Emission spectrographic analysis of a pale green calcite crystal from Helwan, Egypt, indicated the presence of various impurities including manganese and iron. Optical and paramagnetic resonance absorption spectra before and after X-ray irradiation were measured at room temperature. The results indicated that both the iron and manganese ions are present mainly in their divalent state. The presence of trace Mn(IV) was also indicated. Upon exposure to X-rays, the pale green calcite crystal yielded an orange-red luminescence at room temperature accompanied by a change in its color to brownish-yellow in both its optical and EPR absorption spectra. The observed changes are attributed to the formation of a number of hole and electron trap centers, associated with impurity Fe(II) and Mn(IV) ions forming Fe(III) and excited Mn(II)** centers respectively. Transitions within the Mn(II)** centers, leading to the formation of comparatively more stable Mn(II) ones, are felt responsible for the observed room-temperature phosphorescence. Heating the crystal up to 400°K for one hour caused recombination of the X-ray induced electron and hole trap centers, and produced thermoluminescence by the indirect excitation of emission at the Mn(II) impurity sites.

These data suggest that the calcite crystal precipitated in an environment that was more acidic and with a very low oxidation potential.

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