TEM OBSERVATIONS AND X-RAY-POWDER DATA ON LAMPROPHYLLITE POLYTYPES FROM GARDINER COMPLEX, EAST GREENLAND

Lamprophyllite and barytolamprophyllite from the Tertiary Gardiner Com plex, fan Greenland (JOHNSEN et al. 1994), are investigated by TEM and Rietveld analysis. Results show that the lamprophyllite sample is com posed of both an orthorhombic and a monoclinic modification of lamprop hyllite with minor amounts of natrolite, while the barytolamprophyllit e sample proved to be monoclinic only. Quantitative analysis performed by partial Rietveld analysis show that the orthorhombic modification constitutes >50%mass of the lamprophyllite sample. The two lamprophyll ite modifications are composed of essentially identical layers that ar e stacked in two different ways. Accordingly, it is proposed to introd uce polytype nomenclature in the lamprophyllite group. Based on the pr esent study three polytypes can be identified: lamprophyllite-2M, lamp rophyllite-20 and barytolamprophyllite-2M. Refined unit cell parameter s of the lamprophyllite polytypes are presented.