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Formula: KNa2Li(Mg,Fe)2Ti2Si8O24
Phyllosilicate (sheet silicate), neptunite group,
lithium- and titanium- bearing
mineral
Crystal system: Monoclinic
Specific gravity: 3.15 calculated
Hardness: 5 to 6
Colour: Dark brown to red-brown
Environments
Igneous environments
Metamorphic environments
Magnesioneptunite was approved in 2009, but to date (August 2025) it has been reported only from the type
locality.
Localities
At the type locality, Xenolith no. 5, Lakargi Mountain, Upper Chegem volcanic caldera, Chegemsky District,
Kabardino-Balkaria, Russia, magnesioneptunite has been found in a xenolith of altered
sandstone located between
skarnified carbonate xenoliths and
ignimbrite. Magnesioneptunite occurs as nearly isometric grains
and aggregates up to 0.1 mm in size in the cores of some grains of a magnesium-rich variety of
neptunite. Grains of magnesioneptunite are dark brown to
red-brown, translucent and with a vitreous lustre. It is associated with
quartz, sanidine,
microcline, albite,
magnesium-bearing ilmenite,
monazite-(Ce),
cesium-bearing analcime,
pollucite,
rutile, aegirine and
neptunite. Magnesioneptunite and the magnesium-rich variety of
neptunite were formed as products of
ilmenite alteration
(Geology of Ore Deposits, 53, 775-782, HOM).
Magnesioneptunite from the Lakargi Mountain -
Image
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