granularity
Quick Summary
Igneous rocks are formed from the solidification of molten earth material, known as magma. These rocks are one of the three main types, alongside metamorphic and sedimentary rocks. The texture of an igneous rock is determined by its cooling rate, which influences the size, shape, and arrangement of its mineral grains.12
Crystallinity and granularity are key textural features. Crystallinity describes whether a rock is entirely crystalline (holocrystalline), partly crystalline (hemicrystalline), partly glassy (hemihyaline), or entirely glassy (holohyaline). Granularity refers to grain size, with terms like phaneritic for visible grains and aphanitic for grains only visible under magnification. Rapid cooling, typical of volcanic environments, results in aphanitic or glassy textures, while slower cooling, deep within the Earth, leads to phaneritic textures.31
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