Veins and veinlets of aluminum phosphates with turquoise occur at the Itacupim Island in the coastal plain northeast the state of Pará. A thick mature lateritic iron crust rich in aluminum phosphates developed an apatite-bearing alkaline-ultramafic complex which constitutes the Island. The veins and veinlets are cm-thick, usually constituted by wavellite, fibrous to radialfibrous, with bony or porcelaneous aspect, and can turquoise. Pebbles of these phosphates inside of apatite-bearing ultramafic rocks are very common at the base of the hang wall, and locally form expressive agglomerates. Turquoise forms half spheroids and is bluish-green, microcrystalline, and is intergrown with kaolinite and Mn oxy-hydroxides. The mineral identification was carried out by XRD optic microscopy, chemical analyses by wet methods and by SEM/EDS. The lower CuO values, in comparison to turquoise elsewhere, are compensated by the higher Fe2O3 and ZnO. The spheroids display countless inclusions of micrometric goyazite or svanbergite. The turquoise relation to veins and veinlets with wavellite, goyazite or svanbergite, quartz and clay minerals, its porcelaneous aspect and well-known occurrence of turquoise in hydrothermal environment indicate that the Itacupim turquoise was formed by the same mechanism. It doesn't display any clear relationship to laterite profile. The color and compact aspect of this turquoise make it suitable for use as gems.
Turquoise; aluminum phosphate; apatite; Itacupim island; Amazon region; Brazil
