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

Sector zoned tourmaline, exhibiting striking compositional differences amon g pyramidal sectors, occurs in the cap rock of the salt dome associated wit h the Challenger Knoll, Gulf of Mexico. Trace amounts of fine-grained (10-8 0 mu m) euhedral authigenic tourmaline are included in calcite as part of t he carbonate-rich portion of the cap rock. Compositional, textural and geol ogical data are most consistent with an authigenic origin of the tourmaline in bedded halite under oxidizing conditions prior to incorporation into th e cap rock. The cap rock tourmaline most commonly has a tabular morphology, and sections cut through the crystals display impressive optical and chemi cal 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 ex change vector Fe(Al)(-1). The amount of OH calculated from the structural f ormula is variable (2.9-3.7 apfu), positively correlated with Mg contents a nd negatively correlated with R3+ (= Al + Fe3+). This is consistent with th e 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-de fined 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-se ctor). 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 a nd 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-se ctor at the expense of Al. Within given a-sectors there is a significant am ount of oscillatory zoning involving Fe(AI)(-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 degrees C). The "oxy-dravite"-povondraite solid solution trends found in the cap rock t ourmalines is similar to that developed in other ferrian tourmalines from o xidized meta-evaporites, meta-arkoses, quam-tourmaline veins, stockwork vei ns associated with granitic porphyry metal deposits, and hydrothermal veins in metasediments. However, the great range of compositions found in the ca p rock tourmalines is rarely attained in other ferrian tourmaline occurrenc es. Extrapolation of the cap rock and other ferrian tourmaline data to an A l-free basis results in a composition very similar to the type material use d to define the povondraite end-member.