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Unformatted text preview: slower the cooling rate, the larger the mineral crystals. The five principal textures of igneous rocks are Coarse Grained (or phaneritic) The majority of mineral crystals are of a uniform size and large enough to be identifiable without a microscope. This texture occurs when magma cools slowly inside Earth. Fine Grained (or aphanitic) Very small crystals, which are generally not identifiable without strong magnification, develop when molten material cools quickly on, or very near, the surface of Earth. Porphyritic Two very contrasting sizes of crystals are caused by magma having two different rates of cooling. The larger crystals are termed phenocrysts; and the smaller, surrounding crystals are termed ground-mass (or matrix). Glassy No mineral crystals develop because of very rapid cooling. This lack of crystals causes the rock to have a glassy appearance. In some cases, rapidly es-caping gases may produce a frothy appearance similar to spun glass. Figure 2,20 Tarbuck)...
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