Paratacamite-(Mg) (IMA 2013-014), Cu3(Mg, Cu)Cl2(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 {10} 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–3, respectively. The mineral is optically uniaxial (–) with ε = 1.785(5) and ω > 1.8 and slight pleochroism: O (bluish green) > E (green). Electron-microprobe analyses provided the empirical formula Cu3(Mg0.60Cu0.38Ni0.01Mn0.01)Cl2(OH)6. The mineral is easily soluble in dilute HCl. Paratacamite-(Mg) is trigonal, R, with cell parameters a = 13.689(1), c = 14.025(1) Å, V = 2275.8(3) Å3 and Z = 12. There is a pronounced sub-cell corresponding to a' ≈ ½a, c' ≈ c in space group Rm. The eight strongest lines in the X-ray powder diffraction pattern are [dobs Å(I)(hkl)]: 5.469(87)(021), 4.686(26)(003), 2.904(34)(401), 2.762(100)(22,042), 2.265(81)(404), 1.819(26)(603), 1.710 (34)(440) and 1.380(19)(446). The structure was refined to R1 = 0.039 for 480 Fo > 4σF reflections. Refinement using interlayer Mg-Cu site scattering factors indicated that Mg is distributed statistically between both interlayer octahedra M1O6 and M2O6. A comparison of the distortions associated with M1O6 and M2O6 octahedra suggest that the sample is near the upper compositional limit for stability of the R phase.

Tondiite, with the simplified formula Cu3Mg(OH)6Cl2, occurs as a rare supergene mineral in a phonolitic tephrite from the type locality, Vesuvius volcano, Italy, as well as associated with haydeeite in the Santo Domingo Mine, Arica Province, Chile. It is emerald green to bright green in colour and occurs in irregularly shaped crystals, often with stepped faces. Its calculated density is 3.503 g cm−3. Tondiite crystallizes with the herbertsmithite structure type, space group Rm. Lattice parameters are a = 6.8377(7) Å and c = 14.088(2)Å for the holotype material. The c parameter may vary with Mg/Cu ratio and the presence of impurity atoms. The five strongest lines in the calculated powder diffraction pattern are [d in Å(I)(hkil)]: 5.459(88)(101), 3.419(22)(110), 2.764(100)(112 3), 2.266(54)(024), 1.706(26)(220). Several tondiite crystals have been examined by single-crystal X-ray diffraction and by electron microprobe analysis. The observed Mg content ranges between 0.6 and 0.7 atoms per formula unit. The structural role of Mg is discussed.