Page 270 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

TABLE 8.1 CHARACTERISTICS OF SOME COPPER SILICATE MINERALS

MINERAL FORMULA COLOR MOHS
NAME HARDNESS

chrysocolla (Cu,Al) 2 H 2 Si 2 0 5 (OH) 4 · xH 20 vitreous/earthy green 2 - 4
cuprorivaite CaCuSi 4O 10 vitreous blue 5
dioptase CuSi0 3 -H 2 0 vitreous green 5.5-6.5
plancheite Cu 8 (Si 4 O u ) 2 (OH) 4 .H 2 0 translucent blue 5.5
shattuckite Cu 5 (Si0 3 ) 4 (OH) 2 translucent dark blue 3.5

C H R Y S O C O L L A
The chemical formula for the common copper silicate mineral chrysocolla was formerly ex­
pressed as CuSi0 2 -7zH 2 0, but can be represented as (Cu,Al) 2 H 2 Si 2 0 5 (OH) 4 -.x;H 2 0 (Fleischer,
Wilcox, and Matzko 1984). This is a monoclinic, fibrous or massive mineral of variable proper­
ties that commonly occurs in bands showing varying intensity of blue-green coloration (Garrels
and Dreyer 1952). The bands may be associated with chalcedony, a cryptocrystalline variety of
quartz. The color of chrysocolla in thin section ranges from black through blue green to only a
slight greenish tinge; the birefringence increases with increasing color. Garrels and Dreyer
reported the analyses of ten different chrysocolla samples, from green to black, and none of
them had a composition that matched the shorter formula formerly used for this mineral. They
suggest that chrysocolla may, in fact, be a solution of copper silicate in silica, which would help
to explain the extraordinary variations in properties and composition of the chrysocolla
samples studied. 1
The mineral is sometimes described as being isotropic, but all specimens examined by the
author in the laboratory have been anisotropic. These samples show a low refractive index,
lower than 1.66, which is the refractive index of the mounting medium. Typical examples are
illustrated in PLATE 49. When chrysocolla is ground to a fine powder, it can retain a good color,
although when viewed under the microscope in transmitted bright-field illumination, the green
color of the conchoidal fragments is often difficult to see, and the particles may appear almost
colorless. Most of the mineral particles are biréfringent, often with fibrous or undulóse extinc­
tion. The pigment is stable in light and decomposes only in strongly acidic or strongly alkaline
environments.
X-ray diffraction data for chrysocolla are often unsatisfactory and variable due to hydration
and poor crystallinity of the sample. The ICDD files have two sets of X-ray diffraction entries for

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