Serrabrancaite is a new manganese phosphate from the Alto Serra Branca pegmatite near Pedra Lavrada, Paraiba, Brazil. The mineral occurs as dark brown to dark greenish black isometric crystals up to 0.3 mm size. The crystals have an adamantine luster and are translucent. Chemical analysis yields (in wt%) Mn2O3 = 46.85, P2O5 = 42.72, H2O = 9.8, total = 99.37. The empirical formula is Mn0.98P1.00O3.98·0.90 H2O, simplified to MnPO4·H2O. Serrabrancaite is monoclinic, space group C2/c, with unit-cell parameters a = 6.914(2), b = 7.468(2), c = 7.364(2) Å, β = 112.29(3)°, V = 351.8(1) Å3. The measured density is 3.17(1) g/cm3, the calculated density for Z = 4 is 3.16 g/cm3. Serrabrancaite is isostructural with the sulfates of the kieserite group. The mineral is an alteration product from a phosphate pegmatite and is accompanied by vernadite and phosphosiderite.

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