The new mineral coutinhoite, ideally ThxBa(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, α 1.620(3), β 1.627(3), γ 1.629(3), 2Vmeas. = 40(5)°, 2Vcalc. = 56.1°, dispersion r < v strong, orientation Y = c. Pleochroism is Z > Y, X yellow. The empirical formula (based on Si + P = 5) is (Th0.30Ba0.19K0.07Ca0.04)∑0.60(UO2)2.00(Si4.92P0.08)∑5.00O12.91·2.86H2O. 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) Å3, and Z = 16. It is probably isostructural with weeksite. Both the description and name were approved by the CNMMN-IMA (Nomenclature Proposal 2003–025).
