ATACAMITA DEL PÓRFIDO CUPRÍFERO TACA TACA BAJO, SALTA (ARGENTINA)

COLOMBO, FERNANDO; LIRA, RAÚL

Río Cuarto

Congreso; 7º Congreso de Mineralogía y Metalogenia; 2004

Departamento de Ciencias Geológicas (UBA) y Departamento de Geología (Univ. Nac. de Río Cuarto)

Atacamite, Cu2(OH)3Cl, was found as isolated crystals and crystal groups lining a fissure in a drilling core of Taca Taca Bajo porphyry copper. Crystals reach up to 6 mm, with a length-to-width ratio of » 4:1, and show dominant {120} and {011}, modified by {010}, {142}, {111} and {992}. There are also some {hkl} forms that could not be measured. Some faces are rounded. Crystals are blackish green, translucent and show intense vitreous luster. Streak is light green, and they have conchoidal fracture and perfect cleavage in one direction. Orientation is a = Z, b = X, c = Y; pleochroism is very weak, b = yellowish green, g = a = emerald green. Cell dimensions (in Å), refined in space group Pnam, are a = 6.0252(5), b = 9.1113(7), c = 6.8615(7), V = 376.67(5) Å3. The powder X-ray diagram has a good match with PDF file 25-0269. No lines other than those of atacamite were found. EDS analyses show only Cl and Cu. Published data on atacamite genesis indicate that conditions prevailing during its precipitation were low temperature (<200ºC), a pH of 4 or higher, a content of CuCl+ around 20% of total dissolved Cu2+ and absence of important quantities of higher chloro-complexes.