| Full text | |
| Author(s): |
Andrade, M. B.
;
Yang, H.
;
Atencio, D.
;
Downs, R. T.
;
Chukanov, N. V.
;
Lemee-Cailleau, M. H.
;
Persiano, A. I. C.
;
Goeta, A. E.
;
Ellena, J.
Total Authors: 9
|
| Document type: | Journal article |
| Source: | MINERALOGICAL MAGAZINE; v. 81, n. 3, p. 555-564, MAY 2017. |
| Web of Science Citations: | 3 |
| Abstract | |
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. (AU) | |
| FAPESP's process: | 11/22407-0 - New and rare Brazilian mineral species characterization |
| Grantee: | Daniel Atencio |
| Support Opportunities: | Regular Research Grants |
| FAPESP's process: | 13/03487-8 - Center of characterization of new mineral species: Raman spectroscopy, electron microprobe and X-ray and neutron diffraction |
| Grantee: | Marcelo Barbosa de Andrade |
| Support Opportunities: | Research Grants - Young Investigators Grants |