Introduction
The nomenclature of the minerals belonging to the pyrochlore supergroup was revised by Atencio et al. (Reference Atencio, Andrade, Christy, Gieré and Kartashov2010) and is approved by International Mineralogical Association Commission on New Minerals, Nomenclature and Classification (IMA–CNMNC). The new nomenclature uses the occupation of the A, B and Y sites and the general formula is written as A2-mB2X6-wY1-n, where m = 0 to 1.7, w = 0 to 0.7 and n = 0 to 1 (Lumpkin and Ewing, Reference Lumpkin and Ewing1992, Reference Lumpkin and Ewing1995; Ercit and Robinson, Reference Ercit and Robinson1994; Brugger et al., Reference Brugger, Gieré, Graeser and Meisser1997). The names are composed by two prefixes and one root name. The first prefix stands for the predominant ion of the dominant valence (or H2O or vacancy ‘□’) at the Y site while the second prefix represents the predominant cation of the dominant valence (or H2O or □) at the A site. The root names were defined according to the predominant cation of the dominant valence at the B site as: pyrochlore (if Nb5+ is the predominant cation of the dominant valence), microlite (Ta5+), roméite (Sb5+), betafite (Ti4+) or elsmoreite (W6+) (Atencio et al., Reference Atencio, Andrade, Christy, Gieré and Kartashov2010). Hydrokenoralstonite and fluornatrocoulsellite were designated recently as unassigned members of the pyrochlore supergroup (Atencio et al., Reference Atencio, Andrade, Bastos Neto and Pereira2017). In oxycalciomicrolite, the Y site is dominated by O2–, the A site by Ca2+ and the B site by Ta5+, presenting the ideal formula Ca2Ta2O7. In natural crystals it is common to observe other elements occupying these sites, such that other authors have described similar samples but misclassified them as stibiomicrolite (Černý et al., Reference Černý, Chapman, Ferreira and Smeds2004) or microlite (Guastoni et al., Reference Guastoni, Diela and Pezzotta2008).
Oxycalciomicrolite, (Ca,□,Na)2Ta2O7, is a new mineral of the microlite group of the pyrochlore supergroup (IMA2019-110, Menezes da Silva, et al., Reference Menezes da Silva, Neumann, Ávila, Faulstich, Alves and de Almeida2020). It occurs in the Fumal pegmatite, which is located 18 km north of the city of Nazareno, Minas Gerais state, Brazil. The type material is deposited at Museu Nacional, Universidade Federal do Rio de Janeiro, Quinta da Boa Vista, s/n°, 20940-040, Rio de Janeiro, Brazil, under the registration code MN 7601-M. It is the first mineral described and deposited in the Museu Nacional after the September 2, 2018 fire.
Occurrence
Oxycalciomicrolite occurs as an accessory phase in the saprolite of the Fumal pegmatite (21°04’08.23”S, 44°33’59.63”W), Nazareno, Minas Gerais, Brazil (Fig. 1). The pegmatite body is narrow (apparent thickness of 4 metres), deeply weathered and contains quartz, kaolinised albite and microcline, muscovite, columbite-subgroup minerals, cassiterite, hematite, ilmenite, monazite-(Ce), epidote-group minerals, xenotime-(Y), zircon, beryl, spinel and garnet-group minerals. The Fumal pegmatite belongs to the Sn–Ta–Nb–Li-rich São João del Rei Pegmatite Province (Pereira et al., Reference Pereira, Ávila, Neumann, Netto and Atencio2003); the evolution of this province is associated with the Paleoproterozoic granitoids of the Mineiro Belt (Ávila et al., Reference Ávila, Teixeira, Cordani, Moura and Pereira2010, Reference Ávila, Teixeira, Bongiolo, Dussin and Vieira2014; Teixeira et al., Reference Teixeira, Ávila, Dussin, Corrêa Neto, Bongiolo, Santos and Barbosa2015). Another important pegmatite of this province is the Volta Grande body (Lagache and Quéméneur, Reference Lagache and Quéméneur1997; Alves et al., Reference Alves, Neumann, Ávila and Faulstich2019) which is currently mined for Li, Ta and Sn as well as feldspar for the ceramic industry. The oxycalciomicrolite characterised here was obtained through the concentration of heavy minerals from the pegmatite saprolite. The original material was concentrated using a pan to eliminate excess quartz, feldspar and mica-group minerals (commonly ‘biotite’ and/or muscovite). The heavy-mineral concentrate was then separated using a dense liquid (methylene iodide S.D. = 3.32 kg/L) and ferrite hand magnet. A Frantz isodynamic separator with magnetic coil current of 0.1 A was then used to remove any residual ferromagnetic minerals and then ranging from 0.3 to 1.8 A, to separate the different para- and diamagnetic minerals. Pyrochlore-supergroup minerals can be found in the concentrates separated from the 0.8 to 1.8 A range. The crystals chosen for characterisation are predominantly homogeneous and without evidence of weathering. Rarely some crystals were found to be compositionally altered to other species of microlite-group minerals, usually at the edges and close to fractures; these were not used for this study. Other crystals with different lustre and colours were also identified as microlite-group minerals in the same concentrate.
Fig. 1. Location map of the Fumal pegmatite: (a) map of Brazil, (b) detail showing location (hatched area) of (c) the position of the Fumal pegmatite in southern Minas Gerais state (modified from Faulstich, Reference Faulstich2016).
Appearance and physical properties
Oxycalciomicrolite occurs as octahedra, modified occasionally to rhombododecaedra with crystals generally ranging in size from 0.2 to 0.5 mm (Fig. 2). It is brownish-yellow to brownish-red in colour with a white streak. The mineral is translucent to transparent with a vitreous to resinous lustre. It is non-fluorescent under longwave and shortwave ultraviolet light. It has a conchoidal fracture and no parting or cleavage was observed. The tenacity is brittle and the Mohs’ hardness corresponds to 5–5½. The calculated density, based on the empirical formula and unit-cell parameters, is 6.333 g/cm3. The mineral is isotropic and the calculated refraction index, based on the empirical formula, is n calc. = 2.037 using the Gladstone–Dale relationship N = K d + 1 (K from Mandarino, Reference Mandarino1981, d: density).
Fig. 2. Oxycalciomicrolite crystals, 0.2 mm in size from the same heavy minerals concentrate as the type specimen. (a) Stereomicroscope (b) back-scattered scanning electron and secondary electron composite image.
Composition
The composition of oxycalciomicrolite was determined using a JEOL JXA-8230 electron probe microanalyser (wavelength dispersive mode, 15 kV, 20 nA and 3 μm beam diameter). Analytical data from 14 spots are given in Table 1. The empirical formula calculated based on 2 cations at the B site is (Ca1.57□0.26Na0.06Sn0.03Sr0.03U0.02Mn0.02Fe0.01Ce0.01)∑2.00(Ta1.79Nb0.18Ti0.03)∑2.00O6.00[O0.64F0.19□0.17]∑1.00 and the simplified formula is Ca2Ta2O7. The mineral was named oxycalciomicrolite, based on nomenclature rules proposed by Atencio et al. (Reference Atencio, Andrade, Christy, Gieré and Kartashov2010) and the dominant-valencey rules (Bosi et al., Reference Bosi, Hatert, Hålenius, Pasero, Miyawaki and Mills2019), because O2– predominates in the Y site, Ca2+ predominates in the A site and Ta5+ predominates in the B site.
Table 1. Chemical analyses of oxycalciomicrolite.
S.D. = standard deviation and n.d. = not detected.
A grain of oxycalciomicrolite was tested for reactions with water, nitric acid (70%), hydrochloric acid (37%), aqua regia and sulfuric acid (98%), all at room temperature. The grain did not react to any of these tests. When tested with hydrofluoric acid (48%), the mineral went through dissolution and showed a colour change from brownish to white.
Infrared data
The infrared data spectrum of oxycalciomicrolite was obtained from a powdered sample (6 mg) mixed, homogenised and pressed with anhydrous KBr (300 mg), using a PerkinElmer Spectrum 400 FT-IR/FT-NIR spectrometer, at the resolution of 16 cm–1 and accumulation of 30 scans. A pure KBr disk was used as a background for the analyses.
The spectrum of oxycalciomicrolite (Fig. 3) shows bands in the range 400–700 cm–1 and bands at 917 and 1000 cm–1. The first group are associated with vibrations of the microlite type framework and the second one related to Ta–O octahedron vibrations (Andrade et al., Reference Andrade, Atencio, Chukanov and Ellena2013a, Reference Andrade, Atencio, Persiano and Ellenab; Biagioni et al., Reference Biagioni, Orlandi, Nestola and Bianchin2013; Andrade et al., Reference Andrade, Yang, Atencio, Downs, Chukanov, Lemée-Cailleau, Persiano, Goeta and Ellena2017; Guang et al., Reference Guang, Ge, Li, Yu and Shen2017). No significant absorption attributed to H–O–H bending vibrations or O–H stretching vibrations, characteristically at 1600 to 1700 cm–1 and 2900 to 3700 cm–1, respectively, were observed. This is indicative of the absence of structural H2O in oxycalciomicrolite.
Fig. 3. Infrared spectrum of oxycalciomicrolite.
Raman data
The Raman spectrum of oxycalciomicrolite was collected from a randomly oriented crystal using a Horiba Jobin Yvon LabRAM800 HR spectrometer coupled to Olympus BX41 microscope, and a thermoelectrically cooled CCD detector (at 203.15 K). Excitation was provided by a 632.8 nm wavelength He–Ne laser, while a 600 lines/mm diffraction grid ensured a spectral resolution of 1 cm–1.
The Raman spectrum shows three bands between 100 and 1100 cm–1 with vibrations at 295, 652 and 791 cm–1 (Fig. 4). Based on previous studies (Glerup et al., Reference Glerup, Nielsen and Poulsen2001; Arenas et al., Reference Arenas, Gasparov, Wei Qiu Nino, Patterson and Tanner2010), we assign the 295 and 652 cm–1 bands to the B–X octahedral stretching and to the X–B–X bending, respectively. The 791 cm–1 band could not be associated to any known vibrational mode of the pyrochlore supergroup, but it is possible that this band results from the combination, or overtone, of the 295 and 652 cm–1 bands (Arenas et al., Reference Arenas, Gasparov, Wei Qiu Nino, Patterson and Tanner2010; Bahfenne and Frost, Reference Bahfenne and Frost2010).
Fig. 4. Raman spectrum of oxycalciomicrolite.
X-ray diffraction
Powder X-ray data were obtained using a Bruker-AXS D8 Advance ECO equipment with an energy-discriminant LynxEye XE detector, operating with unfiltered CuKα (λ = 1.54056 Å) radiation, collected from 4° to 105°2θ. Oxycalciomicrolite is cubic with space group 
$Fd\bar{3}m$. The powder X-ray lines and diffractogram are given in Table 2 and Fig. 5. Unit-cell parameters, obtained by the Pawley fitting from powder data are a = 10.4325(4) Å, V = 1135.46(14) Å3 and Z = 8.
Table 2. Powder X-ray diffraction data (d in Å) for oxycalciomicrolite, indexed with a = 10.4325 (4) Å, V = 1135.46 (14) Å3 and Z = 8.
The strongest lines are given in bold.
Fig 5. Powder X-ray diffraction pattern of oxycalciomicrolite showing the principal Miller indices compatible with those of minerals of the pyrochlore supergroup.
Discussion
According to the nomenclature and classification of the minerals belonging to the pyrochlore supergroup revised by Atencio et al. (Reference Atencio, Andrade, Christy, Gieré and Kartashov2010), the mineral described in this study was named as oxycalciomicrolite, although other authors described and named similar minerals based on the original classification and nomenclature of the pyrochlore group (Hogarth, Reference Hogarth1977). Černý at al. (Reference Černý, Chapman, Ferreira and Smeds2004) described a mineral named ‘stibiomicrolite’ in the Varuträsk pegmatite, northeastern Sweden, while Guastoni et al. (Reference Guastoni, Diela and Pezzotta2008) identified a ‘microlite’ in the pegmatites of Vigezzo Valley (western Alps in Italy). Considering the available chemical data for these occurrences and following the classification scheme of Atencio et al. (Reference Atencio, Andrade, Christy, Gieré and Kartashov2010), these minerals should be renamed as oxycalciomicrolite.
The small size of the crystal used for structure determination jeopardised its preparation for chemical analyses. Differences were observed between the electrons per formula unit (epfu) based on the chemical formula that satisfied the crystal-structure refinement and the product referring to the chemical analyses. As the composition and the crystal structure analyses could not be obtained from the same crystal, we chose not to present the data obtained by single-crystal diffraction.
Acknowledgements
VHRMS and SSCGS thanks Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for postgraduate grant, TPC thanks Fundação Carlos Chagas Filho de Amparo à Pesquisa do Rio de Janeiro for postgraduate grants, FEAA thanks Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for postgraduate grant. The authors acknowledge the X-Ray Laboratory of Federal Fluminense University (Niterói, RJ) for single-crystal analysis, the Regional Centre for Innovation and Technological Development (CRTI), Federal University of Goiás (Goiania, GO) for electron probe microanalysis and the Gemmological Laboratory (LAPEGE) at the Centre for Mineral Technology (CETEM) for FT-IR analysis. CAA (grants 478805/2010-1 and 30377/2014-3) and RN (grants 302828/2015-0 and 315472/2018-9) thank CNPq for financial support, as well as for research grant 406853/2013-4. We also greatly thank the anonymous referees, Associate Editor and the Principal Editor Stuart Mills for the helpful suggestions and guidelines.
