Examples of Results from Our SEM Lab: St Peter Sandstone--Colorful Quartz in CL!

The St Peter Sandstone is very old, from the Ordovician period (485-443 million years ago, give or take)

This sandstone originated as a sheet of sand in clear, shallow water near the shore of a Paleozoic sea 
and consists of fine-to-medium-size, well-rounded quartz grains with frosted surfaces. Quartz is rather 
"boring" seen in an SEM as secondary electron images (SEI) or backscattered electron images (BSE), but 
is very photogenic when imaged with cathodoluminescence (CL). The electron beam of the SEM excites the 
weakly bound outer shell electrons, which produce different colored light, depending upon trace elements 
present in the quartz. As the sand grains came from many different sources, we see a variety of colors.

SE Image

BSE Image

CL image. Notice that now you can discern the "overgrowths" of quartz cementing the grains, which are "invisible" in the SE and BSE images

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