Morphology, Twinning, and Optical Orientation of Gismondine

Rab Nawaz
Department of Geology, Ulster Museum, Belfast BT9 5AB

Summary: Gismondine is a monoclinic mineral but prior to X-ray determination of its unit cell it was variously regarded as orthorhombic, monoclinic, or triclinic on account of its intricate twinning. A study of gismondine crystals from Osa Quarry, near Rome, indicates that the morphologically dominant form is {¯232} which by twinning about the normal to {100} produces penetration-twinned octahedral crystals united on (100) and (001). Gismondine has poor cleavage on {¯232s optic plane is (010) and γ:[001] = 42.5° (in obtuse β).

Mineralogical Magazine; September 1980 v. 43; no. 331; p. 841-844; DOI: 10.1180/minmag.1980.043.331.03
© 1980, The Mineralogical Society
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