Anorthoclases or plagioclases? A problem of nomenclature.

In this paper we intended to examine the results of X-ray investigations, by means of the high temperature preccssion method, on four feldspars of varying composition which come from a trachyte of the Euganean Hills (North Italy). The curves of the variation of alfa* as a function of temperature were obtained, and the inversion temperatures to geometrically monoclinic symmetry were extrapolated. A comparison between these and the melting temperatures shows that there are three feldspars which, according to the nomenclature proposed by SMITH & MACKENZIE (1958), are clearly definable-two as plagioclases, and one as an orthoclase; with the fourth case, however, the definition of the feldspar using the above mentioned nomenclature is not in agreement with the commonly-held mineralogic-petrographic meaning of the significance of the term plagioclase. It is for this reason that we recommend the use of the exclusively chemical criterion proposed by SMITH (1974) for an unambiguous definition of plagioclases and an-orthoclases.