Images
Formula: Ca7(SiO4)3(OH)2
Nesosilicate (insular SiO4 groups), chegemite subgroup,
humite group, forms a series with
fluorchegemite
Crystal System: Orthorhombic
Specific gravity: 2.86 measured, 2.892 calculated
Hardness: 5½ to 6
Colour: Pink, yellow-pink to white
Common impurities: Ti
Environments
Volcanic igneous environments
Metamorphic environments
Chegemite is a product of sanidinite facies
metamorphism of calcareous xenoliths in ignimbrite. Associated
minerals include larnite, spurrite,
rondorfite,
reinhardbraunsite, wadalite,
lakargiite and srebrodolskite
(HOM).
Localities
At the type locality, Xenolith no. 1, Lakargi Mountain, Upper Chegem volcanic caldera, Baksan Valley,
Kabardino-Balkaria, Russia, chegemite was discovered in 2007 the in altered calcareous silicate xenolith in
ignimbrite. The central part of xenolith 1 (visible thickness 2–3 m)
is composed of bluish-grey marble with relict banding. Dark gray massive
spurrite-calcite rocks are developed
at the southern part of the xenolith. Calcium minerals of the humite group,
such as chegemite and reinhardbraunsite are prominent in these
rocks. Small fragments of lightly coloured spurrite rock, up to 50 cm in size,
with pink spots of reinhardbraunsite and yellow egg-like segregations
of rondorfite are considerably less common.
Wadalite, rondorfite,
hydroxylellestadite and
lakargiite are widespread in different rock types within xenolith 1.
Perovskite, srebrodolskite
and garnet of close to kimzeyite
composition are rare constituents. Among secondary
low-temperature minerals, calcium hydrosilicates, such as hillebrandite
and afwillite, minerals of the
ettringite group,
hydrocalumite and hydrogarnets
are more common.
Relics of larnite and
magnesioferrite are characteristic in
cuspidine- and chegemite- bearing rocks, and
bultfonteinite is also common in
cuspidine-bearing rocks. Brucite
grains with periclase relics, associated with
calcite, are found in the spurrite
rock.
The size of commonly cracked chegemite grains typically does not exceed 300 microns.
Xenomorphic and ovoid relics of larnite,
spurrite,
hydroxylellestadite and inclusions of isometric
wadalite crystals with zones of a potential new mineral of the
mayenite group occur within the chegemite grains.
Lakargiite and
srebrodolskite inclusions are also occasionally detected within
chegemite grains. Minerals of the mayenite group are replaced by
katoite, which also forms rims on
reinhardbraunsite. Rounded relics of chegemite with thin rims
of a calcium hydrosilicate are enclosed in large grains of
reinhardbraunsite
(EJM 21.1045-1059).
Chegemite from the Lakargi Mountain - Image
At Xenolith no. 7, Lakargi Mountain, Upper Chegem volcanic caldera, Baksan Valley, Kabardino-Balkaria, Russia,
chegemite was discovered in the skarned carbonate-silicate xenolith
more than 5 m across. The contact of this xenolith with the hosting
ignimbrite is marked by an almost 1 m thick
cuspidine rock with fluorite. A
light-yellow rock with dark-pink spots underlies the
fluorite-cuspidine zone. The pink
patches proved to be monomineralic aggregates of chegemite reaching 30 cm in maximum extension comprising
individual chegemite grains up to 5 mm in size. Larnite relics and
abundant inclusions of rondorfite replaced by calcium hydrosilicates with
magnesium impurities occur within chegemite grains. The yellow part of the rock with the pink chegemite
spots comprises a hydrogarnet -
bultfonteinite - ettringite
groundmass with abundant calcium hydrosilicates. Here, rare lakargiite
crystals, and larnite grains replaced on the periphery by chegemite, and
hydroxylellestadite are noted. In addition, chegemite is
replaced by reinhardbraunsite
(EJM 21.1045-1059).
Chegemite from Xenolith no. 7 - Image
Back to Minerals