Description of igneous rocks requires information about crystallinity, grain size, shape of grains, and mutual relations of grains, in addition to that dealing with the composition of the constituent materials. The rocks can be entirely of crystals ( holocrystalline ), crystals and glass ( hypocrystalline or merocrystalline ), or entirely of glass ( holohyaline ). There is no unanimity concerning sizes for fine-, medium-, and coarse-grained rocks but <0.05 mm, 0.05–1 mm (or 2 mm—the IUGS recommendation) and >1 mm (or 2 mm) correspond generally with grain sizes of many effusive (volcanic), hypabyssal, and plutonic rocks, respectively (0.05 mm is the approximate size at which the human eye ceases to resolve individual particles). However, the variation in range of sizes used is illustrated by the ranges of <1 mm, 1–5 mm and >5 mm used by some authors.
Coarse-grained igneous rocks are referred to as being phaneritic(dyscrystalline); the term for rocks whose crystal grains...
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Bibliography
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© 1989 Van Nostrand Reinhold
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Gavasci, A.T. (1989). Textures of igneous rocks . In: Petrology. Encyclopedia of Earth Science. Springer, Boston, MA. https://doi.org/10.1007/0-387-30845-8_240
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