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

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), 2V_meas = 60(2)°, and 2V_calc = 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, H₂O_calc 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 (K_2.98Na_0.04)_Σ3.02(Fe³⁺_2.08Al_0.05Mg_0.01)_Σ2.14S_3.93O₁₆(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) ų, 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, $\overline{1}$ 32, 023, and $\overline{1}$ 13; 3.085, strong, 202, 150, and $\overline{1}$ 33; 3.006, medium, 004, $\overline{1}$ 51, and 151; 2.704, medium, 152 and $\overline{2}$ 23; 2.597, medium-weak, $\overline{2}$ 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 F_o > 4σ(F_o) and 144 refined parameters. The crystal structure is isotypic with that of alcaparrosaite, K₃Ti⁴⁺Fe³⁺(SO)₄O(H₂O)₂.