The causes and petrological significance of cathodoluminescence emissions from alkali feldspars |
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Authors: | Adrian A Finch Jeanette Klein |
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Institution: | Department of Environmental Sciences, University of Hertfordshire, College Lane, Hatfield, Hertfordshire, AL10 9AB, UK; E-mail: a.finch@herts.ac.uk, GB Institut für Mineralogie, Universit?t Marburg, Hans-Meerweinstrasse, Marburg, D-35032, Germany; E-mail: Biophotonik.KL@t-online.de, DE
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Abstract: | The cathodoluminescence (CL) of a variety of alkali feldspars from South Greenland has been examined in an attempt to understand
the causes of the CL and its petrological significance. Analytical methods have included CL spectroscopy, secondary ion mass
spectrometry (SIMS) and electron paramagnetic resonance (EPR) to correlate the presence of certain CL emissions to the presence
of certain trace element and point defects. Where possible, blue and red luminescent fractions of the same rock samples have
been separated and analysed separately. Blue CL appears to relate to the presence of electron holes on bridging oxygens, particularly
on the Al-O-Al bridge, as determined from EPR studies. No correlation with other proposed activators for blue CL such as Eu2+, Ga3+ or Ti4+ was observed. Some blue luminescent feldspars also have an emission in the infra-red (IR), invisible during normal visible
CL petrography. The red and IR CL emissions correspond to features in EPR spectra attributed to Fe3+ and support previous suggestions that Fe3+ is related to this emission. However, our studies indicate that the visible red CL relates specifically to Fe3+ on the T1 site, whereas the equivalent CL from disordered feldspars lies in the IR. The difference between red and IR CL
emissions therefore relates to the state of Fe3+ order across the tetrahedral sites. These data allow more meaningful interpretations of CL as a petrographic tool in alkali
feldspar-bearing rocks.
Received: 5 March 1998 / Accepted: 23 November 1998 |
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