Tan crystals of the rare silicate mineral grumantite from the type locality and only one of the two known localities worldwide: Lovozero Massif, Kola, Russian Federation. — Photo: David Hospital · CC BY-SA 4.0
Grumantite
Grumantite is an IMA-recognized mineral species in the silicate class, with the chemical formula NaSi₂O₄(OH)·H₂O.
It crystallizes in the orthorhombic crystal system (space group Fdd2).
Key properties: Mohs hardness 4–5; specific gravity 2.21; White streak; Vitreous, silky to dull in aggregates luster; optically biaxial.
IMA symbol Gmt, IMA status: approved, named in 1985.
Found in veins cutting alkalic feldspathoid-bearing pegmatites in a differentiated alkalic massif.
It is found associated with Ussingite, makatite, kazakovite, tisinalite, nordite, sodalite, nepheline, potassic feldspar, arfvedsonite, aegirine, eudialyte, loparite.
The name comes from an old Russian name for the Spitsbergen Archipelago.