Paper Number: 1462
New microlite-group minerals from Volta Grande Pegmatite, Minas Gerais, Brazil
Andrade, M.B.1 and Atencio, D.2
1São Carlos Institute of Physics, University of São Paulo, Brazil, mabadean@terra.com.br
2Instituto de Geociências, Universidade de São Paulo, Brazil.
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Hydrokenomicrolite, fluorcalciomicrolite and hydroxycalciomicrolite are new microlite-group minerals from the pyrochlore supergroup (A2-mB2X6-wY1-n) [1] occurring as accessory phases in the rare-element granitic Volta Grande pegmatites, Nazareno, Minas Gerais state, Brazil [2].
Hydrokenomicrolite, (□,H2O)2Ta2(O,OH)6(H2O), is cubic, Fd-3m, a = 10.454(1) Å, V = 1142.5(2) Å3, Z = 8 [3]. Hydrokenomicrolite is H2O-dominant in Y–site and vacancy-dominant in A-site. The presence of H2O was investigated using crystal structure refinements and infrared spectroscopy.
Fluorcalciomicrolite, (Ca,Na,□)2Ta2O6F, is the second mineral species with F-dominant on the Y-site described in the microlite-group [4]. The first one was fluornatromicrolite (Na,Ca,Bi)2Ta2O6F [5]. The crystal structure parameters are Fd-3m, a = 10.4191(6) Å, V = 1131.07 Å3 and Z = 8.
Hydroxycalciomicrolite is the first described microlite group-mineral which crystallizes in space group P4332, instead of Fd-3m [6]. The simplified formula is Ca15Ta2O6(OH). The unit cell parameters are a = 10.4205(1) Å, V = 1131.53 Å3 and Z = 8. The ordering of Ca cations and vacancies at the A-site are related to the lowering of symmetry from the F- to P-lattice.
References:
[1] Atencio D et al. (2010) Can Min 48: 673-698
[2] Lagache M and Quéméneur J (1997) Can Min 35: 153-165
[3] Andrade MB et al. (2013) Am Min 98: 292-296
[4] Andrade MB et al. (2013) Min Mag 77: 2989-2996
[5] Witzke T et al. (2011) Can Min 49: 1105-1110
[6] Andrade MB et al. (2013) CNMNC Newsletter 18:3252; Min Mag 77:3249-3258