Fluorcalciomicrolite, (Ca,Na,☐)₂Ta₂O₆F, a new microlite-group mineral from Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil

Abstract

Fluorcalciomicrolite, (Ca, Na, ☐)2Ta2O6F, is a new microlite-group, pyrochlore supergroup mineral approved by the CNMNC (IMA 2012-036). It occurs as an accessory mineral in the Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil. Associated minerals include: microcline, albite, quartz, muscovite, spodumene, “lepidolite”, cassiterite, tantalite-(Mn), monazite-(Ce), fluorite, “apatite”, beryl, “garnet” , epidote, magnetite, gahnite, zircon, “tourmaline” , bityite, hydrokenomicrolite, and other microlite-group minerals under study. Fluorcalciomicrolite occurs as euhedral, untwinned, octahedral crystals 0.1–1.5 mm in size, occasionally modified by rhombododecahedral faces. The crystals are colourless and translucent; the streak is white, and the lustre is adamantine to resinous. It does not fluoresce under ultraviolet light. Mohs' hardness is 4½–5, tenacity is brittle. Cleavage is not observed; fracture is conchoidal. The calculated density is 6.160 g/cm3. The mineral is isotropic, ncalc. = 1.992. The Raman spectrum is dominated by bands of B–X octahedral bond stretching and X–B–X bending modes. The chemical composition (n = 6) is (by wavelength dispersive spectroscopy, H2O calculated to obtain charge balance, wt.%): Na2O 4.68, CaO 11.24, MnO 0.01, SrO 0.04, BaO 0.02, SnO20.63, UO20.02, Nb2O53.47, Ta2O576.02, F 2.80, H2O 0.48, O=F–1.18, total 98.23. The empirical formula, based on 2 cations at the B site, is (Ca1.07Na0.81☐0.12)Σ2.00(Ta1.84Nb0.14Sn0.02)Σ2.00[O5.93(OH)0.07]6.00[F0.79(OH)0.21]. The strongest eight X-ray powder-diffraction lines [d in Å (I)(hkl)] are: 5.997(59)(111), 3.138(83)(311), 3.005(100)(222), 2.602(29)(400), 2.004(23)(511), 1.841(23)(440), 1.589(25)(533), and 1.504(24)(444). The crystal structure refinement (R1 = 0.0132) gave the following data: cubic, Fdm, a = 10.4191(6) Å, V = 1131.07(11) Å3, Z = 8.

Hydroxycalciomicrolite, Ca1.5Ta2O6(OH), a new member of the microlite group from Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil

“…Such symmetry change has also been observed in the synthetic compounds Ca 1.5 Ta 2 O 6 F and Ca 1.5 Nb 2 O 6 F (Le Berre et al, 2007). In contrast, fluorcalciomicrolite has the space group Fd 3m (Andrade et al, 2013b) and does not show the same number of weak reflections violating the F-lattice symmetry. The lack of ordering in the type fluorcalciomicrolite is probably due to the low vacancy concentration in the A site: the empirical formula is (Ca 1.07 Na 0.81 □ 0.12 ) Σ2.00 (Ta 1.84 Nb 0.14 Sn 0.02 ) Σ2.00 [O 5.93 (OH) 0.07 ] Σ6.00 [F 0.79 (OH) 0.21 ] (Andrade et al, 2013b).…”

“…Hydroxycalciomicrolite (10 crystals) was separated from an old concentrate, labelled 'djalmaite', from the Museu de Geociências, Universidade de São Paulo, without the details of donor registration. Hydrokenomicrolite (Andrade et al, 2013a) and fluorcalciomicrolite (Andrade et al, 2013b) are also from the same concentrate. So no paragenetic data can be presented for these minerals.…”

“…In general, H-O-H angle bending is associated with absorption peaks in the range 1500-1700 cm -1 ; their absence is consistent with all the hydrogen in hydroxycalciomicrolite present in OH groups. Recently, Andrade et al (2013b) performed IR analysis and detected the bands at similar positions within experimental error, 3600 and 3581 cm -1 for fluorcalciomicrolite (Fd 3m).…”

Oxybismutomicrolite, a new pyrochlore-supergroup mineral from the Malkhan pegmatite field, Central Transbaikalia, Russia

“…The current IMA list of minerals (updated in November, 2019; ) includes nine valid species belonging to the microlite group. Three of them, hydroxykenomicrolite, oxystannomicrolite and oxystibiomicrolite have been redefined by Atencio et al (2010) from former species ‘cesstibtantite’, ‘sukulaite’ (= stannomicrolite) and ‘stibiomicrolite’, respectively, while six others were published as new species: fluorcalciomicrolite (Andrade et al , 2013a), fluornatromicrolite (Witzke et al , 2011), hydrokenomicrolite (Andrade et al , 2013b), hydroxycalciomicrolite (Andrade et al , 2017), kenoplumbomicrolite (Atencio et al , 2018) and oxynatromicrolite (Fan et al , 2016). Thus oxybismutomicrolite is the tenth member of the microlite group.…”

Espectroscopia Raman e redes neurais artificiais na identificação de minerais dos grupos do pirocloro e da microlita

“…vibrações da rede (100-200 cm -1 ), deformação angular (200-500 cm -1 ), estiramento simétrico e antissimétrico (500-800 cm -1 ), sobretons (800-1400 cm -1 ), deformação angular de H 2 O (1400-1800 cm -1 ) e estiramento de OH (2800-4000 cm -1 ). 62,[81][82][83] A determinação teórica das bandas Raman prevista por análise de fator de grupo e cálculos ab initio para amostras do supergrupo do pirocloro apresenta seis modos vibracionais. [84][85][86][87] No entanto, como visto nas figuras 14-37, há mais de seis modos vibracionais obtidos experimentalmente.…”