Most metamorphic rocks that we observe at Earth’s surface were derived from the three most common sedimentary rocks—shale, limestone, and quartz sandstone. Shale is the most likely parent of most slate, phyllite, schist, and gneiss. This sequence of metamorphic rocks reflects an increase in grain size, a change in rock texture, and a change in mineralogy.
Limestone, which is composed of the mineral calcite (CaCO3), is the parent rock of marble, while quartz (SiO2) sandstone is the parent of quartzite.
Because calcite and quartz are simple chemical compounds compared to clay minerals, their mineralogy does not change during metamorphism; calcite usually remains calcite, and quartz remains quartz. Rather, these minerals recrystallize to produce larger fused grains that are the main constituents of marble and quartzite, respectively.
The major characteristics of the most common metamorphic rocks are summarized in Figure 1. Notice that metamorphic rocks can be broadly classified by the type of foliation exhibited and, to a lesser extent, the chemical composition of the parent rock. It is worth noting that certain rock names (slate, schist, and gneiss) are also used to describe rock texture.
Foliated Metamorphic Rocks
Slate A very fine-grained (less than 0.5-millimeter) foliated rock composed mainly of minute chlorite and mica flakes (too small to be visible to the human eye) is termed slate. Slate may also contain tiny quartz and feldspar crystals. Thus, slate generally appears dull and closely resembles shale. A noteworthy characteristic of slate is its excellent rock cleavage, or tendency to break into flat slabs.
Slate is most often generated by the low-grade metamorphism of shale, mudstone, or siltstone. Less frequently it is produced when volcanic ash is metamorphosed. Slate’s color depends on its mineral constituents:
Black (carbonaceous) slate contains organic material, red slate gets its color from iron oxide, and green slate usually contains a lot of the mineral chlorite. Phyllite Phyllite represents a degree of metamorphism between slate and schist. Its constituent platy minerals are larger than those in slate but not large enough to be readily identifiable with the unaided eye. Although phyllite appears similar to slate, it can be easily distinguished from slate by its glossy sheen and wavy surface (see Figure 1).
Phyllite exhibits rock cleavage and is composed mainly of very fine crystals of mainly muscovite, chlorite, or both. Schist Medium- to coarse-grained metamorphic rocks in which platy minerals are dominant are called schists. These flat components commonly include muscovite and biotite that display parallel alignments that give the rock its foliated texture (Figure 2).
In addition, schists contain smaller amounts of other minerals, often quartz and feldspar. Some schists are composed mostly of dark minerals (such as amphiboles). As with slate, the parent rock of most schists is shale that has undergone mediumto high-grade metamorphism during a major mountain-building episode.
As you learned in the previous section, the term schist describes the texture of rocks, and as such it is used to name rocks that have a wide variety of chemical compositions. To indicate composition, mineral names are added. For example, schists composed primarily of muscovite and biotite are called mica schist (see Figure 2). Mica-schists often contain accessory minerals, some of which are unique to metamorphic rocks. Some common accessory minerals that occur as porphyroblasts include garnet, staurolite, and andalusite, in which case the rock is called garnet-mica schist, staurolite-mica schist, or andalusite-mica schist (see Figure 3).
In addition, schists may be composed largely of the minerals chlorite or talc, in which case they are called chlorite schist and talc schist, respectively. Both chlorite and talc schists can form when rocks having basaltic compositions undergo metamorphism.
Gneiss is the term applied to medium- to coarse-grained banded metamorphic rocks in which granular and elongated (as opposed to platy) minerals predominate. The most common minerals in gneiss are quartz, potassium feldspar, and plagioclase feldspar. Most gneisses also contain lesser amounts of biotite, muscovite, and amphibole. Some gneisses will split along the layers of platy minerals, but most break in an irregular fashion.
Recall that during high-grade metamorphism, the light and dark components separate, giving gneisses their characteristic banded or layered appearance. Thus, most gneisses consist of alternating bands of white or reddish feldspar-rich zones and layers of dark ferromagnesian minerals (Figure 4). These banded gneisses often exhibit evidence of deformation, including folds and sometimes faults.
Gneisses having a felsic composition may be derived from granite or its fine-grained equivalent, rhyolite. However, most gneisses are generated through high-grade metamorphism of shale. Therefore, gneiss represents the highest-grade metamorphic rock in the sequence of shale, slate, phyllite, schist, and gneiss. Like schists, gneisses may also include large crystals of accessory minerals such as garnet. Gneisses made up primarily of dark minerals also occur. For example, an amphibole-rich rock that exhibits a gneissic texture is called amphibolite.
Nonfoliated Metamorphic Rocks Marble The metamorphism of limestone or dolostone produces the crystalline metamorphic rock called marble (see Figure 1). Pure marble is white and composed essentially of the mineral calcite. Because of its relative softness (3 on the Mohs scale), marble is easy to cut and shape. White marble is particularly prized as a stone from which monuments and statues are carved, such as the Lincoln Memorial in Washington, DC, and the Taj Mahal in India. Unfortunately, when marble is exposed to acid rain, its composition (calcium carbonate) makes it susceptible to chemical weathering. The parent rocks of most marbles contain impurities that color the stone.
Thus, marble can be pink, gray, green, or even black and may contain a variety of accessory minerals (such as chlorite, mica, garnet, and wollastonite). When marble forms from limestone interbedded with shales, it appears banded and exhibits visible foliation. When deformed, these banded marbles may develop highly contorted mica-rich folds that enhance the rocks’ artistic appearance. These decorative marbles have been used as building stones since prehistoric times.
Quartzite is a very hard metamorphic rock formed from quartz sandstone (Figure 5). Under moderate- to high-grade metamorphism, the quartz grains in sandstone fuse together (see the inset in Figure 5). Recrystallization is often so complete that, when broken, quartzite splits across the original quartz grains rather than along their boundaries. In some instances, sedimentary features such as cross-bedding are preserved and give the rock a banded appearance. Pure quartzite is white, but iron oxide may produce reddish or pinkish stains, while dark mineral grains may impart shades of green or gray.
Hornfels is a fine-grained nonfoliated metamorphic rock, and unlike marble and quartzite, it has a variable mineral composition. The parent rock of most hornfels is shale or another clay-rich rock, which has been “baked” by a hot intruding magma body. Hornfels tends to be gray to black in color and quite hard, and it may display conchoidal fracture.
Hornfels is the group name for a set of contact metamorphic rocks that have been baked and hardened by the heat of intrusive igneous masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. These properties are due to fine grained non-aligned crystals with platy or prismatic habits. The term is derived from the German word Hornfels, meaning “hornstone”, because of its exceptional toughness and texture both reminiscent of animal horns. These rocks were referred to by miners in northern England as whetstones.
Most hornfels are fine-grained, and while the original rocks (such as sandstone, shale, slate and limestone) may have been more or less fissile owing to the presence of bedding or cleavage planes, this structure is effaced or rendered inoperative in the hornfels. Though they may show banding, due to bedding, etc., they break across this as readily as along it; in fact, they tend to separate into cubical fragments rather than into thin plates. (www.wikipedia.org)
The most common hornfels (the biotite hornfels) are dark-brown to black with a somewhat velvety luster owing to the abundance of small crystals of shining black mica. The lime hornfels are often white, yellow, pale-green, brown and other colors. Green and dark-green are the prevalent tints of the hornfels produced by the alteration of igneous rocks. Although for the most part the constituent grains are too small to be determined by the unaided eye, there are often larger crystals of cordierite, garnet or andalusite scattered through the fine matrix, and these may become very prominent on the weathered faces of the rock )