Trachyte pyroxenes in the Euganean Hills (Northern Italy)

Wet and microprobe analyses of the green and brown pyroxenes of the trachytes of the Euganean Hills showed them to be augites with small contents of sodium. The green pyroxenes, occurring in almost all the samples, were only slightly or not at all zoned, and could be placed in two groups differentiated by small but significant variations of elements Fe3+, Mn, Al VI, Na, P and Mg. The brown pyroxene, occurring only in four of the six trachyte samples, show appreciable differences in elements Si, Al and Ti, both because of zoning in the same grain and between one grain and another. In some of them, zones of much more aluminifcrous augitic pyroxene and zones of calcium-poor pyroxene were both present. The brown pyroxene generally showed greater contents of Al IV, Al VI and Ti and lower contents of Si, Mn and Fe tot than the green ones. The chemical differences observed may be related to pressure differences at the moment of crystallization.