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BIBTEX RIS

Parisite-(La), ideally CaLa 2 (CO 3 ) 3 F 2 , a new mineral from Novo Horizonte, Bahia, Brazil

2017; Cambridge University Press; Volume: 82; Issue: 1 Linguagem: Inglês

10.1180/minmag.2017.081.028

ISSN

1471-8022

Autores

Luiz Alberto Dias Menezes Filho, Mario Luiz de Sá Carneiro Chaves, Н. В. Чуканов, Daniel Atencio, Ricardo Scholz, Igor Pekov, Geraldo Magela da Costa, Shaunna M. Morrison, Marcelo B. Andrade, Érico Tadeu Fraga Freitas, Robert T. Downs, Dmitriy I. Belakovskiy,

Tópico(s)

Clay minerals and soil interactions

Resumo

ABSTRACT Parisite-(La) (IMA2016-031), ideally CaLa 2 (CO 3 ) 3 F 2 , occurs in a hydrothermal vein crosscutting a metarhyolite of the Rio dos Remédios Group, at the Mula mine, Tapera village, Novo Horizonte county, Bahia, Brazil, associated with hematite, rutile, almeidaite, fluocerite-(Ce), brockite, monazite-(La), rhabdophane-(La) and bastnäsite-(La). Parisite-(La) occurs as residual nuclei (up to 5 mm) in steep doubly-terminated pseudo-hexagonal pyramidal crystals (up to 8.2 cm). Parisite-(La) is transparent, yellow-green to white, with a white streak and displays a vitreous (when yellow-green) to dull (when white) lustre. Cleavage is distinct on pseudo-{001}; fracture is laminated, conchoidal, or uneven. The Mohs hardness is 4 to 5, and it is brittle. Calculated density is 4.273 g cm −3 . Parisite-(La) is pseudo-uniaxial (+), ω = 1.670(2) and ε = 1.782(5) (589 nm). The empirical formula normalized on the basis of 11 (O + F) atoms per formula unit (apfu) is Ca 0.98 (La 0.83 Nd 0.51 Ce 0.37 Pr 0.16 Sm 0.04 Y 0.03 ) Σ1.94 C 3.03 O 8.91 F 2.09 . The IR spectrum confirms the absence of OH groups. Single-crystal X-ray studies gave the following results: monoclinic (pseudo-trigonal), space group: C 2, Cm, or C 2/ m, a = 12.356(1) Å, b = 7.1368(7) Å, c = 28.299(3) Å, β = 98.342(4)°, V = 2469.1(4) Å 3 and Z = 12. Parisite-(La) is the La-dominant analogue of parisite-(Ce).

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