Magnesian aubertite, (Cu,Mg)Al(SO_4)_2Cl centre dot 14H_2O, from mine gold mine of Furtei in Sardinia, Italy

摘要

Magnesian aubertite, an intermediate term between aubertite and magnesioaubertite with theoretical formula (Cu,Mg)Al(SO_4)_2C1 centre dot 14H_2O, occurs inside a grotto excavated in hydrothermally altered andesitic rocks at the gold mine of Furtei (Sardinia, Italy). The mineral is strictly associated with tamarugite and alunogen; the complete assemblage also includes natrojarosite, halotrichite-pickeringite, gyp-sum, halite, paratacamite-atacamite, growing on kaolinite-dickite, montruorillon-ite and quartz. A least-squares unit-cell refinement of 52 reflections from X-ray powder diffraction analysis gave the following results for a triclinic cell: a valence 6.302(4) A, b valence 13.195(9) A, c valence 6.288(5) A, #alpha# valence 91.77(6) deg, #beta# valence 94.53(5) deg, #gamma# valence82.63(6) deg, V valence 516.8(4) A~3. Quantitative wet-chemical analysis gave the following composition on the basis of Cu+Mg valence 1: (Cu_(0.56)Mg_(0.44))A1_(1.12)(SO_4)_(2.34)C1_(1.09) centre dot 14H_2O, with an atomic Cu/Mg ratio of 1.28. The formation of magnesian aubertite is linked to the generation of acidic sulfate waters following oxidation of pyrite disseminated in the country rocks. Interaction of these waters with the hydrothermal alteration min-erals releases Mg and Al into solution, while Cu derives from oxidation of Cu-bear-ing sulfides (mainly enargite).