CHALCOPYRITE or COPPER-PYRITES, a copper iron sulphide an important ore of copper.
Chalcopyrite crystallizes in the tetragonal system with inclined hemihedrism, but the form is so nearly cubic that it was not recognized as tetragonal until accurate measurements were made in 1822. Crystals are usually tetrahedral in aspect but frequently twinned, and they are often complex and difficult to decipher. The fracture is conchoidal, and the material is brittle. Hardness 4; specific gravity 4.2. The colour is brass-yellow, and the lustre metallic ; the streak, or colour of the powder, is greenish-black. The mineral is especially liable to surface alteration, tarnishing with beautiful iridescent colours; a blue colour usually predomi nates, owing probably to the alteration of the chalcopyrite to covellite (CuS).
Chalcopyrite may be readily distinguished from pyrite (or iron pyrites), which it somewhat resembles in appearance, by its deeper colour and lower degree of hardness : the former is easily scratched by a knife, whilst the latter can only be scratched with difficulty or not at all. Chalcopyrite is decomposed by nitric acid with separation of sulphur and formation of a green solution; ammonia added in excess to this solution changes the green colour to deep blue and precipitates red ferric hydroxide.
The chemical formula CuFeS, corresponds with the percentage composition Cu=34.5, Fe=3o•5, 5-35.o. Analyses usually, how ever, show the presence of more iron, owing to the intimate ad mixture of pyrite. Traces of gold, silver, selenium or thallium are sometimes present, and the mineral is sometimes worked as an ore of gold or silver, which are present in solid solution.
Chalcopyrite is of wide distribution and is the commonest of the ores of copper. Extensive deposits are mined in the United States, particularly at Butte in Montana, and in Namaqualand, South Africa. Well-crystallized specimens are met with at many localities ; for example, formerly at Wheal Towan (hence the name towanite, which has been applied to the species) in the St. Agnes district of Cornwall, at Freiberg in Saxony, and Joplin, Missouri. (L. J. S.)