_Tetrahedrite_ (Fahl ore), a very complex sulphide of copper, iron, lead, zinc, with arsenic, etc. It is often rich, and carries silver values. =Oxidised Ores.=—The most important of the oxidised ores are— _Malachite_, CuCO_{3}.Cu(OH)_{2}, containing, when pure, 57·3 per cent. copper (73·7 CuO); is widely distributed, but usually occurs as such in non-paying quantities except in a few particular localities. It is found in the upper parts of the veins. Whenever found with sulphide ores, it is an extremely useful material to mix in the charge, as it supplies oxygen as well as copper. Malachite is still an important source of the metal in Mexico, Chili, and Bolivia, though not quite so much so as formerly, whilst it is specially important in the Tanganyika (Katanga) deposits, of which it constitutes the greater portion so far developed. _Cuprite_, Cu_{2}O, contains 88·8 per cent. copper, when pure. It is widely distributed, but is never found by itself in paying deposits, though in the early days of mining and smelting it was an important source of metal, since it was easily reduced, and consequently was cheaply worked. _Melaconite_, CuO, contains 79·8 per cent. copper, when pure; is fairly widely distributed, although hardly ever in sufficient quantity to pay. In one or two localities, however—viz., Tennessee, North Carolina, and Virginia—it was formerly an important source of the metal. The deposits were at first very promising, as they consisted largely of very rich melaconite ore; this was however, quickly worked out, the ordinary heavy chalcopyrite with 2·5 per cent. copper being struck below. Other oxidised ores include— _Azurite_, 2CuCO_{3}. Cu(OH)_{2}, and _Atacamite_, CuCl_{3}. 3Cu(OH)_{2}, from Chili. In modern work, the chief ore smelted is impure chalcopyrite. Carbonate and oxidised ores, when they can be obtained, are mixed with it, increasing the concentration and shortening the process; except under certain special circumstances. =Preliminary Treatment of Ores.=—The treatment of ores preparatory to smelting includes the processes of sampling, wet concentration, agglomeration of fines, and roasting. =Sampling.=—Since sampling is not a part of the extraction process proper, in copper smelting, it will be convenient to deal with the subject separately here.