Tincalconite

Tincalconite is a hydrous sodium borate mineral closely related to borax, and is a secondary mineral that forms as a dehydration product of borax. Its formula is Na₂B₄O₇·5H₂O or Na₂[B₄O₅(OH)₄]·3H₂O.

Tincalconite
General
Category	Nesoborates
Formula (repeating unit)	Na2[B4O5(OH)4]·3H2O
Strunz classification	6.DA.15
Crystal system	Trigonal
Crystal class	Trapezohedral (32) H-M symbol: (32)
Space group	R32

Tincalconite typically occurs as a fine grained white powder. It crystallizes in the hexagonal crystal system and has been found as primary euhedral di-rhombohedral, pseudo-octahedral crystals. It is also found pseudomorphically replacing borax crystals. It has a specific gravity of 1.88 and a Mohs hardness of 2. Refractive index values are nω=1.460 and nε=1.470.

While most tincalconite is created by man through exposing borax to dry air, there are natural occurrences of tincalconite, as in Searles Lake, California where it was first described in 1878. In addition to several California and Nevada locations it is reported from Argentina, Italy, Turkey and Ukraine.

The name comes from "tincal", Sanskrit for borax, and Greek, "konis", meaning powder, for its composition and typical powdery nature.