ALBERT

Description: Paratacamite-(Mg) (IMA 2013-014), Cu 3 (Mg,Cu)Cl 2 (OH) 6 , is the new Mg-analogue of paratacamite. It was found near the village of Cuya in the Camarones Valley, Arica Province, Chile. The mineral is a supergene secondary phase occurring in association with anhydrite, atacamite, chalcopyrite, copiapite, dolomite, epsomite, haydeeite, hematite, magnesite and quartz. Paratacamite-(Mg) crystals are rhombs and thick to thin prisms up to 0.3 mm in size exhibiting the forms {201} and {001}. Twinning by reflection on {101I} is common. The mineral is transparent with a vitreous lustre, with medium to deep-green colour and light-green streak. Mohs hardness is 3–31/2, the tenacity is brittle and the fracture is conchoidal. Paratacamite-(Mg) has one perfect cleavage on {201}. The measured and calculated densities are 3.50(2) and 3.551 g cm –3 , respectively. The mineral is optically uniaxial (–) with = 1.785(5) and 〉 1.8 and slight pleochroism: O (bluish green) 〉 E (green). Electron-microprobe analyses provided the empirical formula Cu 3 (Mg 0.60 Cu 0.38 Ni 0.01 Mn 0.01 )Cl 2 (OH) 6 . The mineral is easily soluble in dilute HCl. Paratacamite-(Mg) is trigonal, R 3I, with cell parameters a = 13.689(1), c = 14.025(1) Å, V = 2275.8(3) Å 3 and Z = 12. There is a pronounced sub-cell corresponding to a ' 1/2 a , c ' c in space group R 3I m . The eight strongest lines in the X-ray powder diffraction pattern are [ d obs Å( I )( hkl )]: 5.469(87)(021), 4.686(26)(003), 2.904(34)(401), 2.762(100)(223I,042), 2.265(81)(404), 1.819(26)(603), 1.710 (34)(440) and 1.380(19)(446). The structure was refined to R 1 = 0.039 for 480 F o 〉 4 F reflections. Refinement using interlayer Mg-Cu site scattering factors indicated that Mg is distributed statistically between both interlayer octahedra M 1O 6 and M 2O 6 . A comparison of the distortions associated with M 1O 6 and M 2O 6 octahedra suggest that the sample is near the upper compositional limit for stability of the R 3I phase.