MIKE MCNAMEE / SCIENCE PHOTO LIBRARY MIKE MCNAMEE / SCIENCE PHOTO LIBRARY
Cross polarised light micrograph of a thin section of a porphyritic basalt rock from Mount Dore, France. The section is dominated by the mineral plagioclase feldspar which appears as 'zebra- striped twins' in a fine grained basalt matrix. Large crystals, like these, embedded in a fine grained matrix are known as phenocrysts. The brightly coloured crystals in this specimen, also phenocrysts, are the minerals pyroxene & olivine. The size of a phenocryst depends on the rate at which it was cooled out of the magma; the larger the crystals the slower the process. Magnification x2.5 at 35mm size. Microcosmos P. 139 fig.7.22.
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