Page 243 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

THE C H E M I S T R Y OF COPPER S U L F I D E S

The copper sulfides are generally opaque under bright-field microscopy and range in color from
black to blue in reflected light. The refractive indices are all greater than 1.66. Chakrabarti and
Laughlin (i983) give a provisional analysis of the copper-sulfur system that includes a detailed
discussion of the copper-sulfur phase diagram. The major difficulty with this system is created
by many phase regions, which can have variable composition, and relatively few sulfide com­
pounds of fixed stoichiometry. A monoclinic low chalcocite, Cu 2 S, has been identified that is
stable at temperatures up to 103 °C; a hexagonal high chalcocite with the same formula is stable
at temperatures above 103 °C. The face-centered cubic mineral digenite also comes in a high and
low form: high digenite, Cu 7 2 S 4 , and low digenite (synthetic), Cu 9 S 5 . The ICDD files have
another entry for digenite (synthetic), Cu 1-8 S, which is the same formula as the previous entry
divided by 5. Digenite is stable over a wider range of sulfur content than some of the other
phases, and at room temperature, low digenite is thought to be stabilized by the presence of iron
between 0.4 and 1.6 atomic%. Compounds with stoichiometry approximating chalcocite, Cu 2 S,
are stable between 435 °C and 1130 °C. The temperature stability of copper-deficient forms
varies: at about 36.6% copper, they are stable up to 507 °C, and at 35.65% copper, down to 72 °C.
There is an orthorhombic djurleite of nominal composition, Cu 196 S, stable to 72 °C, and
another orthorhombic compound, anilite, Cu 1-75 S, which is stable up to 75 °C. The hexagonal
mineral covellite, CuS, is stable to 507 °C. Blaubleibender ("blue-remaining") covellite has a
slightly different X-ray pattern than ordinary covellite and contains from 2 to 8 atomic % more
copper. It can be formed by oxidation of digenite and chalcocite at room temperature. The ICDD
files regard spionkopite, Cu 132S, as a component of blaubleibender covellite.
Some of these phases have a tendency to undergo thermomechanical alteration. This
can make their identification difficult because the mineral phase can be altered by grinding
during sample preparation, as exhibited, for example, by anilite, which can be altered to digen­
ite by grinding.
The ICDD files recognize the eleven varieties of copper sulfides shown in TABLE 6.1. In addi­
tion, there is a diverse array of mixed metallic sulfides that includes copper-iron sulfides and
copper-lead sulfides. These minerals—particularly chalcopyrite, CuFeS 2—have been used as
ores of copper or other metallic elements since ancient times. Enargite, Cu 3 AsS 4 , and bornite,
Cu 5FeS 4, were also of importance in the ancient smelting of copper (Rapp 1986).

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