At first glance, panchromatic CL images are not easy to interpret. However, with some experience and the use of secondary electron images and element maps, CL images are as easy to understand as transmitted light petrographic images.
A few general observations have been made. Quartz grains are weakly to brightly luminescent (in scanned CL). Authigenic quartz cement is generally non-luminescent to weakly luminescent. Feldspar grains are widely variable from non-luminescent to very bright. Authigenic feldspar cement is non-luminescent. The Fe-rich carbonates typically present as cements in sandstones are weakly luminescent. Of the heavy minerals, zircons are typically very bright and zoned, tourmalines, garnets, and opaque minerals are typically non-luminescent. Clays and detrital micas are generally non-luminescent. Pore space is typically somewhat luminescent due to the green luminescence produced by standard epoxies.
Below is an example of a panchromatic CL image of a sandstone. This image was chosen because it contains a fairly diverse assemblage of grains, cement, and porosity. This sandstone is from the Val Verde Basin of west Texas. Various features are marked.
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1) zoned and fractured carbonate grain/cement
2) typical quartz grain (light gray CL) 3) typical quartz-lined microfracture (dark gray CL) 4) quartz grain with quartz-lined, compaction-related microfractures 5) Na-feldspar grain, probably with authigenic cement 6) quartz grain showing relict high-T deformation 7) CL-zoned quartz cement 8) heavy mineral (probably zircon but atypical low luminescence) |
