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Anais da Academia Brasileira de Ciências

versão impressa ISSN 0001-3765versão On-line ISSN 1678-2690

Resumo

SOARES, Dwight R.; BEURLEN, Hartmut; FERREIRA, Ana C.M.  e  DA-SILVA, Marcelo R.R.. Chemical composition of gahnite and degree of pegmatitic fractionation in the Borborema Pegmatitic Province, northeastern Brazil. An. Acad. Bras. Ciênc. [online]. 2007, vol.79, n.3, pp.395-404. ISSN 0001-3765.  http://dx.doi.org/10.1590/S0001-37652007000300005.

Gahnite electron probe microanalyses from three pegmatites (Mirador, Capoeira and Quintos) of the Borborema Pegmatitic Province in northeastern Brazil allowed to determine molar compositions ranging from 86.22 to 95.41 mol% gahnite (ZnAl2O4), 1.51 to 6.62 mol% hercynite (Fe2+Al2O4), 0.11 to 5.89 mol% spinel (MgAl2O4), 1.48 to 2.74 mol% galaxite (MnAl2O4), and Zn/FeTotal atomic ratios ranging from 11.53 to 60.87. These data compared with gahnite compositions from albite subtype pegmatites from Cap de Creus, Spain (Zn/FeTotal between 10 and 60) and from the source granite of the Separation Rapids pegmatite field, Ontario, Canada (Zn/FeTotal =3.5) indicate a high degree of fractionation of the studied pegmatites. This interpretation agrees with conclusions based on compositional data of tourmaline (vacancy in X-site 0.42 to 0.49), white mica (K/Rb between 8 and 33), feldspar (K/Rb between 8 and 109), garnet (Mn/(Mn+Fe2+)between 0.93 and 0.97) and in Nb-Ta oxides (Mn/(Mn+Fe2+)>0.5) in the same pegmatites.

Palavras-chave : gahnite; rare-element granitic pegmatites; Borborema Pegmatitic Province; mineral chemistry.

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