Abstract
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.
Highlights
IntroductionGahnite is a spinel group mineral (with general formula AB2O4), with its A site being occupied mainly by Zn and the B site mainly by Al. According to Deer et al (1981) gahnite occurs most commonly as accessory mineral in granitic pegmatites, “in metassomatic replacement veins” and as contact metamorphic mineral in marbles
Gahnite is a spinel group mineral, with its A site being occupied mainly by Zn and the B site mainly by Al
They exhibit remarkably high Zn contents in the range of 6.97 to 7.64apfu, corresponding to 86 to 95% of zinc apfu in the A-site or of 86.22 to 95.41 mol% gahnite, considering that the B site is occupied almost completely by Al (> 99% of the B site) and that the presence of other elements detected in this site (Fe3+, Ti, Cr, Si) remains below the detection limit and/or near the analytical error
Summary
Gahnite is a spinel group mineral (with general formula AB2O4), with its A site being occupied mainly by Zn and the B site mainly by Al. According to Deer et al (1981) gahnite occurs most commonly as accessory mineral in granitic pegmatites, “in metassomatic replacement veins” and as contact metamorphic mineral in marbles. An almost complete solid solution of gahnite is registered only with ordinary spinel (MgAl2O4) according to these authors. The most common trace elements present in gahnite are Li, Rb, Ba, Ga, Cu, and Ti (Von Knorring and Dearnley 1960). Cotelo Neiva et al (1955) found up to 13.5 wt. % SnO2, corresponding to 1.5 Sn atoms per formula unit (apfu) from the Cabanas pegmatite in Portugal.
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