Chabazite-offretite epitaxial overgrowths in cornubianite from Passo Forcel Rosso, Adamello, Italy
Passaglia, Elio; Tagliavini, Alessandra
European Journal of Mineralogy Volume 6 Number 3 (1994), p. 397 - 406
published: Jun 7, 1994
manuscript accepted: Jan 21, 1994
manuscript received: Jul 5, 1993
ArtNo. ESP147050603006, Price: 29.00 €
Abstract The first occurrence of the zeolite offretite in Italy is described in a cornubianite rock from Passo Forcel Rosso, Adamello. The mineral occurs 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, showing that the Si/Al ratio and K content are relatively constant, whereas 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.