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Coutinhoite, a new thorium uranyl silicate hydrate, from Urucum mine, Galiléia, Minas Gerais, Brazil

¹ Instituto de Geociências, Universidade de São Paulo, Rua do Lago, 562, 05508-080 São Paulo, SP, Brazil
² Museu Jobas de Ciências Naturais "José Bonifácio de Andrada e Silva," Rua Martim Afonso 24, Biquinha, 11310-010 São Vicente, SP, Brazil

Correspondence: ^* E-mail: datencio{at}usp.br

The new mineral coutinhoite, ideally Th_xBa(_1–2x)(H₂O)_y(UO₂)₂Si₅O₁₃ · H₂O, with 0 x 0.5 and 0 y (2 + x), occurs as a secondary hydrothermal mineral in the Córrego do Urucum granitic pegmatite, Lavra Urucum, Galiléia Co., Minas Gerais, Brazil. Coutinhoite is intimately associated with weeksite, phosphuranylite, meta-uranocircite, and uranocircite on muscovite and microcline. The mineral forms irregular aggregates with very small curved scales, flaky crystals, up to 10 µm long and with a thickness up to about 0.5 µm. Coutinhoite is transparent to translucent and displays a waxy to silky luster; color and streak are yellow. It is non-fluorescent. The hardness is less than 2. It is brittle. Calculated density is 3.839 g/cm³. Coutinhoite is biaxial negative, 1.620(3), ß 1.627(3), 1.629(3), 2V_meas. = 40(5)°, 2V_calc. = 56.1°, dispersion r < v strong, orientation Y = c. Pleochroism is Z > Y, X yellow. The empirical formula (based on Si + P = 5) is (Th_0.30Ba_0.19K_0.07Ca_0.04)_0.60(UO₂)_2.00(Si_4.92P_0.08)_5.00O_12.91·2.86H₂O. The mineral is orthorhombic, probable space group Cmmb (67). Cell parameters were refined from the powder data: a 14.1676(9), b 14.1935(9), c 35.754(2) Å, V = 7189.7(2) ų, and Z = 16. It is probably isostructural with weeksite. Both the description and name were approved by the CNMMN-IMA (Nomenclature Proposal 2003–025).