Colorless platy silky crystals of the very rare niobium mineral epistolite from the type locality in Greenland: Ilímaussaq, Narsaq, Kujalleq. Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY-SA 4.0
Epistolite
Epistolite is an IMA-recognized mineral species in the silicate class, with the chemical formula (Na )Nb₂Na₃Ti(Si₂O₇)₂O₂(OH)₂(H₂O)₄.
It crystallizes in the triclinic crystal system (space group P-1). It is a member of the Seidozerite-Lamprophyllite structural group.
Key properties: Mohs hardness 1.5–3; specific gravity 2.65–2.89; Pearly to silky luster; optically biaxial.
IMA symbol Epi, IMA status: approved, named in 1901.
A low temperature mineral in alkalic pegmatites, albitites, sodalite xenoliths, and hydrothermal veins.
It is found associated with Aegirine, albite, murmanite, sphalerite, manganoan pectolite, neptunite, steenstrupine, sodalite, eudialyte, nenadkevichite.
Its name comes from the Greek for letter, in allusion to the flat rectangular crystal habit and white color.