Gabbro microcrystals. Polarised light micrograph of a section through a sample of gabbro rock, showing the phaneritic (coarse-grained) microcrystals. Gabbro is a coarse-grained igneous rock, formed by the slow and gradual crystallisation of basaltic magma at large depth under the Earth's surface. Gabbros are distinguished from other plutonic rocks by their high content of plagioclase-feldspars and pyroxene. Magnification: x57, when printed 10 centimetres wide.
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