Abstract
Beta‐uranophane from Perus, SP, Brazil‐ has been heated progressively up to 1.350°C, and its thermal behaviour has been investigated thoroughly by several methods: thermogravimetry, differential thermal analysis, optical measurements (variation of refraction indices), infra‐red spectroscopy, and X‐ray diffraction (powder photographs at room temperature of previously heated material, and powder photographs at high temperatures in special cameras). – The mineral seems to be stable up to a temperature of 200°C, confirmed by the several experimental procedures. The differential thermal analysis gives a sharper point of transformation at 165°C, which should be ascribed to water evaporation (zeolitic water). At this temperature the beta‐uranophane should change to another isostructural compound, containing a small amount of crystallization water. Alternatively, crystal structure of the mineral could be transformed into a random layer structure, due probably to loss of the structural water. This kind of crystal disorder induces, as a rule, in the X‐ray powder diagrams, absences of the hkl reflections, leaving merely the prismatic (hk__0 and 0__kl) and the pinacoidal reflections (h__00, 0__k__0, 00__l, and h__0__l). This could explain the small number of observed lines in the powder photographs of the material heated above 200°C.
A new phase transformation is reported in the temperature interval of 700 to 800°C. This transformation should be related to the beta‐uranophane chemical breakdown into two or more new crystalline phases. The chemical composition of such products is discussed in the paper. As the temperature rises to a higher level, a phase transformation takes place between 1000 and 1200°C and yet another one in the range of 1200 to 1350°C. The latter transition has been detected with the high temperature camera, and could be ascribed to the formation of an alloy between the platinum holder and the breakdown products of beta‐uranophane.
Infra‐red espectroscopy has been proved inadequate for the study of the phase transformations of beta‐uranophane. However, some conclusions could be reached in connection with the water content at several temperature levels.