Paratacamite-(Mg), Cu₃(Mg,Cu)Cl₂(OH)₆; a new substituted basic copper chloride mineral from Camerones, Chile

<jats:title>Abstract</jats:title><jats:p>Paratacamite-(Mg) (IMA 2013-014), Cu<jats:sub>3</jats:sub>(Mg, Cu)Cl2(OH)<jats:sub>6</jats:sub>, 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 {10<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S0026461X00012299_inline01.png" />} is common. The mineral is transparent with a vitreous lustre, with medium to deep-green colour and light-green streak. Mohs hardness is 3–3½, 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<jats:sup>–3</jats:sup>, respectively. The mineral is optically uniaxial (–) with ε = 1.785(5) and ω > 1.8 and slight pleochroism: <jats:italic>O</jats:italic> (bluish green) > <jats:italic>E</jats:italic> (green). Electron-microprobe analyses provided the empirical formula Cu<jats:sub>3</jats:sub>(Mg<jats:sub>0.60</jats:sub>Cu<jats:sub>0.38</jats:sub>Ni<jats:sub>0.01</jats:sub>Mn<jats:sub>0.01</jats:sub>)Cl<jats:sub>2</jats:sub>(OH)<jats:sub>6</jats:sub>. The mineral is easily soluble in dilute HCl. Paratacamite-(Mg) is trigonal, <jats:italic>R</jats:italic><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S0026461X00012299_inline03.png" />, with cell parameters <jats:italic>a</jats:italic> = 13.689(1), <jats:italic>c</jats:italic> = 14.025(1) Å, <jats:italic>V</jats:italic> = 2275.8(3) Å<jats:sup>3</jats:sup> and <jats:italic>Z</jats:italic> = 12. There is a pronounced sub-cell corresponding to <jats:italic>a'</jats:italic> ≈ <jats:italic>½a, c'</jats:italic> ≈ <jats:italic>c</jats:italic> in space group <jats:italic>R</jats:italic><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S0026461X00012299_inline03.png" />m. The eight strongest lines in the X-ray powder diffraction pattern are [<jats:italic>d</jats:italic><jats:sub>obs</jats:sub> Å<jats:italic>(I)(hkl)</jats:italic>]: 5.469(87)(021), 4.686(26)(003), 2.904(34)(401), 2.762(100)(22<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S0026461X00012299_inline03.png" />,042), 2.265(81)(404), 1.819(26)(603), 1.710 (34)(440) and 1.380(19)(446). The structure was refined to <jats:italic>R</jats:italic><jats:sub>1</jats:sub> = 0.039 for 480 <jats:italic>F</jats:italic><jats:sub>o</jats:sub> > 4σ<jats:italic>F</jats:italic> reflections. Refinement using interlayer Mg-Cu site scattering factors indicated that Mg is distributed statistically between both interlayer octahedra <jats:italic>M</jats:italic>1O<jats:sub>6</jats:sub> and <jats:italic>M</jats:italic>2O<jats:sub>6</jats:sub>. A comparison of the distortions associated with <jats:italic>M</jats:italic>1O<jats:sub>6</jats:sub> and <jats:italic>M</jats:italic>2O<jats:sub>6</jats:sub> octahedra suggest that the sample is near the upper compositional limit for stability of the <jats:italic>R</jats:italic><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S0026461X00012299_inline03.png" /> phase.</jats:p>