Determination and estimation of magnesium content in the single phase magnesium-calcite [Ca(1-x)MgxCO3(s)] using electron probe micro-analysis (EPMA) and X-ray diffraction (XRD)

Mg-calcite (MgCc) is the term used for calcite containing variable magnesium content. The correct determination of the Mg-content in the calcite is of great interest for many fields of research. This study presents the potential, accuracy and the limitations of determining Mg-content in MgCc by means of electron probe micro-analysis (EPMA) coupled with the energy dispersive and wave dispersive spectrometry (EDS/WDS), and X-ray diffractometry (XRD). These techniques were used to examine the distribution of Mg (in mol% MgCO3) in six calcite marble samples from different locations of Peshawar basin (Pakistan), a part of Lesser Himalayas. Results showed variable Mg-content averaging 0.944-1.740 mol% from the chemical analysis with EPMA/EDS of the whole rock sample. This was almost consistent with the XRD findings of 0.750-1.690 mol%. The sample NO13 with heteroblastic grain structure showed disequilibrium geometry due to the contact metamorphism, characterized by the relatively high temperature and low pressure. This caused predominantly quick re-crystallization of the carbonate phase, thus showing lower Mg-content in the sample. It was assumed that the observed small variability in the Mg-content of the investigated calcites even with in the sample is due to the temperature dependency of the Mg incorporation into the calcites. The highest degree of accuracy in Mg-content determination was observed based on the lattice parameter a and cell volume V. For the Mg-content obtained by XRD, best correlation was observed between the lattice parameter a, cell volume V and Mg-content, with r (2) = 0.991 and 0.990 respectively. The difference between the d (104) values from the Rietveld refinement and the observed XRD patterns were generally < 0.002 .