Anatacamite from La Vendida mine, Sierra Gorda, Atacama desert, Chile: a triclinic polymorph of Cu_2(OH)_3CI

摘要

The new mineral anatacamite (IMA-CNMNC No. 2008-042) has the ideal chemical formula Cu_2(OH)_3Cl. It is triclinic with space group P1. Unit-cell data from single-crystal X-ray studies gave a = 9.1646(9), b = 9.2029(8), c = 9.2102(8) A, á = 95.858(6)°, ? = 96.290(7)°, ? = 96.507(2)°, V = 761.97(12) A~3, Z = 8. Anatacamite is the triclinic polymorph of atacamite, botal-lackite and clinoatacamite. The strongest reflections in the X-ray powder diffraction data, calculated from single crystal data, are as follows d_(obs). (A) (I): 5.432 (100), 2.747 (94), 2.257 (56), and 2.889 (31). Anatacamite was found in La Vendida mine, Sierra Gorda mining district, northeast of Antofagasta, Chile. It occurs as triangular, tabular crystals up to 2 mm in size grown on prismatic atacamite crystals. Anatacamite shows emerald green colour with a vitreous luster. The crystals show a good cleavage parallel to {111}. The calculated density is 3.725 g/cm~3, the calculated mean refractive index is 1.80, pleochroism is not apparent. In La Vendida mine anatacamite is closely associated with atacamite, further minerals of this mineralization are alunite, clinoatacamite, chalcanthite, coquimbite, eriochalcite, jarosite, krohnkite, magnesioaubertite, tamarugite and voltaite. Chemical analyses gave (wt.%) CuO 74.24, ZnO 0.02, NiO 0.99, Cl 16.98, H_2O_(calc) 12.74, O = Cl -3.84, sum 101.13. The resulting empirical formula based on 4 anions pfu is Cu_(1.97)Ni_(0.03)(OH)_(2.99)Cl_(1.01). Anatacamite is named for its triclinic symmetry and its polymorphism with atacamite. Type material is deposited in the collection of the Mineralogical Museum Hamburg, Germany.