Reconstruction of low-temperature binary phase diagrams using a constrained least squares method; application to the H (sub 2) O-CsCl system

A method is proposed for drawing an X-T projection from scarce data, e.g. for the system H (sub 2) O-CsCl whose binary curves are completely unknown. Using a solution of CsCl with salinity close to the roughly estimated eutectic composition, synthetic fluid inclusions were made in order to measure the eutectic T by microthermometry (-23.7 degrees C). Raman spectroscopy confirms that no hydrate exists. A general mathematical model based on a constrained least squares method was set up to fit the two curves, taking into account the eutectic T and the triple point of water. The eutectic composition was thus calculated by minimizing the global variance of the model (56.87 wt.% CsCl).