Temperature effect on cathodoluminescence of high-Mn carbonate

Carbonate minerals such as calcite show typical concertation quenching (self-quenching) in their cathodoluminescence (CL), which has been widely applied in earth science, extensively used in carbonate sedimentology. In most cases, the carbonate minerals involve two types of impurity centers for activator of Mn²⁺ and quencher of Fe²⁺. If the content of Fe²⁺ is low in the carbonates, the concentration of Mn²⁺ almost controls the emission intensity of the carbonate CL. It is believed that the CL activated by Mn²⁺ should be quenched above around 50,000 ppm of Mn due to concertation quenching (e.g., Machel et al., 1991), whereas some carbonates with high Mn contents have been reported so far (e. g., Polgári et al., 2007). However, the mechanism of CL emission in high-Mn carbonates has not yet been elucidated. In this study, we have conducted to clarify the emission mechanism in kutnohorite with high Mn content by spectral analysis of CL data obtained at various temperature.

The single crystals of several kutnohorite from various localities were employed for CL measurements. An SEM-CL was used to measure CL spectra ranging from 300 to 800 nm under control of sample temperature in the range from -196 to 17 °C with a cryo- and heating-stage.

Most samples show bright to dull red CL, where they have high content of Mn up to around 15 wt.%. The CL spectra at 17 °C exhibit almost a similar pattern with a single broad peak at 646 nm assigned to the electronic transition from the excited ⁴G to the ground state ⁶S in Mn²⁺ activator. When rising sample temperature from near liquid nitrogen temperature, the emission peak shits to higher wavelength side, suggesting a strong effect of crystal field (Mn-ligands distance). The intensity of the emission component evaluated by CL spectral analysis shows an intense reduction with an increase in sample temperature, corresponding to thermal quenching reported in luminescent materials. The activation energy in this process is estimated at 0.32 eV, of which energy might be transferred to neighbor Mn activators possibly as unit equivalent to integral multiples of activation energy. Therefore, even when the kutnohorite occurs a significant concentration quenching, it can still emit bright CL. It might conclusively suggest the relation of CL emission to specific Mn sites occupied in kutnohorite structure.