The Canadian Mineralogist
Volume 34, pages 615-621 (1996)
TWINNED HAMBERGITE FROM THE GILGIT DISTRICT, NORTHERN PAKISTAN
R. PETER RICHARDS
Morphogenesis, Inc., 154 Morgan Street, Oberlin, Ohio 44074, U.S.A.
Abstract
Hambergite crystals from a locality in northern Pakistan occur attached to elbaite, and show two different habits on the same specimen.
Neither habit has been previously described. Crystals of the first habit are twinned by reflection on {110} and consist of plates
flattened parallel to the twin plane, and bounded by {001}, {100}, {010}, {210}, {110}, and {341}. Crystals of the second habit, including the main
crystal on the specimen, are also twinned and have a conspicuously hemimorphic habit. They are double twins by reflection on {110}, composed
of a large central crystal in twinned relationship to two platy crystals, one on each side. Twin boundaries are marked by re-entrant grooves,
and by optical discontinuities observable even in unpolarized light at low magnification under a binocular microscope. The forms present include those
on the platy twins, plus {241}. The difference between the two habits results from differences in the effect of twinning on crystal
growth in the presence of one or two twin planes. The observed hemimorphy is a property of the twined aggregate, and does not call into
question the holohedral symmetry of untwinned hambergite.