Cathodoluminescence Image 2
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Cathodoluminescence (Image 2)

June 11, 2010
Cathodoluminescence (Image 2) A cathodoluminescence (or CL) image from granite (Town Mountain Granite, Llano, Texas -- ~1.12 to- 1.07 billion years old) and shows two minerals intergrown with each other. 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). [One of two related images. See Next Image.] More about this Image This photo is a CL image from granite (Town Mountain Granite, Llano, Texas -- ~1.12 to 1.07 billion years old) and shows two minerals intergrown with each other. The bluish cross-hatched area is occupied by a grain of potassium feldspar (microcline), and the purple red-rimmed mineral in the upper half of the image is a grain of sodium feldspar (albite). Both mineral grains grew as the granite slowly melted, cooled and crystallized. The black areas were altered by low temperature fluids that circulated through the granite after it had solidified, and may also be late filled fractures and cavities (fluid inclusions). The reddish areas probably were altered and invaded by sodium feldspar at a later time, when younger granites intruded the already solidified Town Mountain Granite and led to circulation of hot water (hydrothermal fluids) along fractures and grain boundaries. The 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 research was supported by National Science Foundation grant EAR 03-18769, awarded to Juergen Schieber, Department of Geological Sciences, Indiana University. (Date of Image: Aug. 17, 2005)

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