Crystallochemical characterization of polyoxometalate new minerals

New mineral characterization evolved a lot over time and nowadays requires a combination of good data from advanced analytical methods and techniques such as crystal structure refinement, Raman and IR spectroscopy, electron microprobe and synchrotron X-ray diffraction.Polyoxometalate new minerals are dominated by the Lindqvist hexaniobate ion, (Nb₆O₁₉)⁸ˉ, a super-octahedron formed by mutual six edge-sharing of NbO₆ octahedra.Polyoxometalate structural features and chemical composition can led to the development of new materials that can be applied to antiviral, antitumoral and antibacterial studies.There are three hexaniobate minerals approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA): melcherite [1], peterandresenite [2] and hansesmarkite [3].The crystallographic parameters and chemical composition are presented below: I) melcherite, ideally Ba₂Na₂Mg[Nb₆O₁₉].6H₂O,(IMA 2015-018).The mineral is trigonal, R-3, with a = 9.0117(6), c = 23.3986(16)Å, β = 118.650(4)˚,V = 1645.64(19)ų and Z = 3. Empirical formula based on electron microprobe analyses is: (Ba₁.₇₅K₀.₁₉)₁.₉₄(Na₁.₈₀Ca₀.₁₉)₁.₉₉(Mg₀.₉₆Mn₀.₀₂Al₀.₀₂)₁.₀₀Nb₆.₀₂O₁₉•6H₂OII) peterandresenite, (IMA 2012-084), ideally Mn₄Nb₆O₁₉.14H₂O,monoclinic, C2/m, with a = 15.329(1),b = 9.4121(5), c = 11.2832(9)Å, β = 118.650(4)˚,V = 1428.6(2)ų and Z = 2. Empirical formula based on electron microprobe analyses [2]: (Mn₃.₉₂Ca₀.₀₅Na₀.₀₃)₄.₀₀(Nb₅.₇₁Mn₀.₁₃Fe₀.₁2Si₀.₀₃)₅.₉₉O₁₈.₅₇•14H₂O.III) hansesmarkite, (IMA 2015-067), ideally Ca₂Mn₂Nb₆O₁₉•20H₂O, triclinic, P-1, with a = 9.081(4), b = 9.982(8), c = 10.60(1)Å, α = 111.07(8),β = 101.15(6),γ = 99.39(5)˚,V = 850.8(13)ų and Z = 1.Empirical formula based on electron microprobe analyses [3]: (Ca₁.₉₃Na₀.₀₂K₀.₀₁)₁.₉₆(Mn₁.₇₉Fe₀.₁₁)₁.₉₀Nb₆O₁₈.₈₄•20H₂O.We will discuss the mineral chemical composition, the amount of cation substitution in crystallographic sites, and its influence on polyoxometalate mineral crystallization in three different crystallographic space groups.Understanding of these complex mechanisms is crucial to the prediction of new mineral species and development of new technological applications.