SUMMARY. Plots of partition coefficient (mineral/matrix)

against ionic radius of the elements inseventeen biotites from trachytes and rhyolites of the Euganean Hills do not show differences in the type of diagram between the biotites with tetrahedral sites completely filled by Si and A1, and those in which filling is only possible with the intervention of Fe 3+ (or Ti 4+) since these sites are incompletely filled by Si and A1. This behaviour seems to exclude, for the latter, the presence ofFe 3 § (or Ti a§ in the tetrahedral site. Similar indications are given by structural analysis. The possibility of existence of [(OH)4] 4- groups or, more probably, vacancies inthese sites is therefore suggested. The consequences that these results would have on the possibility of correct calculation of the crystaUo-chemical formula re discussed. PLOTTING the partition coefficients between phenocryst pyroxenes and groundmass, Onuma et al. (1968) produced iagrams which revealed the dominant role of phenocryst crystal structure in determining the partition coefficient patterns between the phenocryst and its lava matrix. The later data of Higuchi and Nagasawa (i969), Higuchi et al. (I97O), Jensen (I973), Matsui et al. (I977) , and De Pieri and Quareni (I978) confirmed the occurrence in these diagrams of families of subparallel curves, having approximately the same general slope and peaking in the same points, probably indicating the optimum ionic radius for a cation in a given structural site. As regards the biotites, the data of Matsui et al, (1977) regarding major and trace elements indicate the presence of two peaks, one representing the site of the cations in sixfold coordination and the other the K site in approximately twelvefold coordina-tion. The greater or lesser deviations from these curves, shown by Co 2+ and Cr ~+, indicate the presence of effects due to the properties of the magmas. De Pieri et al. (I978) have recently reported ata on the chemistry of seventeen biotites from trachytes and alkali rhyolites from the Euganean Hills (northern Italy). The analytical results clearly show that there are notable differences between th