Hydroxycalciomicrolite, Ca_1.5Ta₂O₆(OH), a new member of the microlite group from Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil

Abstract

Hydroxycalciomicrolite, Ca 1.5 Ta 2 O 6 (OH) is a new microlite-group mineral found in the Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil. It occurs as isolated octahedral and as a combination of octahedral and rhombic dodecahedral crystals, up to 1.5 mm in size. The crystals are yellow and translucent, with a white streak and vitreous to resinous lustre. The mineral is brittle, with a Mohs hardness of 5-6. Cleavage is not observed and fracture is conchoidal. The calculated density is 6.176 g cm -3 . Hydroxycalciomicrolite is isotropic, n calc. = 2.010. The infrared and Raman spectra exhibit bands due to O-H stretching vibrations. The chemical composition determined from electron microprobe analysis (n = 13) is (wt.%): Na 2 O 0.36 (8) Hydroxycalciomicrolite is cubic, with unit-cell parameters a = 10.4205(1) Å, V = 1131.53(2) Å 3 and Z = 8. It represents a pyrochlore supergroup, microlite-group mineral exhibiting P4 3 32 symmetry, instead of Fd 3m. The reduction in symmetry is due to long-range ordering of Ca and vacancies on the A sites. This is the first example of such ordering in a natural pyrochlore, although it is known from synthetic compounds. This result is promising because it suggests that other species with P4 3 32 or lower-symmetry space group can be discovered and characterized.

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

“…The strong IR bands at 570 cm –1 (with the shoulders at 650 and 705 cm –1 ) and 375 cm –1 correspond to stretching and bending vibrations of the octahedral framework, respectively, and are typical for microlite-group minerals (Andrade et al ., 2013 b , 2017; Chukanov, 2014; Geisler et al ., 2004). The shoulder at 880 cm –1 corresponds to a combination mode (a combination of stretching vibrations of the octahedral framework and lattice vibrations involving extra-framework cations).…”

“…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.…”

Oxycalciomicrolite, (Ca,Na)2(Ta,Nb,Ti)2O6(O,F), a new member of the microlite group (pyrochlore supergroup) from the Paleoproterozoic São João del Rei Pegmatite Province, Minas Gerais state, Brazil

“…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 ., 2013 a , b ; Biagioni et al ., 2013; Andrade et al ., 2017; Guang et al ., 2017). 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.…”

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

“…[55][56][57][58][59][60][61] Entretanto, sua aplicação na caracterização de minerais do supergrupo do pirocloro parece ser ainda ser inicial com relação à identificação de minerais brasileiros. 7,[62][63] Até o presente momento não foi identificado trabalhos sobre a utilização de redes neurais artificiais como ferramenta na interpretação quantitativa de espectros Raman de minerais do supergrupo do pirocloro, o que reforça a motivação para o desenvolvimento desse trabalho.…”

“…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.…”