along contacts of xenolith 1. They are light coloured from white to yellow or red and contain abundant calcium minerals of the humite group, including reinhardbraunsite and kumtyubeite. More rarely, small fragments of light spurrite rocks, 50 cm in size, with striking pink spots of reinhardbraunsite and yellow oval rondorfite patches are noted. At the contact of the xenolith with ignimbrites, cuspidine rocks with larnite, rondorfite, rustumite and wadalite are developed. Reinhardbraunsite, rondorfite, P- and As- bearing hydroxylellestadite, wadalite, Sn- and U- bearing lakargiite, srebrodolskite and magnesioferrite are commonly distributed in different types of skarn rocks; perovskite, kimzeyite-type garnet, periclase and sphalerite are considerably less abundant; secondary calcium hydrosilicates (bultfonteinite, hillebrandite and afwillite), minerals of the ettringite group, hydrocalumite, hydrogarnets, calcite and brucite are fairly common.
Kumtyubeite is noted in spurrite - rondorfite - ellestadite zones of skarn. In hand specimens of the greyish-reddish matrix, kumtyubeite forms oval spots up to 1 cm across, composed of light pink grains 250 µm in maximum dimension. Rarely, kumtyubeite contains inclusions of lakargiite and kimzeyite.
Rondorfite is also characteristically found in this association, occurring as rounded, isometric inclusions in kumtyubeite, partly replaced by calcium hydrosilicates. P- and As-bearing hydroxylellestadite associated with kumtyubeite is often replaced by minerals of the ettringite group and hydrogarnets. Accessory minerals are srebrodolskite and magnesioferrite, forming aggregates reaching 500 µm in size, and also kimzeyite-type garnet, wadalite and sphalerite. Reinhardbraunsite is confined to spurrite zones of the skarn, which change to kumtyubeite - spurrite - rondorfite - ellestadite zones toward the xenolith centre. Larnite relics are absent within kumtyubeite and reinhardbraunsite (AM 94.10.1361-1370).
Kumtyubeite from the Lakargi Mountain - Image

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