Tennantite

Tennantite
General
Category Sulfosalt minerals
Formula
(repeating unit)		 Cu6[Cu4(Fe,Zn)2]As4S13[1]
Strunz classification 2.GB.05
Crystal system Cubic
Crystal class Hextetrahedral (43m)
H-M symbol: (4 3m)
Space group I43m
Identification
Color Flint-gray to iron-black, cherry-red in transmitted light
Crystal habit massive to well formed crystals
Twinning Contact and penetration twins
Cleavage None
Fracture Subconchoidal to uneven
Tenacity Somewhat brittle
Mohs scale hardness 3 - 4.5
Luster Metallic, commonly splendent
Streak reddish gray
Diaphaneity Opaque, except in very thin fragments
Specific gravity 4.65
Polish luster gray, inclining to black to brown to cherry-red
Optical properties Isotropic
Refractive index n greater than 2.72
References [1][2][3]

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]

The mineral was first described for an occurrence in Cornwall, England in 1819 and named after the English chemist Smithson Tennant (1761-1815).[1]

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.[4]

See also

References

  1. 1 2 3 4 5 Mindat.org
  2. 1 2 3 Handbook of Mineralogy
  3. ↑ Webmineral data
  4. ↑ 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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