3.4.5 Igneous Textures: Pyroclastic Rocks and Pegmatites

What is Pyroclastic, or fragmental, texture. Some igneous rocks are formed from the consolidation of individual rock fragments that are ejected during a violent volcanic eruption. The ejected particles might be very fine ash, molten blobs, or large angular blocks torn from the walls of the vent during the eruption. Igneous rocks composed of these rock fragments are said to have a pyroclastic texture, sometimes referred to as fragmental texture.

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A common type of pyroclastic rock called welded tuff is composed of fine fragments of glass that remained hot enough during their flight to fuse together upon impact. Other pyroclastic rocks are composed of fragments that solidified before impact and became cemented together at some later time. Because pyroclastic rocks are made of individual particles or fragments rather than interlocking crystals, their textures often appear to be more similar to sedimentary rocks than to other igneous rocks.

Pegmatitic Texture. Under special conditions, exceptionally coarse-grained igneous rocks, called pegmatites, may form. These rocks, which are composed of interlocking crystals all larger than a centimeter in diameter are said to have a pegmatitic texture. Most pegmatites are found around the margins of large plutons as small masses or thin veins that commonly extend into the adjacent host rock.

Pegmatites form in the late stages of crystallization, when water and other volatiles, such as chlorine, fluorine, and sulfur, make up an unusually high percentage of the melt. Because ion migration is enhanced in these fluid rich environments, the crystals that form are abnormally large. Thus the large crystals in pegmatites are not the result of inordinately long cooling histories. Rather, they are the consequence of the fluid rich environment that enhances crystallization.

The composition of most pegmatites is similar to that of granite. Many pegmatites contain large crystals of quartz, feldspar, and muscovite. However, some contain significant quantities of comparatively rare and valuable minerals.