Buddingtonite is an ammonium feldspar with formula: NH₄AlSi₃O₈ (note: some sources add 0.5H₂O to the formula). It forms in hydrothermal areas by alteration of primary feldspar minerals. It is an indicator of possible gold and silver deposits, as they can become concentrated by hydrothermal processes. It crystallizes in the monoclinic crystal system and is colorless to white with a vitreous luster. Its structure is analogous to that of high sanidine (KAlSi₃O₈). Buddingtonite has a hardness of 5.5 and a specific gravity of 2.32.