Phase Relations of the Alkali Feldspars: I. Introductory Remarks

Monoclinic K-feldspars and triclinic Na-feldspars at high temperatures form a "continuous" series of mix crystals. The apparent lack of a discontinuity in the monoclinic-triclinic solid-solution series has been puzzling. A transition from monoclinic to triclinic symmetry was found to exist and is here described. The composition at which the transition occurs was found to be a function of temperature: $Or_{40}Ab_{60}$ at ~0° C; $Or_{5}Ab_{95}$ at ~1,000° C. (extrapolated). The transition considered at a fixed composition as a function of temperature has the characteristics of a displacive transformation. The investigation was carried out with natural crystals homogenized at high temperature and quenched. Winchell's term "analbite" is redefined and used for the high-temperature modification of $NaAlSi_{3}O_{8}$. Besides the displacive transformation, another type, or diffusive transformation, is considered. A detailed discussion of available data led to the conclusion that the series $KAlSi_{3}O_{8}-NaAlSi_{3}O_{8}$, stable at high temperature (as a sanidine-barbierite, analbite series), differs from those members stable at low temperature (micro-cline-albite) by a different distribution of the Al and Si ions-disordered at high and ordered at low temperature. A slow and (at least on the potash side) continuous process of ordering in crystals originally disordered is considered to be responsible for the variable appearance of intermediate states (orthoclase, adularia). Besides the displacive transformation, another type, or diffusive transformation, is considered. A detailed discussion of available data led to the conclusion that the series $KAlSi_{3}O_{8}-NaAlSi_{3}O_{8}$, stable at high temperature (as a sanidine-barbierite, analbite series), differs from those members stable at low temperature (micro-cline-albite) by a different distribution of the Al and Si ions-disordered at high and ordered at low temperature. A slow and (at least on the potash side) continuous process of ordering in crystals originally disordered is considered to be responsible for the variable appearance of intermediate states (orthoclase, adularia). The process of K/Na exchange is also discussed in connection with an investigation of different types of perthites, such as those with exsolved albite and those with exsolved analbite. The meaning of the mineral name "anorthoclase" is considered in connection with some remarks on nomenclature of the alkali feldspars. The extension of this nomenclature to ternary feldspars is illustrated, with some results of an investigation of feldspars from rhombporphyries.