Paper Number: 1438
Polyoxoniobates from Cajati, São Paulo, Brazil
Atencio, D.1 and Andrade, M.B.2
1Instituto de Geociências, Universidade de São Paulo, Brazil, datencio@usp.br
2São Carlos Institute of Physics, University of São Paulo, Brazil
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Menezesite and melcherite are two polyoxoniobates described from the contact zone between dolomite carbonatite and “jacupirangite” (=a pyroxenite) at the Jacupiranga mine, in Cajati Co., São Paulo state, Brazil.
Menezesite is the first natural heteropolyniobate [1]. The second natural heteropolyniobate, aspedamite, isostructural with menezesite, was described in the Herrebøkasa Quarry, Aspedammen, Østfold, Southern Norway [2]. The simplified formula is (□,Ba,K)12(□,Mg)3Zr4(BaNb12O42).12H2O. The end-member formula was originally expressed as Ba2MgZr4(BaNb12O42).12H2O, but it should be more correctly expressed as □12Mg3Zr4(BaNb12O42).12H2O [2]. It is cubic, Im-3, a = 13.017(1) Å, V = 2206(1) Å3, Z = 2. The structure is based on the heteropolyanion [BaNb12O42]22−, which consists of twelve face- and corner-sharing NbO6 octahedra that surround the 12-coordinated Ba cation. This type of heteropolyanion was originally described by Dexter and Silverton [3].
Melcherite is the second natural hexaniobate [4]. The first one is peterandresenite [5] and the third is hansesmarkite [6], both from in Tvedalen, Larvik, Vestfold, Norway. Its simplified formula is (Ba,K)2(Na,Ca)2Mg[Nb6O19].6H2O. Chemical composition varies from Ba2Na2Mg[Nb6O19].6H2O to (BaK)(NaCa)Mg[Nb6O19].6H2O. It is trigonal, R-3, a = 9.0117(6) Å, c = 23.3986(16) Å, V = 1645.64(19) Å3, Z = 3. The structure is based on the [Nb6O19]8− anion, which consists of clusters of mutually 6 edge-sharing NbO6 octahedra forming a super-octahedron. This type of polyanion was originally described by Lindqvist [7].
References:
[1] Atencio D et al. (2008) Am Mineral 93:81-87
[2] Cooper MA et al. (2012) Can Mineral 50:793-804
[3] Dexter DD and Silverton JV (1968) J Am Chem Soc 90:3589-3590
[4] Andrade MB et al. (2015) Min Mag 79:547
[5] Friis H et al. (2014) Eur J Min 26:567-576
[6] Friis H et al. (2015) IMA Nomenclature Proposal 2015-067
[7] Lindqvist I (1953) Arkiv Kemi 5(3):247-250
Figure 1: Melcherite from Cajati