Long-range-short-range order in calcic plagioclases as a continuous and reversible function of temperature

A continuous structural change in anorthite (CaALSi2Os) and contiguous plagioclases is described as a function of temperature and NaA1Si30 s content.In pure anorthite, this transformation takes place over a temperature range of approximately 450 ° C., and is complete or nearly complete at the melting point (1554 ° C.).The presence of NaA1Si30 s in solid solution greatly extends this temperature range.The transformation is manifested in single-crystal X-ray photographs by the variation in diffuseness of certain X-ray reflections, as a continuous and reversible function of temperature.To interpret the observations it is suggested that two structurally equivalent Ca( + Na) atom positions permit the development of out-of-step domains; the size (and shape) of these Ca(+Na) atom domains is an equilibrium function of temperature.The ~ime required for the establishment of domain equilibrium varies with temperature, from a matter of minutes in the upper temperature range, to several days at approximately 800 ° C. The transition is observable in plagioclase crystals of composition ranging from pure anorthite to approximately 27 % NaA1SiaO 8.