The systems albite‐orthoclase‐water and albite‐orthoclase‐quartz‐water: Chemographic phase relationships

Phase relationships in the systems NaAlSi3O8‐KAlSi3O8‐H2O and NaAlSi3O8‐KAlSi3O8‐SiO2‐H2O to 10 kbars pressure are discussed using pressure‐temperature projections and isobaric, polythermal projections. Complexities in these systems result at about 4.2 and 2.5 kbar, respectively, as the result of critical endpoints generated by the intersections of the vapor‐saturated solidi and the feldspar solvi, previously investigated experimentally by Seek and by Morse. A little‐known method derived by Hillert is used to distinguish eutectic and peritectic reactions involving alkali feldspars and water‐undersaturated liquids.