A sample stage and a control system have been developed to vary the humidity and temperature of the atmosphere surrounding a powder sample during an X-ray-diffraction experiment. The use of this stage and a position-sensitive detector creates the opportunity to conduct experiments that determine the stability fi elds and reaction mechanisms of related sulfates of different states of hydration, such as melanterite, siderotil and rozenite. The sample is dusted onto a zero-background plate, and this small amount of dispersed sample ensures maximum interaction among all of the crystallites of the sample and the atmosphere. Determination of the phase boundary for the dehydration of melanterite to rozenite is compared to that determined by Chou et al. (2002). The rate of dehydration of cuprian melanterite to cuprian siderotil is shown to depend on the relative humidity in the chamber, which is consistent with a diffusion-controlled process. Rozenite is observed to form by very rapid dehydration of cuprian melanterite. The crystallization of melanterite from liquid under controlled humidity is demonstrated.
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