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(searched for: doi:10.1127/ejm/2019/0031-2892)
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Published: 5 December 2020
by MDPI
Abstract:
The occurrence of sulfate minerals associated with the pyrite ores of the southern Apuan Alps has been known since the 19th century but modern mineralogical studies started only in the last decade. Sulfate assemblages were identified in all the pyrite ore deposits from the studied area but the more impressive associations were discovered in the Fornovolasco and Monte Arsiccio mines. Their study allowed to improve the knowledge of the sulfate crystal-chemistry and to achieve a better understanding of the acid mine drainage (AMD) systems associated with pyrite oxidation. More than 20 different mineral species were identified and, among them, four sulfates (volaschioite, giacovazzoite, magnanelliite, and scordariite) have their type localities in the pyrite ore deposits of the Apuan Alps. A review of the mineralogical results of a ten-year-long study is given here.
Published: 12 December 2019
by MDPI
Abstract:
The new mineral species magnanelliite, K3Fe3+2(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 α = 1.628(2), β = 1.637(2), γ = 1.665(2) (white light), 2Vmeas = 60(2)°, and 2Vcalc = 59.9°. It exhibits a strong dispersion, r > v. The optical orientation is Y = b, X ^ c ~ 25° in the obtuse angle β. 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, H2Ocalc 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)Σ3.02(Fe3+2.08Al0.05Mg0.01)Σ2.14S3.93O16(OH)(H2O)2. The ideal end-member formula can be written as K3Fe3+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) Å, β = 94.064(1)°, V = 1543.95(10) Å3, Z = 4. Strongest diffraction lines of the observed X-ray powder pattern are [d(in Å), estimated visual intensity, hkl]: 6.9, medium, 021 and 110; 4.91, medium-weak, 022; 3.612, medium-weak, 1¯32, 023, and 1¯13; 3.085, strong, 202, 150, and 1¯33; 3.006, medium, 004, 1¯51, and 151; 2.704, medium, 152 and 2¯23; 2.597, medium-weak, 2¯42; 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 > 4σ(Fo) and 144 refined parameters. The crystal structure is isotypic with that of alcaparrosaite, K3Ti4+Fe3+(SO)4O(H2O)2.
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