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IMA 2016-031 = parisite-(La)

Posted by Marco E. Ciriotti  
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Marco E. Ciriotti April 18, 2018 09:27AM
Reference:
▪ Menezes Filho, L.A.D., Chaves, M.L.S.C., Chukanov, N.V., Atencio, D., Scholz, R., Pekov, I.V., da Costa, G.M. Morrison, S.M., Andrade, M.B., Freitas, E.T.F., Downs, R.T., Belakovskiy, D.I. (2018): Parisite-(La), CaLa2(CO3)3F2, a new mineral from Novo Horizonte, Bahia, Brazil. Mineralogical Magazine, 82, 133-144.

Abstract:
Parisite-(La) (IMA number 2016-031), ideally CaLa2(CO3)3F2, 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 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), ε = 1.782(5) (589 nm). The empirical formula normalized on the basis of 11 (O + F) pfu is Ca0.98(La0.83Nd0.51Ce0.37Pr0.16Sm0.04Y0.03)Σ=1.94C3.03O8.91F2.09. 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) Å, b = 7.1368(7) Å, c = 28.299(3) Å, β = 98.342(4)º, V = 2469.1(4) Å3, Z = 12. Parisite-(La) is the La-dominant analogue of parisite-(Ce).
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Ronnie Van Dommelen August 10, 2018 12:41PM
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