Magnesian aubertite, an intermediate term between aubertite and magnesioaubertite with theoretical formula (Cu,Mg)Al(SO4)2Cl · 14H2O, 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, gypsum, halite, paratacamite-atacamite, growing on kaolinite-dickite, montmorillonite 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 = 6.302(4) Å, b = 13.195(9) Å, c = 6.288(5) Å, α = 91.77(6)°, β = 94.53(5)°, γ = 82.63(6)°, V = 516.8(4) Å3. Quantitative wet-chemical analysis gave the following composition on the basis of Cu + Mg= 1: (Cu0.56 Mg0.44)Al1.12 (SO4)2.34Cl1.09 · 14H2O, 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 minerals releases Mg and Al into solution, while Cu derives from oxidation of Cu-bearing sulfides (mainly enargite).
Magnesian aubertite, (Cu,Mg)Al(SO4)2Cl . 14H2O, from the gold mine of Furtei in Sardinia, Italy
DA PELO, STEFANIA;FRAU, FRANCO
2000-01-01
Abstract
Magnesian aubertite, an intermediate term between aubertite and magnesioaubertite with theoretical formula (Cu,Mg)Al(SO4)2Cl · 14H2O, 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, gypsum, halite, paratacamite-atacamite, growing on kaolinite-dickite, montmorillonite 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 = 6.302(4) Å, b = 13.195(9) Å, c = 6.288(5) Å, α = 91.77(6)°, β = 94.53(5)°, γ = 82.63(6)°, V = 516.8(4) Å3. Quantitative wet-chemical analysis gave the following composition on the basis of Cu + Mg= 1: (Cu0.56 Mg0.44)Al1.12 (SO4)2.34Cl1.09 · 14H2O, 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 minerals releases Mg and Al into solution, while Cu derives from oxidation of Cu-bearing sulfides (mainly enargite).
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