Small brown crystal aggregates of the rarest member of the neptunite group, magnesioneptunite, from the type and only known locality worldwide in Lakargi Mountain ( Lakargi Mountain, Upper Chegem Caldera, Baksan Valley, Kabardino-Balkaria, Russian Federation). Ex. Erich Laskowski collection (USA). — Photo: David Hospital · CC BY 4.0
Magnesioneptunite
Magnesioneptunite is an IMA-recognized mineral species in the silicate class, with the chemical formula KNa₂Li(Mg,Fe)₂Ti₂Si₈O₂₄.
It crystallizes in the monoclinic crystal system (space group C2/c). It is a member of the Neptunite structural group.
Key properties: Mohs hardness 5–6; specific gravity 3.152; Vitreous luster; optically biaxial.
IMA symbol Mnpt, IMA status: approved, named in 2009.
Found in volcanic ejecta of hornfelsed fine-grained sandstone as reaction zones between a skarn carbonate xenolith and host ignimbrite by alteration of ilmenite.
It is found associated with Quartz, sanidine, microcline, albite, aegirine, rutile, Mg-ilmenite, monazite-(Ce), Cs-analcime, pollucite.
As the magnesium analog of neptunite.