MIKE MCNAMEE / SCIENCE PHOTO LIBRARY MIKE MCNAMEE / SCIENCE PHOTO LIBRARY
Cross polarised light micrograph of a thin section of a porphyritic basalt rock from Aberdeen, Scotland. The section is dominated by the large specimens of the minerals quartz & orthoclase feldspar. The quartz grains are clear grey, while those of orthoclase feldspar are darker grey. Large crystals, like these, embedded in a fine grained matrix are known as phenocrysts. A large twin crystal, known as a Carlsbad twin, is visible towards the upper right of the picture. A Carlsbad twin is a simpler variation of the multiple twins formed by plagioclase feldspar. Magnification x2.5 at 35mm size. Microcosmos P. 139 fig.7.23.
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