Crystal-Chemistry of Sulfates from the Apuan Alps (Tuscany, Italy). VII. Magnanelliite, K3Fe3+2(SO4)4 (OH)(H2O)2, a New Sulfate from the Monte Arsiccio Mine

The new mineral species magnanelliite, K3Fe(2)(3+)(SO4)(4)(OH)(H2O)(2), was discovered in the Monte Arsiccio mine, Apuan Alps, Tuscany, Italy. It occurs as steeply terminated prisms, up to 0.5 mm in length, yellow to orange-yellow in color, with a vitreous luster. Streak is pale yellow, Mohs hardness is ca. 3, and cleavage is good on {010}, fair on {100}. The measured density is 2.82(3) g/cm3. Magnanelliite is optically biaxial (+), with ff = 1.628(2), fi = 1.637(2), = 1.665(2) (white light), 2Vmeas = 60(2)ffi, and 2Vcalc = 59.9ffi. It exhibits a strong dispersion, r > v. The optical orientation is Y = b, X-boolean AND c 25ffi in the obtuse angle fi. It is pleochroic, with X = orange yellow, Y and Z = yellow. Magnanelliite is associated with alum-(K), giacovazzoite, gypsum, jarosite, krausite, melanterite, and scordariite. Electron microprobe analyses give (wt.%): SO3 47.82, TiO2 0.05, Al2O3 0.40, Fe2O3 25.21, MgO 0.07, Na2O 0.20, K2O 21.35, H(2)Ocalc 6.85, total 101.95. On the basis of 19 anions per formula unit, assuming the occurrence of one (OH) and two H2O groups, the empirical chemical formula of magnanelliite is (K2.98Na0.04) (Sigma 3.02)(Fe3 + 2.08Al0.05Mg0.01) Sigma 2.14S3.93O16(OH)(H2O)(2). The ideal end-member formula can be written as K(3)Fe3+ (2)(SO4)(4)(OH)(H2O)(2). Magnanelliite is monoclinic, space group C2/c, with a = 7.5491(3), b = 16.8652(6), c = 12.1574(4) angstrom, beta = 94.064(1)degrees, V = 1543.95(10) angstrom(3), Z = 4. Strongest diffraction lines of the observed X-ray powder pattern are [d(in angstrom), estimated visual intensity, hkl]: 6.9, medium, 021 and 110; 4.91, medium-weak, 022; 3.612, medium-weak, 132, 023, and 113; 3.085, strong, 202, 150, and 133; 3.006, medium, 004, 151, and 151; 2.704, medium, 152 and 223; 2.597, medium-weak, 242; 2.410, medium-weak, 153. The crystal structure of magnanelliite has been refined using X-ray single-crystal data to a final R1 = 0.025, on the basis of 2411 reflections with Fo > 4(Fo) and 144 refined parameters. The crystal structure is isotypic with that of alcaparrosaite, K3Ti4 + Fe3 +(SO)(4)O(H2O)(2).