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

Sector zoned tourmaline, showing striking compositional variations in pyramidal sectors, is described from the cap rock of the salt dome associated with the Challenger Knoll, Gulf of Mexico. Trace amounts of fine-grained (10-80 mu m) euhedral authigenic tourmaline are included in calcite forming part of the carbonate-rich portion of the cap rock. Compositional, textural and geological data are 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 commonly has a tabular morphology, with optical and chemical zoning, this tourmaline is magnesian but surprisingly rich in Fe (super 3+) , poor in Ca and deficient in Al, and is best described as a solid solution between ferrian 'oxy-dravite' and povondraite. Four discrete pyramidal sectors display chemical sector zoning of a magnitude rarely recorded in minerals. The influence of surface energies at distinct faces of cap rock tourmaline apparently dramatically affects cation incorporation at the relatively low T of formation (< 150 degrees C). This 'oxy-dravite'-povondraite solid solution trend is similar to that developed in other ferrian tourmalines from oxidized meta-evaporites, meta-arkoses, quartz-tourmaline veins, stockwork veins associated with granite porphyry metal deposits and hydrothermal veins in metasediments.