Synonyms
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Peacock ore ; Erubescite ; Purple copper ore ; Horseflesh ore.
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BORNITE was named after the Austrian mineralogist Ignatius von Born (1742-1791)
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Composition
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A sulphide of copper and iron. Cu5FeS4.
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| Analysis of Bornite |
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| | 1 | 2 | 3 | 4 | 5 |
| Cu | 63.33 | 63.90 | 63.24 | 63.20 | 63.24 |
| Fe | 11.12 | 10.79 | 11.12 | 11.34 | 11.20 |
| S | 25.55 | 25.17 | 25.54 | 25.65 | 25.54 |
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| | 100.00 | 99.86 | 99.90 | 100.19 | 99.98 |
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| S.G. | 5.074 | | 5.079 | | 5.072 |
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| | 1. Theoretical composition. |
| | 2. Guilford Co. N.C. (with about 3 per cent. chalcocite). |
| | 3. Messina, Transvaal. |
| | 4. Fanambana, Madagascar. |
| | 5. Bristol, Conn. |
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The copper content of bornite may in practice range up to 70 per cent. owing to the presence of chalcocite.
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Alteration
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Bornite readily alters to chalcocite and to covellite, cuprite, chrysocolla, malachite and azurite.
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Crystallography
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Isometric; hexoctahedrel, but rarely euhedral or idiomorphic. Commonly granular, compact or massive. Twin plane {III} ; often with penetrating twins.
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Physical Properties
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H = 3, Sp. Gr. = 5.06-5.08. Lustre, metallic; colour, brownish bronze on freshly fractured surfaces but rapidly tarnishes to variegated purple, blue and nearly to black; streak, greyish black; fracture, conchoidal; cleavage, absent or imperfect.
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Tests and Diagnosis
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Fusibility, 2½. Heated on charcoal, gives an odour of sulphur dioxide. Heated in a closed tube, gives very little sulphur. In the reducing flame bornite fuses to a magnetic globule. Soluble in nitric acid with separation of sulphur.
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The colour of freshly fractured surfaces and the tarnish are characteristic of bornite.
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Occurrence
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Bornite is a copper ore of very wide occurrence and is often associated with other copper minerals in hypogene deposits especially in the zones of primary sulphides and of secondary sulphide enrichment. It is less common as a supergene mineral in the upper zones of secondary oxide enrichment and of leaching in copper deposits.
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Among the ore minerals of copper, bornite is probably exceeded in importance only by chalcocite and chalcopyrite with which it is often associated.
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Bornite occurs in dykes as disseminations in basic rocks ; in contact metamorphic rocks; in replacement deposits; and in pegmatites. Rarely are bornite bearing veins of economic importance.
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Among the very large number of occurrences of bornite, mention may be made of the well-known ones in Cornwall, in N. Rhodesia, at Ookiep (Namaqualand), Messina (Transvaal) and other parts of Africa, Butte (Montana), Bisbee (Arizona), Kennecott (Alaska), Braden and Chuquicamata (Chile) and in the syngenetic sedimentary copper deposits of the Kupferschiefer at Mansfeld in Germany.
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