Aluminous, alkali-deficient schorls occur with dumortierite in hydrothermally altered tuffs at Jack Creek, Jefferson County, Montana, and Ben Lomond, Hervey Range, North Queensland, Australia. The composition of tourmaline from both deposits, e.g., that of a schorl from Jack Creek,
closely approaches that of the alkali-defect end-member,
in response to the alkali-defect substitution, R+ + R2+ = R3+. In addition, tourmalines with a small alkali defect component frequently show significant ratAl substitution, as shown by the composition of this crystal from the Jack Creek deposit,
In the schorls from Jack Creek, substitution of Al for Si in the six-membered ring is chargebalanced by substitution of Al for divalent cations in the octahedral sites (Tschermak's subsitution). In the schorls from the external zone of the Ben Lomond deposit, Al substitution is primarily charge-compensatedb y either substitution of (Na,Ca) for a vacancy or Ca for Na in the 9-coordinated alkali-site. This accounts for the limited alkali-site defect found in Ben Lomond tourmalines, which have the most extensive Al substitution. The coupling oftetrahedral- and alkali-site occupancies appears to help satisfy the chargesaturation requirements of the oxygen atoms shared by these sites.
The sensitivity of chemistry of tourmaline to the chemical and physical conditions that prevail during its crystallization emphasize its potential as a petrologic indicator mineral.