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The mineralogy of secondary assemblages deriving from the alteration of pyrite ore deposits from the southern sector of the Apuan Alps (Tuscany, Italy) has been investigated. Twenty-five mineral species have been identified so far; among them, eight, showing interesting crystal-chemical features, were fully characterized using X-ray diffraction, chemical analyses, and spectroscopic techniques [1]. The structural description of some previously known minerals (epsomite, wilcoxite, coquimbite, khademite, alum-(K), and voltaite) allowed to improve our knowledge about H-bond systems in sulfates and to assess their role in hosting potentially toxic elements. Moreover, this study permitted the identification of two new mineral species, giacovazzoite, $K_5Fe^{3+}_3O(SO_4)_6(H_2O)_9·H_2O$, from the Monte Arsiccio mine, and bohuslavite, $Fe^{3+}_4(PO_4)_3(SO_4)(OH)(H_2O)_{10}·nH_2O$ (5 < n < 24), from the Buca della Vena mine [2, 3].
Giacovazzoite (Figure 1) is the natural counterpart of the β-Maus’ Salt. It is monoclinic, space group P21/c, with a = 9.48, b = 18.44, c = 18.05 Å, β = 92.6°, V = 3153.6 Å$^3$. Its crystal structure is characterized by a heteropolyhedral $[Fe^{3+}_3O(SO_4)_6(H_2O)_3]^{5-}$ cluster and by an interstitial complex with composition $[K_5(H_2O)_7]^{5+}$. The heteropolyhedral cluster is known in other minerals (metavoltine, scordariite, carlsonite) as well as in some synthetic compounds.
Bohuslavite (Figure 1) is triclinic pseudo-hexagonal, space group P-1, with unit-cell parameters a = 13.38, b = 13.34, c = 10.86 Å, α = 92.8, β = 91.0, γ = 119.9°, V = 1675.7 Å$^3$. This mineral can be considered a new kind of microporous compound, with heteropolyhedral layers with composition $[Fe^{3+}_4(PO_4)_3O(OH)(H_2O)_{10}]$, decorated on both sides by $SO_4$ groups. Additional $H_2O$ groups are hosted in channels running along c as well as in the interlayers.
This work stressed the complexity of sulfates, improving the knowledge of their systematics.
[1] C. Biagioni, D. Mauro, M. Pasero Minerals. 2020, 10, 1092.
[2] C. Biagioni, L. Bindi, D. Mauro, M. Pasero Phys Chem Miner. 2020, 47, 7.
[3] D. Mauro, C. Biagioni, E. Bonaccorsi, U. Hålenius, M. Pasero, H. Skogby, F. Zaccarini, J. Sejkora, J. Plášil, A.R. Kampf, J. Filip, P. Novotný, R. Škoda, T. Witzke Eur J Mineral, 2019, 31, 1033.
