The crystal structure of normandite and its crystal-chemical relationshipswith lavenite

The crystal structure of normandite from Amdrup Fjord, Greenland and from M ont Saint-Hilaire, Quebec, was solved and refined to a residual R-1 of 0.03 3 and 0.047, respectively. Normandite is a member of the cuspidine group an d, more precisely, the Ca-Ti-dominant analogue of lavenite. Consequently, i ts structure can be described in terms of "walls" of octahedra, interconnec ted by corner-sharing and through disilicate groups. These structural "modu les" are arranged with the same topology as in cuspidine, hiortdahlite-II, janhaugite and niocalite. EPMA and SEM studies of the Amdrup Fjord material show distinct chemical zoning due to a wide Variation of the major element s Na, Ca, Mn, Zr, and Ti, such that a "lavenitic" core gives way to a "norm anditic" margin. From these data, the following ideal crystal-chemical form ulae are derived: NaCaMnTiSi2O7OF for normandite and NaNaMZrSi2O7F2 (M = Mn , Fe, Ca, Ti) for lavenite.