Characteristics of emission centers in alkali feldspar: A new approach by using cathodoluminescence spectral deconvolution

Abstract

Alkali feldspars in syenite from the Cerro Balmaceda pluton in the Patagonian Andes, Chile, show various petrographic microtextures formed during the magmatic to high-and low-temperature hydrothermal stages in which cathodoluminescence (CL) shows a wide range of blue, violet, and pink to red colors with variable brightness. Their CL spectra exhibit two emission bands: one at 405-420 nm in the blue region and the other at 700-760 nm in the red-infrared (IR) region. Asymmetrically shaped spectral peaks in energy units suggest overlapping of each individual emission, which corresponds to various luminescence centers. Blue emission bands were separated into two spectral peaks fitted by Gaussian curves centered at 3.055-3.076 and 2.815-2.845 eV. A positive correlation is found between emission intensities at 3.055-3.076 eV and TiO 2 contents, suggesting the activation of a Ti 4+ impurity as an emission center. The intensities at 2.815-2.845 eV, where clear and featureless feldspar (CF; not affected by hydrothermal metasomatism) is shown under optical microscopy, which have intensities appreciably higher than those showing patched microperthite (PMP), formed during low-temperature hydrothermal reactions, correlate reciprocally with the intensities of red-IR emission caused by a Fe 3+ impurity center. The peak at 2.815-2.845 eV can be attributed to oxygen defects associated with Al-O-Al and Al-O-Ti bridges. Most of the areas show CL emissions at 700-760 nm in the red-IR region, in which intensities increase with an increase in Fe 2 O 3 contents as impurities. The Fe 3+ ion acts as an activator for the red-IR emission. The Ab-rich and Or-rich phases of PMP have emission components at 1.644 eV (754 nm) and 1.727 eV (717 nm), respectively. The red-IR emission from CF consists of emission components at 1.677 eV (739 nm) and 1.557 eV (796 nm), according to an Fe 3+ impurity center in the Or-rich phase and in the Ab-rich phase as cryptoperthite, respectively. Both components are centered at a wavelength longer than the emission band of Ab-rich and Or-rich phases of PMP, suggesting a change in configurational state around the Fe 3+ ion from the T2 to the T1 site by low-temperature hydrothermal metasomatic reactions. Accordingly, the peak positions of the red-IR emission are controlled by the ordering state of Fe 3+ ion into the T1 site, the existence of multiphase perthite and chemical composition.

Response of cathodoluminescence of alkali feldspar to He+ ion implantation and electron irradiation

“…Unimplanted, He + -ionimplanted and electron irradiated anorthoclase and adularia have an emission band at ~420 nm in blue region (Figs. 2 and 4), which is activated by Ti 4+ impurity centers (Mariano et al, 1973;Finch and Klein, 1999;Götze et al, 2000;Lee et al, 2007;Parsons et al, 2008;Kayama et al, 2010). CL intensities of these UV and blue emissions decrease with increases in radiation dose of He + ion implantation and electron irradiation time.…”

“…Highresolution CL images were observed using a Gatan: MiniCL imaging system under the same conditions as CL spectral analysis using SEM-CL. More details of the equipment construction and analytical procedures can be found in Ikenaga et al (2000) and Kayama et al (2010).…”

“…3). According to Kayama et al (2010), the emission band in blue spectral region consists of an overlap of two emission components, namely the oxygen defect centers associated with Al-O-Al and Al-O-Ti bridges (Al-O --Al/Ti defect center) and the Ti 4+ impurity center. The component assigned to Al-O --Al/Ti defect center is located at longer peak wavelength (2.815-2.845 eV) than that to Ti 4+ impurity centers (3.055-3.076 eV).…”

“…2 and 3), and are attributed to a Fe 3+ impurity that substitutes for Al 3+ ions in tetrahedral sites for the luminescence caused by the radiative transition of the electrons from 4 T 1 to 6 A 1 (Telfer and Walker, 1978;Finch and Klein, 1999;Götze et al, 2000). According to Finch and Klein (1999) and Kayama et al (2010), two components assigned to Fe 3+ impurity centers on T1 and T2 sites constitute the red-IR CL emission. There is an increase in the red-IR emission intensities of the anorthoclase and amazonite with enhanced radiation dose of He + ion implantation and electron irradiation time.…”

Cathodoluminescence of synthetic zircon implanted by He⁺ ion

“…Therefore, a spectral deconvolution should be required for the characterization of constituent emission components in zircon CL. Recently, Tsuchiya et al (2015) have demonstrated that a spectral deconvolution of natural zircon CL can be successfully carried out using a Gaussian function in energy units by the peak-fitting method proposed by Stevens-Kalceff (2009) and Kayama et al (2010). We deconvolute the CL spectra of unimplanted and He + -ion implanted samples with initial peak energies determined by Tsuchiya et al (2015) using the peak-fitting software (Peak Analyzer) implemented in Origin Pro 8.1J SR2.…”