Parisite-(La), ideally CaLa2(CO3)(3)F-2, a new mineral from Novo Horizonte, Bahia, Brazil

Abstract

Parisite-(La) (IMA2016-031), ideally CaLa2(CO3)(3)F2, occurs in a hydrothermal vein crosscutting a metarhyolite of the Rio dos Remedios 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 bastnasite-(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 (+), omega = 1.670(2) and epsilon = 1.782(5) (589 nm). The empirical formula normalized on the basis of 11 (O + F) atoms per formula unit (apfu) is Ca0.98(La0.83Nd0.51Ce0.37Pr0.16Sm0.04Y0.03)(Sigma 1.94)C3.03O8.91F2.09. The IR spectrum confirms the absence of OH groups. Single-crystal X-ray studies gave the following results: monoclinic (pseudo-trigonal), space group: C2, Cm, or C2/m, a = 12.356(1) angstrom, b = 7.1368(7) angstrom, c = 28.299(3) angstrom, beta = 98.342(4)degrees, V = 2469.1(4) angstrom(3) and Z = 12. Parisite-(La) is the La-dominant analogue of parisite-(Ce).