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Cathodoluminescent image of sedimentary rock with tiny quartz crystals
A cathodoluminescence image from a ~370 million-year-old sedimentary rock (Devonian New Albany Shale from Indiana) consisting largely of tiny quartz crystals that grew early in sediment history, probably within 20 to 50 centimenters of burial below the seafloor.
More about this Image
Typically, quartz that grows under such low temperature conditions shows none or only minimal CL. In this picture, such quartz is represented by areas with a bluish hue (high contrast exposure). There are also however, areas where the quartz luminesces with a pinkish-red color, even though all of the quartz in the sample formed at the same time. These pinkish-red areas are all adjacent to a dark line that winds through the central portion of the image, representing the organic wall of an algal cyst. The likely reason for the pinkish-red colors associated with this cyst is a mass of bacterial slime that initially surrounded the cyst during decay processes, and produced conditions for quartz pecipitation that differed from those further away.
Many minerals emit visible light (they luminesce) when they are bombarded with electrons. The light is produced at crystal defects and by impurities (foreign atoms) within the crystals. The wave-length (or color) of the emitted light is therefore related to the growth conditions of the mineral. The phenomenon of light emission by electron bombardment is commonly known as cathodoluminescence, or CL.
This photo was taken with an electron microscope where an electron beam scans over the surface of the sample and, one spot at a time, excites CL that is recorded by a special color-sensitive detector (GATAN Chroma-CL).
This image is part of research supported by National Science Foundation grant EAR 0318769, awarded to Juergen Schieber, Department of Geological Sciences, Indiana University. (Date of Image: Aug. 17, 2005)
Credit: Image by Juergen Schieber, Department of Geological Sciences, Indiana University. Instrumentation: SEM FEI Quanta-FEG 400 with Gatan Chroma CL.
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