The term texture is used to describe the size, shape, and arrangement of the mineral grains within a rock. Recall that texture is one way in which igneous and sedimentary rocks are classified. Most igneous and many sedimentary rocks consist of mineral grains or crystals that have a random orientation and thus appear uniform when viewed from any direction.
By contrast, metamorphic rocks that contain platy minerals (such as micas) and/or elongated minerals (such as amphiboles) typically display some kind of preferred orientation in which the mineral grains exhibit a parallel to subparallel alignment.
Like a fistful of pencils, rocks containing elongated mineral grains that are oriented parallel to each other appear different when viewed from the side than when viewed head-on. A rock that exhibits a preferred orientation of its mineral constituents is said to possess foliation.
The term foliation refers to any planar (nearly flat) arrangement of mineral grains or crystals within a rock. Although foliation may occur in some sedimentary and even a few types of igneous rocks, it is a fundamental characteristic of metamorphosed rocks that have been strongly deformed, mainly by folding.
In metamorphic environments, foliation is ultimately driven by compressional stress that shortens rock units, causing mineral grains in preexisting rocks to develop parallel, or nearly parallel, alignments. Examples of foliation include the parallel alignment of platy minerals through rotation, recrystallization, and flattening of mineral grains or pebbles.
Foliated textures include rock cleavage in which rocks can be easily split into tabular slabs, and compositional banding in which the separation of dark and light minerals generates a layered appearance. These diverse types of foliation can form in many different ways.
Rotation of Platy Mineral Grains
The rotation of existing mineral grains is the easiest of the foliation mechanisms to envision. Figure 1 illustrates the mechanics by which platy or elongated mineral grains are rotated. Note that the new alignment is roughly perpendicular to the direction of maximum stress. Although physical rotation of platy minerals contributes to the development of foliation in low-grade metamorphism, other mechanisms dominate in more extreme environments.
That Produces New Minerals Recall that recrystallization is the creation of new mineral grains from preexisting ones. When recrystallization occurs as rock is being subjected to differential stress, any elongated minerals (such as amphiboles) and platy minerals (such as micas) that form tend to recrystallize perpendicular to the direction of maximum stress. Thus, the newly formed mineral grains exhibit a distinct layering, and the metamorphic rocks containing them exhibit foliation.
Flattening Spherically Shaped Grains
Mechanisms that flatten existing mineral grains are important in the metamorphism of rocks that contain minerals such as quartz, calcite, and olivine. These minerals normally develop roughly spherical crystals and have a rather simple chemical composition.
A change in grain shape can occur as distinct units of a mineral’s crystalline structure slide relative to one another along discrete planes, thereby distorting the grain, as shown in Figure 2. This type of gradual solidstate flow involves slippage that disrupts the crystal lattice as atoms shift positions by breaking existing chemical bonds and forming new ones.
The shape of a mineral may also be altered by a process in which individual atoms move from a location along the margin of the grain that is highly stressed to a less-stressed position on the same grain (Figure 3). This mechanism, called pressure solution, is significantly aided by hot, ion-rich water. Mineral matter (ions) dissolves where grains are in contact with each other (areas of high stress) and is deposited in pore spaces (areas of low stress).
As a result, the mineral grains tend to become shortened in the direction of maximum stress and elongated in the direction of minimum stress. While both of these mechanisms flatten mineral grains, the mineralogy remains the same.
Various types of foliation exist, depending largely upon the grade of metamorphism and the mineralogy of the parent rock. We will look at three: rock, or slaty, cleavage; schistosity; and gneissic texture, or banding.
Rock, or Slaty, Cleavage – Rocks that split into thin slabs when hit with a hammer exhibit rock cleavage. Rock cleavage develops in various metamorphic rocks but is best displayed in slates that exhibit an excellent splitting property called slaty cleavage (Figure 4).
Because it splits easily, slate is used for building materials such as roof and floor tiles as well as billiard table surfaces. In low-grade metamorphic environments, slaty cleavage is known to develop where beds of shale (and related sedimentary rocks) are strongly folded and metamorphosed to form slate (Figure 5). The process begins when compressional stress begins to deform rock units, producing broad folds.
With further deformation the clay minerals in shale, which initially aligned roughly parallel to the bedding surfaces, begin to recrystallize into tiny flakes of chlorite and mica. However, these new platy mineral grains grow so they are aligned roughly perpendicular to the maximum directional stress. Because slate typically forms during the low-grade metamorphism of shale, evidence of the original sedimentary bedding surfaces is often preserved. However, as Figure 5C illustrates, the orientation of slate’s cleavage usually develops at an angle to the sedimentary beds.
Thus, unlike shale, which splits along bedding planes, slate often splits across bedding surfaces. Other metamorphic rocks, such as schists and gneisses, sometimes split along planar surfaces and exhibit rock cleavage.
Schistosity – At higher temperatures and pressures, the minute mica and chlorite flakes in slate begin to recrystallize into larger muscovite and biotite crystals. When these platy minerals are large enough to be discernible with the unaided eye, they exhibit planar or layered structures called schistosity.
Rocks that have this type of foliation are referred to as schist. In addition to containing platy minerals, schist often contains deformed quartz and feldspar crystals that appear flattened or lens-shaped and are embedded among the mica grains.
Gneissic Texture, or Banding – During high-grade metamorphism, ion migration can result in the segregation of minerals, as shown in Figure 6. Notice that the dark biotite and amphibole crystals and light silicate minerals (quartz and feldspar) have separated, giving the rock a banded appearance called gneissic texture, or gneissic banding.
Metamorphic rocks with this texture are called gneiss (pronounced “nice”). Although they are foliated, gneisses do not usually split as easily as slates and some schists.
Gneissic Texture, or Banding – During high-grade metamorphism, ion migration can result in the segregation of minerals, as shown in Figure 6. Notice that the dark biotite and amphibole crystals and light silicate minerals (quartz and feldspar) have separated, giving the rock a banded appearance called gneissic texture, or gneissic banding. Metamorphic rocks with this texture are called gneiss (pronounced “nice”). Although they are foliated, gneisses do not usually split as easily as slates and some schists.
Other Metamorphic Textures
Metamorphic rocks that do not exhibit foliated textures are referred to as nonfoliated. Nonfoliated metamorphic rocks typically develop in environments where deformation is minimal and the parent rocks are composed of minerals that exhibit equidimensional crystals, such as quartz or calcite.
For example, when a fine-grained limestone (made of calcite) is metamorphosed by the intrusion of a hot magma body, the small calcite grains recrystallize to form larger interlocking crystals.
The resulting rock, marble, exhibits large, intergrown calcite crystals, similar in appearance to those in coarse-grained igneous rocks.
Metamorphic rocks may also contain some unusually large grains, called porphyroblasts, that are surrounded by a fine-grained matrix of other minerals.
Porphyroblastic textures develop in a wide range of rock types and metamorphic environments when minerals in the parent rock recrystallize to form new minerals. During recrystallization, certain metamorphic minerals, such as garnet, tend to develop a small number of very large crystals.
By contrast, minerals such as muscovite and biotite typically form a large number of smaller grains. As a result, metamorphic rocks that contain large crystals (porphyroblasts) of, for example, garnet embedded in a finer-grained matrix of biotite and muscovite, are relatively common (Figure 7).
By E. J. Tarbuck, F. K. Lutgens, Illustrated by D. Tasa