Leverettite from the Torrecillas mine, Iquique Provence, Chile : the Co-analogue of herbertsmithite

The new mineral leverettite (IMA 2013-011), ideally Cu3CoCl 2(OH)6, was found at the Torrecillas mine, Salar Grande, Iquique Province, Chile, where it occurs as a supergene alteration phase in association with akaganéite, anhydrite, chalcophanite, goethite, halite, manganite, pyrite, quartz and todorokite. Crystals of leverettite are steep rhombohedra to 1 mm with {101} prominent and modified by {001}, sometimes forming V-shaped twins by reflection on {102̄}. The crystals can also form finger-like, parallel stacked growths along the c axis. The new mineral is medium to deep green in colour and has a light green streak. Crystals are transparent with a vitreous lustre. Mohs hardness is ~3 and the crystals have a brittle tenacity, a perfect cleavage on {101} and a conchoidal fracture. The measured density is 3.64(2) g cm-3 and calculated density based on the empirical formula is 3.709 g cm-3. Optically, leverettite is uniaxial (-) with o and ε > 1.8 and exhibits pleochroism with O (bluish green) > E (slightly yellowish green). The empirical formula, determined from electron-microprobe analyses is Cu3(Co0.43Cu 0.40Mn0.17Ni0.07Mg0.01) Σ1.08Cl1.87O6.13H6. Leverettite is trigonal (hexagonal), space group R3̄m, unit-cell parameters a = 6.8436(6) and c = 14.064(1) Å, V = 570.42(8) Å3, Z = 3. The eight strongest X-ray powder diffraction lines are [dobs Å(I)(hkl)]: 5.469(90)(101), 4.701(18)(003), 2.905(22)(021), 2.766(100)(113), 2.269(66)(024), 1.822(26)(033), 1.711(33)(220), 1.383(23)(128). The structure, refined to R1 = 0.023 for 183 Fo > 4sF reflections, shows leverettite to be isostructural with herbertsmithite and gillardite.