This sample of tincalconite is displayed in the Smithsonian Museum of Natural History. Tincalconite is a oxide mineral of boron and sodium. It has the standard composition Na2(B4O7).5H2O. However, this sample has the indicated composition Na2(B4O5)(OH)4.8H2O. This sample is about 10x15 cm and is from Boron, California.
The sample is described as tincalconite after borax, suggesting that its form is typical of borax. Its name is from "tincal", a Sanskrit name for borax, plus the Greek "konis" for powder.