Aluminocoquimbite, AlFe(SO4)3·9H2O, a new aluminum iron sulfate from Grotta dell’allume, Vulcano, Aeolian Islands, Italy

Aluminocoquimbite, AlFe(SO4)3•9H2O, is a new mineral species formed as a product of low-temperature (<100°C) fumarolic activity at Grotta dell’Allume (Alum Grotto), Vulcano, Aeolian Islands, Sicily, Italy. It occurs as colorless to pale pink tabular or short prismatic crystals up to 0.5 mm in size in association with alunogen, coquimbite, krausite, tamarugite, voltaite, pertlikite, yavapaiite, pickeringite and metavoltine. The mineral is trigonal, space group P31c (no. 163), with a 10.7065(7), c 17.3077(11) Å, V 1718.2(2) Å3 and Z = 4. The strongest six reflections in the X-ray powder-diffraction pattern [dobs in Å(I)(hkl)] are: 9.251(100)(010), 5.310(83)(110), 3.152(43)(224), 4.087(33)(022), 1.765(20)(553), and 2.112(17)(251). Chemical analyses yielded the empirical formula Al1.07Fe0.92S2.92H18.49O21. The simplified formula is AlFe(SO4)3•9H2O. The measured density is 2.03(1) g/cm3, and the calculated density is 2.044 g/cm3. The mineral is uniaxial (+), with v = 1.53 and  = 1.56 (589 nm). Using single-crystal diffraction data, the structure was refined to a final R(F) = 0.0218, and wR2 = 0.0685. The structure of aluminocoquimbite is different from that of coquimbite, and contains isolated Al(H2O)6 octahedral units and infinite [Fe(SO4)3]∞ columns running along [001], consisting of octahedrally coordinated iron atoms and sulfate ions, similar to those observed in ferrinatrite, Na3(H2O)3[Fe(SO4)3]. Interstitial H2O molecules are arranged in a cyclohexane-like chair conformation held together by hydrogen bonding, as in the structure of coquimbite.