17:15 〜 18:30
[SMP26-P06] Unique luminescence of rhodochrosite at low temperature
キーワード：ルミネッセンス、菱マンガン鉱、温度消光、濃度消光
Cathodoluminescence (CL) of carbonate minerals has been widely applied in earth science as it has become a common method in carbonate sedimentology. In most cases of calcite-type carbonate minerals, their CL involve two types of impurity centers as activator of Mn2+ and quencher of Fe2+. If the content of Fe2+ is low in the carbonates, the concentration of Mn2+ almost controls the CL emission intensity. It has been believed that the CL activated by Mn2+ should be quenched above ~50,000 ppm of Mn due to concertation quenching (e.g., Machel et al., 1991). Walker et al. (1989) reported that rhodochrosite (MnCO3) emits an intense luminescence below 77 K, but extinction at room temperature. 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 rhodochrosite by spectral analyses of CL data obtained at various temperatures.
The single crystals of fourteen rhodochrosite from various localities were employed for CL measurements. The CL spectra were obtained by using an SEM-CL system (SEM combined with a grating monochromator) ranging from 300 to 800 nm under a control of sample temperature from −192 to 25 ºC with a cryo-stage. The digitized data were totally sensitive-corrected for all optical paths including grating and photomultiplier tube.
Most samples begin to emit CL in a red region below −100 ºC and increase rapidly during cooling down to near liquid nitrogen temperature, whereas no emission was detected in all samples at room temperature. The CL data were converted into energy unit from wavelength unit for quantitative spectral analysis. Each spectrum is composed of a broad emission band at ~700 nm, which shows clearly higher than previously reported values of calcite: Mn (~620 nm) and magnesite: Mn (~650 nm). It implies that the crystal-field strength (Dq) should be quite higher than other calcite-type carbonates due to a shorter distance between metal (Mn) and ligand in rhodochrosite. A drastic reduction of the CL intensity with increasing temperature can be well explained by a temperature quenching theory known as Mott-Seitz model based on an increase in the probability of non-radiative transition with the rise of temperature (calculated activation energy of ~0.03 eV). The fact suggests that a sample temperature strongly controls the travelling range of the energy, which causes concertation quenching by transfer to the adjacent Mn ions. Consequently, a unique CL in rhodochrosite should be understood as the result of the competition between the effects of temperature quenching and concertation quenching possibly by Förster energy transfer (FRET) model.
<references>
Machel, G., Mason, R., Mariano, N. and Mucci, A. (1991) Causes and emission of luminescence in calcite and dolomite, SEPM Short Course Notes, 25, 9-25.
Walker, G., Abumere, O. and Kamaluddin, B. (1989) Luminescence spectroscopy of Mn2+ centers in rock-forming carbonates, Mineralogical Magazine, 53, 103-140.
The single crystals of fourteen rhodochrosite from various localities were employed for CL measurements. The CL spectra were obtained by using an SEM-CL system (SEM combined with a grating monochromator) ranging from 300 to 800 nm under a control of sample temperature from −192 to 25 ºC with a cryo-stage. The digitized data were totally sensitive-corrected for all optical paths including grating and photomultiplier tube.
Most samples begin to emit CL in a red region below −100 ºC and increase rapidly during cooling down to near liquid nitrogen temperature, whereas no emission was detected in all samples at room temperature. The CL data were converted into energy unit from wavelength unit for quantitative spectral analysis. Each spectrum is composed of a broad emission band at ~700 nm, which shows clearly higher than previously reported values of calcite: Mn (~620 nm) and magnesite: Mn (~650 nm). It implies that the crystal-field strength (Dq) should be quite higher than other calcite-type carbonates due to a shorter distance between metal (Mn) and ligand in rhodochrosite. A drastic reduction of the CL intensity with increasing temperature can be well explained by a temperature quenching theory known as Mott-Seitz model based on an increase in the probability of non-radiative transition with the rise of temperature (calculated activation energy of ~0.03 eV). The fact suggests that a sample temperature strongly controls the travelling range of the energy, which causes concertation quenching by transfer to the adjacent Mn ions. Consequently, a unique CL in rhodochrosite should be understood as the result of the competition between the effects of temperature quenching and concertation quenching possibly by Förster energy transfer (FRET) model.
<references>
Machel, G., Mason, R., Mariano, N. and Mucci, A. (1991) Causes and emission of luminescence in calcite and dolomite, SEPM Short Course Notes, 25, 9-25.
Walker, G., Abumere, O. and Kamaluddin, B. (1989) Luminescence spectroscopy of Mn2+ centers in rock-forming carbonates, Mineralogical Magazine, 53, 103-140.