Ammonium hexafluorosilicate, ammonium fluosilicate or ammonium silicofluoride) has the formula (Nh4)2sif6. It is a toxic chemical, like all salts of fluorosilicic acid. [2] it is made of white crystals,[3] which have at least three polymorphs[4] and appears in nature as rare minerals cryptohalite or bararite.