Quantitative evaluation of attenuation contrast of minerals by using a medical X-ray CT scanner

Abstract

CT values (numerical expression for contrast of CT images) of 66 minerals, were measured by using a medical X-ray CT (computed tomography) scanner. These values were compared with the calculated X-ray linear attenuation coefficients of the minerals. The comparison permitted to evaluate the attenuation contrast resolution by polychromatic X-ray beam of the scanner and to estimate the practical restrictions to use the method for mineralogical purposes. Plots of the CT values vs. monochromatic linear attenuation coefficients calculated for different photon energies revealed the existence of the most efficient photon energy giving the best linear correlation. The energies are about 70 and 80 keV for accelerating voltages of 120 and 130 kV, respectively. These values roughly coincide with the peaks of the photon energy spectra of the incoming X-ray beams. The existence of the most efficient energies (hitherto called "effective energies") shows that CT values can be evaluated quantitatively for minerals in a medical CT scanner with polychromatic X-ray beams. Resolution of the attenuation contrast is about 0.05 cm-1 for these two effective energies in the scanner used. It is expected from the contrast resolution that the degree of Mg-Fe substitution in ferromagnesian silicate minerals is easily estimated if the Fe contents are sufficiently different. However, in the Mg-rich compositional range of ultramafic rocks (XMg≈0.9), olivine and Ca pyroxene may not be discriminated due to the overlap of the CT values. The CaAl-NaSi substitution in plagioclase solid solution is also detectable whereas the CT values of quartz and alkali feldspar are within the plagioclase range.