Sector-zoned tourmaline from the cap rock of a salt dome

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Sector zoned tourmaline, exhibiting striking compositional differences among pyramidal sectors, occurs in the cap rock of the salt dome associated with the Challenger Knoll, Gulf of Mexico. Trace amounts of fine-grained (10-80 μm) euhedral authigenic tourmaline are included in calcite as part of the carbonate-rich portion of the cap rock. Compositional, textural and geological data are most consistent with an authigenic origin of the tourmaline in bedded halite under oxidizing conditions prior to incorporation into the cap rock. The cap rock tourmaline most commonly has a tabular morphology, and sections cut through the crystals display impressive optical and chemical zoning. The tourmaline crystals are magnesian, but surprisingly rich in Fe3+, poor in Ca and deficient in Al. Individual tourmaline crystals have extensive compositional ranges, the most prominent substitution being Fe3+ (0.9-3.4 apfu) for Al (3.5-5.7 apfu) i.e. principally represented by the exchange vector Fe(Al)-1. The amount of OH calculated from the structural formula is variable (2.9-3.7 apfu), positively correlated with Mg contents and negatively correlated with R3+ (= Al + Fe3+). This is consistent with the exchange vector R3+O(MgOH)-1. The cap rock tourmaline is best described as a solid solution between ferrian 'oxy-dravite' and povondraite. Four discrete pyramidal sectors display chemical sector zoning, two well-defined sectors occur at the (+) c-pole of the crystal (o-sector and r-sector) and two sectors occur at the (-) c-pole of the crystal (o-sector and r-sector). The (-) c-pole pyramidal sectors are strongly enriched in Fe3+, Ti, Ca and O, whereas the (+) c-pole pyramidal sectors are enriched in Al, Mg and OH. At the (+) c-pole there are also well-developed triangular-shaped o-sectors separated by r-sector bands with relative Fe-enrichment at the r-sector at the expense of Al. Within given o-sectors there is a significant amount of oscillatory zoning involving Fe(Al)-1 exchange. The magnitude of the chemical sector zoning in the cap rock tourmaline is one of the largest recorded in minerals. The influence of surface energies at distinct faces of cap rock tourmaline apparently dramatically affects cation incorporation at the relatively low temperature of formation (< 150°C). The 'oxy-dravite'-povondraite solid solution trends found in the cap rock tourmalines is similar to that developed in other ferrian tourmalines from oxidized meta-evaporites, meta-arkoses, quartz-tourmaline veins, stockwork veins associated with granitic porphyry metal deposits, and hydrothermal veins in metasediments. However, the great range of compositions found in the cap rock tourmalines is rarely attained in other ferrian tourmaline occurrences. Extrapolation of the cap rock and other ferrian tourmaline data on Al-free basis results in a composition very similar to the type material used to define the povondraite end-member.

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European Journal of Mineralogy

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