Tennantite

Tennantite is a copper arsenic sulfosalt mineral with an ideal formula Cu12As4S13. Due to variable substitution of the copper by iron and zinc the formula is Cu6[Cu4(Fe,Zn)2]As4S13.[1] It is gray-black, steel-gray, iron-gray or black in color. A closely related mineral, tetrahedrite (Cu12Sb4S13) has antimony substituting for arsenic and the two form a solid solution series. The two have very similar properties and is often difficult to distinguish between tennantite and tetrahedrite. Iron, zinc, and silver substitute up to about 15% for the copper site.[1][2]

Tennantite
General
CategorySulfosalt minerals
Formula
(repeating unit)		Cu6[Cu4(Fe,Zn)2]As4S13[1]
Strunz classification2.GB.05
Crystal systemCubic
Crystal classHextetrahedral (43m)
H-M symbol: (4 3m)
Space groupI43m
Identification
ColorFlint-gray to iron-black, cherry-red in transmitted light
Crystal habitmassive to well formed crystals
TwinningContact and penetration twins
CleavageNone
FractureSubconchoidal to uneven
TenacitySomewhat brittle
Mohs scale hardness3 – 4.5
LusterMetallic, commonly splendent
Streakreddish gray
DiaphaneityOpaque, except in very thin fragments
Specific gravity4.65
Polish lustergray, inclining to black to brown to cherry-red
Optical propertiesIsotropic
Refractive indexn greater than 2.72
References[1][2][3]

The mineral was first described for an occurrence in Cornwall, England in 1819, where it occurs as small crystals of cubic or dodecahedral form, and was named after the English chemist Smithson Tennant (1761–1815).[1][4]

Tennantite from Ireland

It is found in hydrothermal veins and contact metamorphic deposits in association with other Cu–Pb–Zn–Ag sulfides and sulfosalts, pyrite, calcite, dolomite, siderite, barite, fluorite and quartz.[2]

The arsenic component of tennantite causes the metal smelted from the ore to be harder than that of pure copper, because it is a copper-arsenic alloy. In the later 20th century it was found that arsenical coppers had been more widely used in antiquity than had been previously realised, and it has been proposed that discoveries made by smelting ores like tennantite were significant steps in the progress towards the Bronze Age.[5]

See also

References
  1. Mindat.org
  2. Handbook of Mineralogy
  3. Webmineral data
  4. Spencer, Leonard James (1911). "Tetrahedrite" . In Chisholm, Hugh (ed.). Encyclopædia Britannica. 26 (11th ed.). Cambridge University Press. pp. 670–671.
  5. Penhallurick, R.D. (1986), Tin in Antiquity: its mining and trade throughout the ancient world with particular reference to Cornwall, London: The Institute of Metals, p. 4, ISBN 0-904357-81-3


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