Chemistry of alkali-deficient schorls from two tourmaline-dumortierite deposits

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. In the schorls from Jack Creek, substitution of Al for Si in the six-membered ring is charge-balanced by substitution of Al for divalent cations in the octahedral sites (Tschermak's substitution). In the schorls from the external zone of the Ben Lomond deposit, [4]Al substitution is primarily charge- compensated by either substitution of (Na,Ca) for a vacancy or Ca for Na in the 9-coordinated alkali-site.