Description
Atacamite is a green orthorhombic copper chloride hydoxide mineral with composition Cu2(OH)3Cl. It commonly occurs as euhedral or fibrous crystals (although it may be massive or granular in form) and forms in the weathering zone around copper lode deposits, particularly when the erosion occurs in a desert environment. It is named after its type locality at Atacama, Chile, in the Atacam Desert, and is also known as remolinite after Los Remolinos in South America where it occurs. Other associated historical mineral terms include halochalzit and smaragdocalcite. Atacamite is also found at Botallack mine (StJust, Cornwall, England), Linares (Spain) and Burra mine (south Australia) and is polymorphous with botallackite, paratacamite and the recently assigned clinoatacamite (qq.v.).
History
The relative rarity of the mineral has led some authors to conclude that the synthetic analogue (copper chloride hydroxide, atacamite type, q.v.) was used as pigment and in fact there are numerous mediaeval recipes for preparing a green copper pigment with common salt. Of these perhaps the best known is that of Theophilus for viride salsum (q.v.).
Atacamite has been identified as a major component in green pigments in Egyptian wall paintings by El Goresy et al. (1986), although it is possible that this is a synthetic form. Green (2001) also reports that copper chloride has been identified as a component in ancient Egyptian green pigments. Atacamite has been identified on eighth century paintings from Dunhuang by Delbourgo (1980), although the method used to identify it (X-ray fluorescence) would not be able to distinguish between the polymorphs. It was also apparently found in wall paintings of almost all dynasties at Dunhuang, and the report by Wainwright et al. (1993) further states that a quarry for atacamite existed at Dunhuang. Atacamite has also been identified on several eleventh to fifteenth century paintings (Van’T Hul-Ehrnreich and Hallebeek, 1972; Kerber et al., 1972; Banik et al., 1982) and in thirteenth century Romanesque frescos where it was present as an alteration product of artificial azurite (Richter, 1988). Additional identifications are given by Naumova and Pisareva (1994).
Physical properties
Habit: acicular, striated prisms, euhedral crystals, fibrous, granular.
Color: green, dark
green, or blackish green.
Luster: adamantine.
Diaphaneity: transparent to translucent.
Streak: apple green.
Cleavage: [010] perfect.
Fracture: conchoidal.
Occurrence: arid
climates with oxidizable copper minerals.
