Morphology, twinning, and optical orientation of gismondine

SUMMARY. Gismondine is a monoclinic mineral but prior to X-ray determination of its unit cell it was variously regarded as orthorhombic, monoclinic, or tri-clinic on account of its intricate twinning. A study of gismondine crystals from Osa Quarry, near Rome, indi-cates that the morphologically dominant form is {~32} which by twinning about he normal to {Ioo} produces penetration-twinned octahedral crystals united on (ioo) and (oo0. Gismondine has poor cleavage on {~.32}; optic plane is (010) and ~:[0OI] = 42.5 ~ (in obtuse/~). GISMONDINE is a monoclinic zeolite mineral, which on account of its penetration twinning (fig. Ia, b) shows tetragonal or orthorhombic pseudosymmetry. It was named by K. C. yon Leonhard (1817) after Professor Gismondi wh