Hydroxycalciomicrolite, Ca1.5Ta2O6(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 56. 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 OH stretching vibrations. The chemical composition determined from electron microprobe analysis (n = 13) is (wt.%): Na2O 0.36(8), CaO 15.64(13), SnO2 0.26(3), Nb2O5 2.82(30), Ta2O5 78.39(22), MnO 0.12(2), F 0.72(12) and H2O 1.30 (from the crystal structure data), O = F -0.30, total 99.31(32), yielding an empirical formula, (Ca1.48Na0.06Mn0.01)(Sigma 1.55)(Ta1.88Nb0.11Sn0.01)S2.00O6.00[(OH)(0.76)F0.20O0.04]. Hydroxycalciomicrolite is cubic, with unit-cell parameters a = 10.4205(1) angstrom, V = 1131.53(2) angstrom(3) and Z = 8. It represents a pyrochlore supergroup, microlite-group mineral exhibiting P4(3)32 symmetry, instead of Fd (3) over barm. 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.
| Identifer | oai:union.ndltd.org:arizona.edu/oai:arizona.openrepository.com:10150/625066 |
| Date | 01 May 2017 |
| Creators | Andrade, M. B., Yang, H., Atencio, D., Downs, R. T., Chukanov, N. V., Lemée-Cailleau, M. H., Persiano, A. I. C., Goeta, A. E., Ellena, J. |
| Contributors | Univ Arizona, Dept Geosci |
| Publisher | MINERALOGICAL SOC |
| Source Sets | University of Arizona |
| Language | English |
| Detected Language | English |
| Type | Article |
| Rights | © 2017 The Mineralogical Society |
| Relation | http://www.ingentaconnect.com/content/10.1180/minmag.2016.080.116, http://minmag.geoscienceworld.org/content/81/3/555 |
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