Growth texture and symmetry of heulandite-Ca from Poona, India

Heulandite-Ca specimens from Poona, India consist mainly of {(1) over bar 1 1}, {001}, {100} growth sectors. The growth sectors are optically triclinic , though the optical extinction angles relative to the b axis are larger in the {(1) over bar 11} and {001} sectors than in the {100} sector. All cell dimensions, determined by single-crystal X-ray methods, indicate that the {(1) over bar 11} and {001} sectors are triclinic and the {100} sector is m onoclinic or nearly so. The triclinic structures are explained by the order ing of Al and Si on the crystal faces.