6.3: Metamorphic Textures
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Metamorphic texture is the description of the shape and orientation of mineral grains in a metamorphic rock. Metamorphic rock textures are foliated, non-foliated, or lineated are described below.
Metamorphic rock identification table. (Source: Belinda Madsen)
Foliation and Lineation
Foliation is a term used that describes minerals lined up in planes. Certain minerals, most notably the mica group, are mostly thin and planar by default. Foliated rocks typically appear as if the minerals are stacked like pages of a book, thus the use of the term ‘folia’, like a leaf. Other minerals, with hornblende being a good example, are longer in one direction, linear like a pencil or a needle, rather than a planar-shaped book. These linear objects can also be aligned within a rock. This is referred to as a lineation. Linear crystals, such as hornblende, tourmaline, or stretched quartz grains, can be arranged as part of a foliation, a lineation, or foliation/lineation together. If they lie on a plane with mica, but with no common or preferred direction, this is foliation. If the minerals line up and point in a common direction, but with no planar fabric, this is lineation. When minerals lie on a plane AND point in a common direction; this is both foliation and lineation.
Foliated metamorphic rocks are named based on the style of their foliations. Each rock name has a specific texture that defines and distinguishes it, with their descriptions listed below.
Slate is a fine-grained metamorphic rock that exhibits a foliation called slaty cleavage that is the flat orientation of the small platy crystals of mica and chlorite forming perpendicular to the direction of stress. The minerals in slate are too small to see with the unaided eye. The thin layers in slate may resemble sedimentary bedding, but they are a result of directed stress and may lie at angles to the original strata. In fact, original sedimentary layering may be partially or completely obscured by the foliation. Thin slabs of slate are often used as a building material for roofs and tiles.
Phyllite is a foliated metamorphic rock in which platy minerals have grown larger and the surface of the foliation shows a sheen from light reflecting from the grains, perhaps even a wavy appearance, called crenulations. Similar to phyllite but with even larger grains is the foliated metamorphic rock schist, which has large platy grains visible as individual crystals. Common minerals are muscovite, biotite, and porphyroblasts of garnets. A porphyroblast is a large crystal of a particular mineral surrounded by small grains. Schistosity is a textural description of foliation created by the parallel alignment of platy visible grains. Some schists are named for their minerals such as mica schist (mostly micas), garnet schist (mica schist with garnets), and staurolite schist (mica schists with staurolite).
Gneissic banding is a metamorphic foliation in which visible silicate minerals separate into dark and light bands or lineations. These grains tend to be coarse and often folded. A rock with this texture is called gneiss. Since gneisses form at the highest temperatures and pressures, some partial melting may occur. This partially melted rock is a transition between metamorphic and igneous rocks called a migmatite .
Migmatites appear as dark and light banded gneiss that may be swirled or twisted some since some minerals started to melt. Thin accumulations of light-colored rock layers can occur in a darker rock that is parallel to each other or even cut across the gneissic foliation. The lighter colored layers are interpreted to be the result of the separation of a felsic igneous melt from the adjacent highly metamorphosed darker layers, or injection of a felsic melt from some distance away.
Non-foliated textures do not have lineations, foliations, or other alignments of mineral grains. Non-foliated metamorphic rocks are typically composed of just one mineral and, therefore, usually show the effects of metamorphism with recrystallization in which crystals grow together, but with no preferred direction. The two most common examples of non-foliated rocks are quartzite and marble. Quartzite is a metamorphic rock from the protolith sandstone. In quartzites, the quartz grains from the original sandstone are enlarged and interlocked by recrystallization. A defining characteristic for distinguishing quartzite from sandstone is that when broken with a rock hammer, the quartz crystals break across the grains. In a sandstone, only a thin mineral cement holds the grains together, meaning that a broken piece of sandstone will leave the grains intact. Because most sandstones are rich in quartz, and quartz is a mechanically and chemically durable substance, quartzite is very hard and resistant to weathering.
Marble is metamorphosed limestone (or dolostone) composed of calcite (or dolomite). Recrystallization typically generates larger interlocking crystals of calcite or dolomite. Marble and quartzite often look similar, but these minerals are considerably softer than quartz. Another way to distinguish marble from a quartzite is with a drop of dilute hydrochloric acid. Marble will effervesce (fizz) if it is made of calcite.
A third non-foliated rock is hornfels identified by its dense, fine-grained, hard, blocky or splintery texture composed of several silicate minerals. Crystals in hornfels grow smaller with metamorphism and become so small that specialized study is required to identify them. These are common around intrusive igneous bodies and are hard to identify. The protolith of hornfels can be even harder to distinguish, which can be anything from mudstone to basalt.