Crystal-chemistry of sulfates from the Apuan Alps (Tuscany, Italy). IV. Giacovazzoite, K5Fe3+3O(SO4)6(H2O)9·H2O, the natural analogue of the β-Maus’s Salt and its dehydration product

Abstract

The new mineral giacovazzoite, K5Fe3+3O(SO4)6(H2O)9·H2O, has been discovered in the Monte Arsiccio mine, Apuan Alps, Tuscany, Italy. It occurs as orange–brown prismatic crystals, up to 0.1 mm long, with yellowish streak and vitreous luster. It is brittle, with a perfect {100} cleavage. Associated minerals are alum-(K), gypsum, krausite, magnanelliite, and scordariite. Electron microprobe analysis (WDS mode) gave (in wt%): SO3 45.82, Fe2O3 22.67, K2O 22.72, H2Ocalc 17.18, total 108.39. The high total, after the addition of calculated H2O (based on structural results), is due to the dehydration of the sample during analysis. The corresponding empirical chemical formula, on the basis of 25 anions and 10 H2O groups, is K5.06Fe3+2.98O(SO4)6.00·10H2O. Giacovazzoite is monoclinic, space group P21/c, with unit-cell parameters a = 9.4797(2), b = 18.4454(5), c = 18.0540(4) A, β = 92.626(2)°, V = 3153.55(13) A3, Z = 4. Its crystal structure was refined to R1 = 0.0587 for 10,254 reflections with Fo > 4σ(Fo) and 511 refined parameters. It is formed by a structural unit, represented by a heteropolyhedral [Fe3+3O(SO4)6(H2O)3]5− cluster, and an interstitial complex, having composition {K5(H2O)7}5+. The [Fe3+3O(SO4)6(H2O)3]5− clusters form layers parallel to {100} which are bonded in this plane by K atoms. Successive layers are bonded, along c, through H-bonds, and by K atoms. The heteropolyhedral cluster occurring in giacovazzoite is known in the crystal structures of metavoltine, scordariite, carlsonite, and a series of synthetic compounds. Among them, the β-Maus’ salt has isotypic relationships with giacovazzoite. This latter mineral easily dehydrates and its dehydrated product has been investigated.