Cathodoluminescence and Raman Spectroscopic Characterization of Experimentally Shocked Plagioclase

Abstract

Cathodoluminescence (CL) spectrum of plagioclase shows four emission bands at around 350, 420, 570 and 750 nm, which can be assigned to Ce 3+ , Al-O-Al or Ti 4+ , Mn 2+ and Fe 3+ centers, respectively. Their CL intensities decrease with an increase in experimentally shock pressure. The peak wavelength of the emission band related to Mn 2+ shifts from 570 nm for unshocked plagioclase to 630 nm for plagioclase shocked above 20 GPa. The Raman spectrum of unshocked plagioclase has pronounced peaks at around 170, 280, 480 and 510 cm-1 , whereas Raman intensities of all peaks decrease with an increase in shock pressure. This result suggests that shock pressure causes destruction of the framework structure in various extents depending on the pressure applied to plagioclase. This destruction is responsible for a decrease in CL intensity and a peak shift of yellow emission related to Mn 2+. An emission band at around 380 nm in the UV-blue region is observed in only plagioclase shocked above 30 GPa, whereas it has not been recognized in the unshocked plagioclase. Raman spectroscopy reveals that shock pressure above 30 GPa converts plagioclase into maskelynite. It implies that an emission band at around 380 nm is regarded as a characteristic CL signal for maskelynite. CL images of plagioclase shocked above 30 GPa show a dark linear stripe pattern superimposed on bright background, suggesting planer deformation features (PDFs) observed under an optical microscope. Similar pattern can be identified in Raman spectral maps. CL and Raman spectroscopy can be expected as a useful tool to evaluate shock pressure induced on the plagioclase in terrestrial and meteoritic samples.

Cathodoluminescence Characterization of Maskelynite and Alkali Feldspar in Shergottite (Dhofar 019)

“…3b) [4]. Similar Raman spectra have been reported in previous Raman study on maskelynite in shergottites [5].…”

“…CL intensities at 380 nm decrease with an increase in electron irradiation time. Such behavior is similar to short-lived luminescence reported in CL spectra of silica minerals [7]. The luminescence center related to emission at 380 nm might be eliminated by electron irradiation.…”

“…3a). Similar UV-blue has been also observed in plagioclase experimentally shocked at 30 and 40 GPa [4]. Maskelynite indicates a rapid decay of its emission during electron irradiation, where CL intensity at 380 nm decreases with an increase in irradiation time at 15 kV accelerating voltage and 2.0 nA beam current.…”

“…Plagioclase in terrestrial rocks often have emission bands at around 420, 560 and 750 nm in blue, yellow and red regions which are assigned to Al-O --Al, Mn 2+ and Fe 3+ center, respectively [6]. Furthermore, an emission band at around 380 nm has not been found in CL spectra of natural and synthetic plagioclase except for experimentally shocked ones at 30 and 40 GPa [4]. According to Kayama et al [4], plagioclase experimentally shocked at 30 GPa represents a dull emission in UV-blue and yellow spectral regions, and that at 40 GPa shows only an emission band with high intensity at around 380 nm [4].…”

“…Furthermore, an emission band at around 380 nm has not been found in CL spectra of natural and synthetic plagioclase except for experimentally shocked ones at 30 and 40 GPa [4]. According to Kayama et al [4], plagioclase experimentally shocked at 30 GPa represents a dull emission in UV-blue and yellow spectral regions, and that at 40 GPa shows only an emission band with high intensity at around 380 nm [4]. This emission band at around 380 nm might be assigned to a shock-induced luminescence center, suggesting a CL signal characteristic of maskelynite.…”