Page 141 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Atacamite Atacamite, the most common of the copper trihydroxychloride
isomers, was named for the Atacama Desert in northern Chile
where it was identified in secondary copper ore deposits. Use of the mineral name can be traced
back to 1801, according to the ICDD files. Atacamite crystals are orthorhombic and vary in color
from emerald to blackish green. The mineral has been observed as a continuous, sugarlike
coating of dark green glistening crystals on many bronze objects from Egypt and Mesopota­
mia (Gettens 1964). These occurrences are similar to those seen on sheet-copper objects from
ancient Peruvian cultures, such as the Vicus and Moche cultures dating to the early centuries
C E . It is also not uncommon to see isolated, rather than continuous, patches of dark green
atacamite on archaeological material. Artificial patination recipes based on sodium chloride or
dilute hydrochloric acid may produce a primary patina of atacamite, but this cannot be confused
with a genuine patina, because atacamite is never found as a continuous layer direcdy adjacent
to the metal surface.

Par atacamite (anakarite) Paratacamite, also named for the Atacama Desert, is more often
and clinoatacamite found as a powdery, light green secondary corrosion layer
on the patina surface or in pustules from the transformation of
nantokite (cuprous chloride); highly developed crystals are less common. The strength of
hydrogen bonding in the copper trihydroxychlorides is highest for paratacamite and lowest for
botallackite, corresponding to the greater stability of paratacamite compared with botallackite.
Confusion has surrounded the classification of paratacamite. Jambor and colleagues (i996)
made the perplexing observation that the most recent entry for paratacamite in the ICDD files 3
was assigned a monoclinic crystal structure by Oswald and Guenter (1971), despite the fact that
the original data from Frondel (1950) placed the mineral in the rhombohedral group. The osten­
sible discovery by Adib and Ottemann (1972) of a zincian paratacamite, which they called ana-
rakite after Anarak province in Iran, only added to the confusion. Until recently, it had been
generally agreed that the original work of Frondel had correcdy established that paratacamite
was rhombohedral. Synthetic studies by Jambor's group, however, showed that the synthetic
end member of the copper-zinc-trihydroxychloride sequence is monoclinic at the copper end
and does not, in fact, have a rhombohedral structure. The ICDD files are, therefore, in error, and
the description of paratacamite first named in 1905 now needs major revision after nearly a cen­
tury of untroubled existence. This means that a new mineral name is necessary for the mono­
clinic form; "clinoatacamite" has been accepted as the appellation and should replace most of
the previously reported occurrences of paratacamite. Jambor and coworkers also found that
paratacamite and clinoatacamite can coexist in the same formation. Their studies of mineral
samples from Chuquicamata, Chile, showed clinoatacamite together with atacamite, paratacam­
ite, gypsum, and alunite on a quartzose matrix.

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