Vanarsite
A very dark blue sodium-calcium vanadate-arsenate forming flattened sub-parallel blades.
Intro adapted from Wikipedia (CC BY-SA).
Vanarsite is an IMA-recognized mineral species in the oxides & hydroxide class, with the chemical formula NaCa₁₂(As³⁺V⁵⁺₈.₅V⁴⁺₃.₅As⁵⁺₆O₅₁)₂·78H₂O.
It crystallizes in the monoclinic crystal system (space group P2_1/c).
Key properties: Mohs hardness 2; specific gravity 2.48; Grayish blue streak; Vitreous luster; optically biaxial.
IMA symbol Vns, IMA status: approved, named in 2014.
Forms secondarily through oxidation of montroseite-corvusite assemblages in a moist environment.
It is found associated with Gatewayite, morrisonite, packratite, pharmacolite, montroseite, corvusite.
An acronym based on the composition and specifically the fact that it contains vanadate, arsenite, and arsenate groups.