CHABAZITE-OFFRETITE EPITAXIAL OVERGROWTHS IN CORNUBIANITE FROM PASSO-FORCEL-ROSSO, ADAMELLO, ITALY

The first occurrence of the zeolite offretite in Italy is described in a cornubianite rock from Passo Forcel Rosso, Adamello. The mineral oc curs as milky-white fibrous overgrowths in structural epitaxy on the ( 0001) crystal faces of hyaline platy chabazite (''herschelite habit'') , associated with large chabazite rhombohedra and yellowish spherules of saponite. Crystal chemical characterization of the zeolitic phases indicates anomalously high Mg contents in the offretite as well as the chabazite overgrowths. The chemistry of the offretites is reviewed, s howing that the Si/Al ratio and K content are relatively constant, whe reas the Ca/Mg ratio varies considerably, there being very few Mg-rich samples. The paragenesis of the newly-formed minerals is interpreted in the light of the host-rock chemistry.