Abstract: The behavior of anorthite under the test conditions is similar to that of other feldspars and quartz. The solubility of anorthite in pure water was measured at 300 bars pressure between 200° and 350°C. Under these conditions, anorthite not only partially dissolves, but also disintegrates by shedding small particles of crystalline material which forms, with matter in true solution, a nonhomogeneous suspension. Two zeolites (afwillite and xonotlite) develop at the higher temperatures. Their stability is determined by the CaO :SiO2 ratio of the solution.
The anorthite fragments react with the solution. They maintain their identity although their composition and structure gradually change over a wide temperature range. Possibly the zeolites inherit their structure from the altered anorthite. It is almost certain that xonotlite inherits its structure from afwillite.