FLUORINE VARIATION IN HAMBERGITE FROM GRANITIC PEGMATITES

Considerable F-for-OH substitution can occur in the structure of hambe rgite, Be-2(BO3)(OH,F). The substitution results in a linear reduction of the indices of refraction. This relation can be used to estimate t he F content of hambergite, subject to the constraint that samples may be compositionally heterogeneous. Previous X-ray-diffraction studies of five crystals of hambergite showed that increasing amounts of F for OH lead to a decrease in the a dimension and an expansion of the b di mension. The F content of hambergite may be readily estimated if the u nit-cell dimensions are known. Although originally described from an a lkaline syenitic pegmatite, the majority of hambergite occurrences are in complex Li-rich granitic pegmatites of the elbaite or transitional elbaite-lepidolite subtypes. Hambergite occurs as a comparatively ear ly phase in massive pegmatites from several localities in the Czech Re public. More commonly, hambergite is a late-crystallizing phase produc ed in miarolitic pockets as the highly evolved magma approaches its so lidus. The F content of hambergite from pegmatites appears to reflect the F activity of the medium (melt or fluid) from which it crystallize s. Hambergite found in miarolitic pockets is typically more homogeneou s in composition and commonly has a lower F content than that found in a massive pegmatite, although F-rich examples from pegmatitic peckers also are observed. It is unclear whether the lower F content of hambe rgite from miarolitic pockets is strictly a result of crystallization in lower-F systems, or involves precipitation in equilibrium with exso lved aqueous fluids.