THE CRYSTAL STRUCTURE OF NORMANDITE AND ITS CRYSTAL-CHEMICAL RELATIONSHIPS WITH LAVENITE

Abstract

The crystal structure of normandite from Amdrup Fjord, Greenland and from Mont Saint-Hilaire, Quebec, was solved and refined to a residual R1 of 0.033 and 0.047, respectively. Normandite is a member of the cuspidine group and, more precisely, the Ca‐Ti-dominant analogue of lavenite. Consequently, its structure can be described in terms of “walls” of octahedra, interconnected by corner-sharing and through disilicate groups. These structural “modules” 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 elements Na, Ca, Mn, Zr, and Ti, such that a “lavenitic” core gives way to a “normanditic” margin. From these data, the following ideal crystal-chemical formulae are derived: NaCaMnTiSi 2O7OF for normandite and NaNaMZrSi2O7F2 (M = Mn, Fe, Ca, Ti) for lavenite.