Tincalconite

Tincalconite is a hydrated sodium borate and one of the family of evaporite borate minerals. It most often appears not as a primary growth but as a fine white powdery coating, formed when the better-known borate borax loses some of its water on exposure to dry air and recrystallises to this more stable, less hydrated phase.

Properties

It is trigonal but is usually seen as a chalky white to colourless crust or as tiny grains; it is soft and dissolves readily in water. Because it forms by dehydration of borax, specimens are often labelled as alteration products.

Occurrence

Tincalconite occurs in dry borate evaporite deposits and playa lake beds, classically in the Mojave Desert of California (Boron, Searles Lake) alongside borax, kernite and ulexite. It typically forms wherever borax specimens are stored in dry conditions.