Romeite-Group Minerals Review: New Crystal Chemical and Raman Data of Fluorcalcioromeite and Hydroxycalcioromeite

Author(s)

Lopes, Gerson A. C. ^{[1, 2]} ; Atencio, Daniel ^[3] ; Ellena, Javier ^[1] ; Andrade, Marcelo B. ^[1]

Affiliation

^[1] Univ Sao Paulo, Sao Carlos Inst Phys IFSC, BR-13566590 Sao Carlos, SP - Brazil

^[2] Amapa State Univ UEAP, Lab Phys LABFIS, BR-68900070 Macapa, AP - Brazil

^[3] Univ Sao Paulo, Inst Geosci IGc, BR-05508080 Sao Paulo, SP - Brazil

• Document type: Review article
• Source: MINERALS; v. 11, n. 12 DEC 2021.
• Web of Science Citations: 0

Abstract

The romeite-group is part of the pyrochlore-supergroup and comprises cubic oxides of A(2)B(2)X(6)Y formula in which Sb5+ predominates in the B-site. The A and Y main occupants determine different minerals in the group and are important for the discovery of new mineral species. Two different romeite-group mineral samples were analysed by electron microprobe analysis (EMPA), Raman spectroscopy and single-crystal X-ray diffraction (XRD). The first sample is from Prabornaz Mine (locality of the original romeite), Saint Marcel, Valle d'Aosta, Italy, whereas the other one occurs in Kalugeri Hill, Babuna Valley, Jakupica Mountains, Nezilovo, Veles, Macedonia. Sample 1 was identified as fluorcalcioromeite, and sample 2 as hydroxycalcioromeite. Both samples belong to the cubic crystal system, space group Fd3 over bar m, Z = 8, where a = 10.2881(13) \& ANGS;, V = 1088.9(4) \& ANGS;(3) for sample 1, and a = 10.2970(13) \& ANGS;, V = 1091.8(4) \& ANGS;(3) for sample 2. The crystal structure refinements converged to (1) R-1 = 0.016, wR(2) = 0.042; and (2) R-1 = 0.023, wR(2) = 0.049. Bond-valence calculations validated the crystal structure refinements determining the correct valences at each crystallographic site. Discrepancies observed in the Sb5+ bond-valence calculations were solved with the use of the proper bond valence parameters. The resulting structural formulas are (Ca1.29Na0.550.11Pb0.05)(sigma=2.00)(Sb1.71Ti0.29)(sigma=2.00){[}O-5.73( OH)(0.27)](sigma=6.00){[}F0.77O0.21(OH)(0.02)](sigma=1.00) for sample 1, and (Ca1.30Ce0.510.19)(sigma=2.00)(Sb1.08Ti0.92)(sigma=2.00)O-6.00{[}(OH)(0. 61)O0.21F0.18](sigma=1.00) for sample 2. The Raman spectra of the samples exhibited the characteristic bands of romeite-group minerals, the most evident corresponding to the Sb-O stretching at around 510 cm(-1). (AU)

• FAPESP's process: 19/23498-0 - Brazilian mineralogy: characterization of new and rare mineral species
• 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