Mn 2+-activated luminescence in dolomite, calcite and magnesite: quantitative determination of manganese and site distribution by EPR and CL spectroscopy

Abstract

Dolomite, calcite and magnesite were studied by EPR and CL spectroscopy. Mn 2+-activated luminescence in calcite and magnesite is characterized by single Gaussian peaks at 16,250 cm −1 (615 nm) and at 15,300 cm −1 (654 nm), respectively. In calcite the Mn 2+ content and the normalized EPR area are linearly correlated when the Mn 2+ content is < 800 ppm. At higher Mn 2+ concentrations, the EPR signal broadens due to ferromagnetic interactions. In contrast to the normalized EPR area, the CL intensity is nonlinearly related to the Mn 2+ concentration. Deconvolution of corrected CL spectra from dolomite shows two overlapping peaks at 17,315 cm −1 (578 nm) and 15,270 cm −1 (655 nm). In rare cases only the 15,270 cm −1 (655 nm) Gaussian peak is present. The CL peak at 17,315 cm −1 (578 nm, I a) is due to Mn 2+ in the Ca site and the peak at 15,270 cm −1 (655 nm, I b) to Mn 2+ in the Mg position. The Mn 2+ content in dolomite is linearly correlated with the normalized EPR areas of the two sites. The distribution ratio ( K D) of Mn 2+ in the two sites of dolomite, i.e. the Ca and the Mg position, was determined by EPR and CL spectroscopy. A linear correlation exists between K D from EPR and the intensity ratio ( I b I a from CL. CL spectroscopy is thus an appropriate method to determine K D in dolomite. Evaporitic dolomites may be differentiated from non-evaporitic dolomites by their K D-values.