A halide is a binary compound of a halogen with oxidation number –1. Metal halides are mostly ionic salts (X^–1), usually very soluble. Nonmetal halides, and a few metal halides such as tin (IV) chloride, are volatile covalent compounds, highly reactive, often violently hydrolyzed by water, and used as halogenating agents. Halide ions form stable ligand complexes. Their reducing power increases down the group. Halide minerals include halite, fluorite, sylvite, apatite, and cryolite. See also alkyl halide.