Temperature effects on cathodoluminescence of Ba-feldspar

Celsian (BaAl₂Si₂O₈: Cn) is an end member of a solid solution with K-feldspar, whereas hyalophane (Hy) is uses for intermediate composition (10-80 mol/% of Cn). Cathodoluminescence (CL) of feldspar has been extensively investigated in the fields of geoscience and planetary science, and as a result the details of the emission centers in K-Feldspar and plagioclase has been reported so far (e.g., Gotze et al., 2000; Kayama et al., 2010). However, the CL of Ba-feldspar have been little studied with respect to the emission mechanism. In this study, we have clarified to assign the emission center and luminescent characteristics upon varied temperature of Ba-feldspar by CL spectral analysis.

CL spectra of celsian have three broad band emissions at around 470 nm in a blue region, at around 560 nm in a yellow region and at around 725 nm in a red region. These emissions are assigned to Al-O^--Al defect center, Mn²⁺ impurity center and Fe³⁺ impurity center, respectively. The emission centers are closely-similar to those of anorthite, though the structure of celsian is thought of as being analogous to K-feldspar. When raising sample temperature, the CL intensity hyalophane shows a gradual enhancement, known as temperature quenching effect reported in luminescent materials. The CL intensity of blue emission of celsian decreases with an increase in sample temperature above -193 ℃. However, the CL intensity of red emission increases with an increase in sample temperature from -193 ℃ to -150 ℃, and decreases with an increase in sample temperature above -150 ℃. According to Petrov et al. (1994), the CL intensity of K-feldspar caused by Al-O^--Al defect center indicates a negative correlation to the intensity related to Fe³⁺ impurity center. However, the results for celsian is not concordant with previous work. On the basis of structural change at low temperature (Calleri and Gazzoni, 1975), when the Ba ions occupy most sites of metal ions in the structure of Ba-K feldspar series, the crystal field around Fe³⁺ ions in T-site might change below -150 ℃ due to the influence of lower bonding angle and shorter distance between Ba and ligand oxygens. It suggests that the CL spectral analysis could explain a crystal chemical behavior during the substitution of large Ba ion in M site of Ba series feldspar.