A turquesa de Itacupim, Pará

Abstract

Na ilha de Itacupim, localizada na região costeira do nordeste do Pará, foram encontrados veios de fosfatos de alumínio contendo turquesa, além de quartzo e argilominerais. A ilha é sustentada por espesso perfil laterítico maturo desenvolvido sobre complexo alcalino-ultramáfico mineralizado em apatita. Os veios e vênulas são de espessura centimétrica, normalmente constituídos de wavellita fibro-radial, onde pode ser observada turquesa verde-azulada, em massas subesferolíticas, microcristalinas, intercrescidas com caulinita e oxi-hidróxidos de Mn, além de quartzo. A identificação mineral foi realizada por DRX, microscopia óptica, análises químicas de rocha total, MEV/SED. Os teores de CuO são inferiores aos das turquesas em geral, compensados por Fe2O3 e ZnO. Os subesferolitos de turquesa contêm inúmeras inclusões micrométricas de goyazita ou svanbergita. A ocorrência da turquesa, na forma de veios e vênulas, seu aspecto porcelanado e a conhecida relação desse mineral com ambiente hidrotermal sugerem que a turquesa de Itacupim também seja de origem hidrotermal, reforçada pela sua associação com wavellita, goyazita ou svanbergita, quartzo e argilominerais. Ela não foi encontrada no perfil laterítico. Seu aspecto compacto e sua cor esverdeada abrem perspectivas para seu uso como mineral de gema.<br>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

Lapidary production in the eastern Caribbean: a typo-technological and microwear study of ornaments from the site of Pearls, Grenada

“…For instance, nephrite sources may be located in the Brazilian Amazon (Costa et al 2002) or in the Sierra Nevada de Santa Marta (Acevedo Gómez et al 2018). Turquoise veins have been reported from St. John (Virgin Islands) (Alminas et al 1994;Knippenberg 2007, 152) and from near the mouth of the Amazon River (Costa et al 2004). Carnelian was arguably sourced in Antigua, mainly worked in Montserrat, and exchanged with other islands (Crock and Bartone 1998;Hofman et al 2014a;Mol 2014;Murphy et al 2000;Watters and Scaglion 1994).…”

Provenancing turquoise: a difficult task

“…Hull & Fayek, 2012;Kievlenko, 2003;Lowry & Lowry, 2010;Pogue, 1915;Shigley et al, 2000;Thibodeau et al, 2015). Some sources of American turquoise originate outside of this area, such as in an area of Brazil (Costa et al, 2004;Shigley et al, 2010), where copper porphyry deposits are also recorded (fig. 2).…”