Tincalconite is a hydrous sodium hydroxy-borate mineral (Na2B4O5(OH)4·3H2O). It often forms as a result of dehydration (removal of some water) of other borate minerals, such as borax (Na2(B4O7)·10H2O). Tincalconite has a nonmetallic luster that is dull to glassy in some specimens. It is quite soft (H=2) and has a white, powdery appearance. It usually takes on the crystal form of the pre-alteration mineral.
Tincalconite taking on the crystal forms of the precursor mineral, borax (i.e., tincalconite pseudomorphs after borax).
Locality: Boron, southeastern Kern County, southern California, USA.
(CSM # 75.28, Colorado School of Mines Geology Museum, Golden, Colorado, USA)