MIKE MCNAMEE / SCIENCE PHOTO LIBRARY MIKE MCNAMEE / SCIENCE PHOTO LIBRARY
Cross polarised light micrograph of a thin section of igneous rock showing a process known as crystal zoning. The section is dominated by large crystals of the mineral plagioclase feldspar. Large crystals, like these, embedded in a fine grained matrix are known as phenocrysts. These phenocrysts are all zoned; the blue & green hexagon at lower right being the most clearly marked. Zoning is an indication of the changing chemical composition of a crystal during its growth, prior to solidification. The crystal is not uniform in composition throughout its volume. Under cross polarised light the crystal appears two-toned. Magnification x2.5 at 35mm size.
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