Melcherite, trigonal Ba2Na2Mg[Nb6O19]·6H2O, the second natural hexaniobate, from Cajati, São Paulo, Brazil: Description and crystal structure

ABSTRACT Melcherite (IMA2015-018), ideally Ba2Na2Mg[Nb6O19]·6H2O, occurs as a vug mineral in the carbonatite of the Jacupiranga mine, Cajati county, São Paulo state, Brazil, associated with dolomite, calcite, magnetite, pyrrhotite, tochilinite, ‘pyrochlore’ and fluorapatite. This is also the type locality for zirkelite, quintinite, menezesite and pauloabibite. The mineral forms irregular, tabular crystals up to 200 µm in maximum dimension. Melcherite is transparent and displays a vitreous lustre; it is beige with a white streak. It is non-fluorescent. The mineral displays perfect cleavage on {001}. Chemical composition varies from Ba2Na2Mg[Nb6O19].6H2O to (BaK)(NaCa)Mg[Nb6O19].6H2O. Empirical formulae for the first and the second compositions are: (Ba1.75K0.19)Σ1.94(Na1.80Ca0.19)Σ1.99(Mg0.96Mn0.02Al0.02)Σ1.00Nb6.02O19.00·6H2O and (Ba0.99K1.00)Σ1.99(Na1.02Ca0.96)Σ1.98(Mg0.95Mn0.05)Σ1.00Nb6.02O19.00·6H2O, respectively. Data for a single crystal with the second composition are: trigonal, R$\bar 3$, a = 9.0117(6) Å, c = 23.3986(16) Å, V = 1645.64(19) Å3 and Z = 3. Calculated density for this formula is 3.733 g/cm3, and the calculated mean refractive index is 1.924. Melcherite is a hexaniobate that has structural layers parallel to the xy plane that stack along the c axis with simultaneous 1/3 [110] displacement so as to produce an R lattice. The melcherite structure is built by layers of [(Ba,K)(O,H2O)9] polyhedra and the [Nb6O19]8− super-octahedron (Lindqvist anion) interconnected by [(Na,Ca)O6] polyhedra. Cations of Mg2+ are bonded to six water molecules each and are not associated with Lindqvist oxygen ions. The mineral is named in honour of Geraldo Conrado Melcher (1924–2011), a pioneer in Jacupiranga carbonatite studies.


Introduction
MELCHERITE is the second natural hexaniobate. The first described was peterandresenite  and hansesmarkite was recently discovered . Polyoxometalates of niobium are dominated by the Linqdvist hexaniobate ion, (Nb 6 O 19 ) 8-, and its synthesis and stability requires alkaline conditions. The crystal structure of these compounds was first described by Lindqvist (1953). Hexaniobates are negatively charged clusters of six mutually edge-sharing NbO 6 octahedra forming a super-octahedron (Nyman, 2011).
Possible polyoxoniobate applications include their use as reagents in the break-down of nerve agents and in the development of filter media protection against chemical warfare agents (Kinnan et al., 2014). Polyoxometalates have also been investigated in coordination chemistry, leading to the development of hybrid organometallic hexametalate complexes (Abramov et al., 2016), and the synthesis of new polyoxoniobates coordinated to copper complexes (Wang et al., 2008).
The mineral is named in honour of Geraldo Conrado Melcher (1924Melcher ( -2011. He was professor at the Department of Mining Engineering at the Polytechnic School, University of São Paulo and was also a pioneer in Jacupiranga carbonatite studies (Melcher, 1966).
Both the description and name were approved by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA2015-018). Type material is deposited in the Museu de Geociências, Instituto de Geociências, Universidade de São Paulo, Rua do Lago, 562, 05508-080 -São Paulo, SP, Brazil. Specimen number: DR982. Part of the cotype sample has been deposited at the University of Arizona Mineral Museum, RRUFF Project (deposition no. R130752).

Occurrence
The mineral occurs in the carbonatite of the Jacupiranga mine (24°43′47″S, 48°06′37″W), Cajati County, São Paulo, Brazil (Menezes Filho and Martins, 1984). For general information about this carbonatite see Menezes . This is also the type locality for zirkelite (Hussak and Prior, 1895), quintinite (Chao and Gault, 1997), menezesite (Atencio et al., 2008) and pauloabibite . Although the joint occurrence of menezesite, pauloabibite and melcherite has not been observed, these minerals may be related genetically. Pauloabibite is trigonal NaNbO 3 , isostructural with ilmenite . The synthetic analogue of pauloabibite was reported by Kinomura et al. (1984) and Kumata et al. (1990) from a two-step synthesis method, involving the preparation of Na 8 Nb 6 O 19 ·13H 2 O (a hexaniobate) followed by hydrothermal reaction with NaOH in a silver-lined vessel at 250°C. Menezesite is a heteropolyoxoniobate, cubic (□,Ba,K) 12 (□, Mg) 3 Zr 4 (BaNb 12 O 42 )·12H 2 O (Atencio et al., 2008). According to Nyman et al. (2002), the heteropolyanions of W, Mo and V are formed simply by acidification of solutions of their oxoanions. Under similar conditions, these oxoanion precursors are not available for Nb, and Nboxo chemistry is dominated by formation of the Lindquist ion [Nb 6 O 19 ] 8-(present in melcherite). However, heteropolyniobate ( present in menezesite) formation is favoured in hydrothermal reactions of aqueous, alkaline precursor mixtures. A competing phase to the formation of polyoxoniobates in hydrothermal aqueous reactions involving Nb and an alkali hydroxide is NaNbO 3 , avoided by using short reaction times (i.e. 24 hours or less) (Nyman et al., 2002). So melcherite could have originally formed under acid conditions, and afterwards, under basic conditions, menezesite and pauloabibite could have formed.

Habit and physical properties
Melcherite forms irregular, tabular crystals up to 200 µm in maximum dimension (Fig. 1) Refractive indices were not measured due to paucity of material. The mean refractive index is estimated as 1.924 using the Gladstone-Dale relationship (Mandarino, 1981).

Mineral chemistry
Melcherite crystals were embedded in epoxy resin and polished. In the back-scattered electron images, we can see that the crystals are zoned (Fig. 2). The chemical analyses (Table 1) were done by means of a Cameca SX100 electron microprobe (wavelength dispersive spectroscopy mode, 15 kV, 10 nA and 20 µm beam diameter). H 2 O was inferred from the crystal structure determination. H 2 O was initially assumedbydifferenceprior to the matrix correction (PAP) and then calculated by stoichiometry post matrix correction due to software limitations. Analyses from the brighter areas of the melcherite crystal, (Fig. 2 Hatert and Burke (2008), where a heterovalent substitution occurs at a given crystallographic site, the charge balance can also be maintained by coupling this substitution to another heterovalent substitution at a different site. At the Ba site, the atom Ba 2+ is replaced progressively by K + , and to maintain charge balance, the atom Na + is replaced progressively by Ca 2+ at the Na site. The substitution mechanism is Ba 2+ + K + ↔ Na + + Ca 2+ . The boundary site occupancies between the two members of the series is ( (Hålenius et al., 2016): "Soon after the approval of the new mineral melcherite (IMA No. 2015-018; see CNMNC Newsletter 25), the authors of the proposal have communicated results of subsequent analytical work on this mineral, which verifies essential contents of sodium. The new data were examined carefully by the CNMNC officers and were found reliable. The revised simplified formula, Ba 2 Na 2 Mg [Nb 6 O 19 ]·6H 2 O, has been approved executively." A fragment of the darker part was extracted from the polished section for crystal structure determination.

Crystal structure determination
Powder X-ray diffraction data (XRD) were obtained using a Siemens D5000 diffractometer equipped with a Göbel mirror and a position-sensitive detector using CuKα radiation and 40 kV and 40 mA at the Instituto de Geociências of the Universidade de São Paulo (Table 2). Unit-cell parameters refined from the powder data are as follows: trigonal, space group: R 3, a = 9.022(2) Å, c = 23.410(6) Å, V = 1650.2(8) Å 3 and Z = 3.    A single-crystal X-ray study was carried-out using a Bruker APEX II CCD diffractometer with graphite-monochromated MoKα (λ = 0.71073 Å) radiation and gave the following data: trigonal, space group: R 3, a = 9.0117(6) Å, c = 23.3986(16) Å, V = 1645.64(19) Å 3 and Z = 3. The X-ray absorption correction was applied to intensity data using the program SADABS from Bruker.  (6) 0.0130 (7) 0.0182 (7) −0.0022 (5) 0.0030 (5) 0.0066 (6) O3 0.66867(17) 0.74208 (18) (7) 0.0107 (7) 0.0327(9) −0.0050 (6)    The SHELXL-97 package (Sheldrick, 2008) was used for the direct methods structure solution and its subsequent refinement. The Ba and Na sites were refined assuming full but joint occupation by Ba/K and Na/Ca respectively, which yielded occupancy values close to those indicated by the empirical formula based on the electron microprobe analysis. A final difference-Fourier synthesis where a small fraction of the oxygen atoms in the hexaniobate polyanion is assumed to be replaced by OH groups in order to balance the slight positive charge deficiency associated with the Ba/K and Na/ Ca sites. Details of the data collection and structure refinement are given in Tables 3 and 4. Selected bond distances and associated bond-valence sum calculations, using the parameters of Brese and O'Keefe (1991), are given in Table 5.
Melcherite is a hexaniobate that has structural layers parallel to the xy plane that stack along the c axis with simultaneous 1/3 [1 1 0] displacement so as to produce an R lattice. The melcherite structure (Figs 4 and 5 (Robinson et al., 1971). The results are comparable to the NbO 6 octahedra present in the crystal structure of peterandresenite and hansesmarkite (Table 6). Ba/K is coordinated by six oxygens and three water molecules. Na/Ca is coordinated by six oxygen atoms in a distorted octahedron and the OAV and OQE values are 354.100°2 and 1.113, respectively. Mg 2+ cations are bonded to six water molecules each and are not associated with Lindqvist oxygen ions. The comparison with MnO 6 in peterandresenite and hansesmarkite shows that the octahedral coordination of the Mg cation is relatively undistorted, as indicated by the values of OAV = 12.285°2 and OQE = 1.003 (Table 6).
The mineral is similar structurally to the synthetic compounds Cs 6 Na 2 (Nb 6 O 19 )·18H 2 O   Table 8.