Arsenic is a ubiquitous element with metalloid properties. Its chemistry is complex and there are many different compounds of both inorganic and organic arsenic. In nature, it is widely distributed in a number of minerals, mainly as the arsenides of copper, nickel, and iron, or as arsenic sulfide or oxide. In water, arsenic is usually found in the form of arsenate or arsenite. Methylated arsenic compounds occur naturally in the environment as the result of biological activity. The most important commercial compound, arsenic(III) oxide, is produced as a by-product in the smelting of copper and lead ores. Arsenic compounds are mainly used in agriculture and forestry as pesticides, herbicides, and silvicides; smaller amounts are used in the glass and ceramics industries and as feed additives.