Pelitic schists

Pelitic schists are formed from fine-grained sedimentary rocks. They are an important metamorphic rock type because they undergo distinct textural and mineralogical changes that are used by geologists to gauge the temperatures and pressures under which the rocks were progressively modified.

Agents of Metamorphism

Metamorphism literally means “the change in form” that a rock undergoes. More precisely, metamorphism is a process by which igneous and sedimentary rocks are mineralogically, texturally, and, occasionally, chemically modified by the effects of one or more of the following agents or variables: increased temperature, pressure, or chemically active fluids. Pelitic schists have long been recognized as one of the best rock types to gauge and preserve the wide range of mineralogical and textural changes that take place as metamorphic conditions progressively increase. It is perhaps easiest to understand the conditions under which metamorphism occurs by excluding those conditions that are not generally considered metamorphic. At or near the earth’s surface, sedimentary rocks form from the physical or chemical breakdown of preexisting rocks at relatively low pressures and temperatures (less than 200 degrees Celsius). Igneous rocks form from molten material at high temperatures (650 to 1,100 degrees Celsius), and at low pressures for volcanic rocks or at high pressures for those formed at great depth. The conditions that exist between these two extremes are those that are considered metamorphic.

Although all the agents of metamorphism work together to produce the distinctive textures and minerals of regionally metamorphosed rocks, each has its own special or distinctive role. Temperature is considered the most important factor in metamorphism. It is the primary reason for recrystallization and new mineral growth. Two types of pressure affect metamorphic rocks. The pressure caused by the overlying material and uniformly affecting the rock on all sides is referred to as the confining pressure. An additional pressure, referred to as directed pressure, is normally caused by strong horizontal forces. While confining pressure has little apparent influence on metamorphic textures, directed pressures are considered the principal reason that several types of metamorphosed rocks develop distinctive textures. Chemically active fluids are important because they act both as a transporting medium for chemical constituents and as a facilitator of chemical reactions during metamorphism. As metamorphic conditions increase, fluids play an important role in aiding metamorphic reactions, even though they are concurrently driven out of the rocks. Most metamorphic rocks preserve mineral assemblages that represent the highest conditions achieved. As metamorphic conditions start to fall, reactions are very slow to occur because adequate fluids are normally not available. If the metamorphism involves significant additions and losses of constituents other than water, carbon dioxide, and other fluids, this process is referred to as metasomatism. For example, if the sodium content of a rock markedly increases during metamorphism, this is referred to as sodium metasomatism.

Types of Metamorphism

Geologists recognize several types of metamorphism, but one, regional metamorphism, is predominant. Regional metamorphism takes place at depth within the vast areas where new mountains are forming. The products of regional metamorphism are exposed in a variety of places. One of the best sites is found on shields—broad, relatively flat regions within continents where the thin veneer of sedimentary rocks has been stripped away, exposing wide areas of deeply eroded ancient mountain belts and their regionally metamorphosed rocks, complexly contorted and intermingled with igneous rocks. The cores of older eroded mountain belts, such as the Appalachians and the Rockies, also commonly expose smaller areas of very old regionally metamorphosed rocks. Pelitic schists mainly occur in regionally metamorphosed rocks. They may also occur in contact metamorphic deposits, which are small, baked zones that formed immediately adjacent to igneous bodies that invaded pelitic rocks as hot molten material. In contrast to the thousands of square kilometers typically occupied by regionally metamorphosed rocks, individual contact metamorphic deposits are rarely more than several kilometers wide.

Foliation

Fine-grained sedimentary rocks (shales and mudstones) that formed from the physical breakdown of other rock types are commonly referred to as pelites. Because pelitic sediments can be hard to distinguish, the collective label “mudrock” has come into widespread use. Pelitic rocks undergo very marked textural and mineralogical changes that geologists use to gauge the conditions of formation in a particular area. “Schist,” as used in the term “pelitic schist,” has several definitions. The name may be applied broadly to any rock that is foliated or contains minerals that have a distinct preferred planar orientation. “Schist” also has a much narrower definition, referring only to foliated, coarse-grained rocks in which most mineral grains are visible to the unaided eye. This foliation, or preferred orientation, is imparted upon the rock because of deformation and recrystallization in response to a pronounced directed pressure at elevated temperatures.

The development of foliation is the most diagnostic feature of rocks that have undergone regional metamorphism. Directed pressure produces a variety of foliations that change as a function of the conditions of formation. An unmetamorphosed pelitic rock is typically a very fine-grained, soft rock that may or may not be finely layered (containing closely spaced planes). At low temperatures and pressures (low grades of metamorphism), pelitic rocks remain fine-grained, but microscopic micaceous minerals form or recrystallize and align themselves perpendicular to the directed pressure. This produces a dense, hard rock called a slate, which readily splits parallel to the preferred orientation. Any layering may be partially or totally obliterated. The accompanying texture is called a flow or slaty cleavage. At slightly higher conditions, micaceous minerals become better developed but remain fine-grained. The foliated rock produced takes on a pronounced sheen and is referred to as a phyllite. As conditions continue to increase, grain size increases until it is visible. This change produces a texture referred to as schistosity, and the rock is called a schist. At very high conditions, the micas that were diagnostic of lower conditions start to become unstable. The rock, referred to as a gneiss, remains foliated but does not readily split as the slates, phyllites, and schists do. The foliation, or gneissosity, takes the form of an alternating light and dark banding. At higher conditions, gneissic rocks gradually grade into the realm where igneous rocks form.

Mineralogic Zones

The first study to show that a single rock type can undergo progressive change on a regional scale was that of British geologist George Barrow toward the end of the nineteenth century in the Highlands of Scotland, southwest of Aberdeen. His work indicated that pelitic schists and gneisses contained three discrete mineralogic zones, each represented by one of three key minerals: staurolite, kyanite, and sillimanite. Barrow suggested that increasing temperature was the controlling agent for the zonation observed. Later work by Barrow and other geologists confirmed this hypothesis and broadened the mapping over the entire Highlands, revealing additional mineral zones that formed at lower metamorphic conditions. From lowest to highest temperatures, several mineralogical zones were recognized. In the chlorite zone, there are slates, phyllites, and mica schists generally containing quartz, chlorite, and muscovite. In the biotite zone, mica schists are marked by the appearance of biotite in association with chlorite, muscovite, quartz, and albite. Biotite occurs in each of the higher zones. A line marking the boundary of the chlorite and biotite zones is referred to as the biotite isograd. (An isograd is a line that marks the first appearance of the key index mineral that is distinctive of the metamorphic zone.) In the almandine (garnet) zone, mica schists are characterized by the presence of almandine associated with quartz, muscovite, biotite, and sodium-rich plagioclase. The garnet first appears along the almandine isograd and occurs through all the higher zones. In the staurolite zone, mica schists typically contain quartz, muscovite, biotite, almandine, staurolite, and plagioclase. In the kyanite zone, mica schists and gneisses contain quartz, muscovite, biotite, almandine, kyanite, and plagioclase. Staurolite is no longer stable in this zone. In the sillimanite zone, mica schists and gneisses are characterized by quartz, muscovite, biotite, almandine, sillimanite, and plagioclase. Sillimanite forms at the expense of kyanite. These zones—also called Barrovian zones—are recognized throughout the world.

Study of Pelitic Rocks

The study of pelitic schists can be conducted from a number of perspectives. The foundation of all studies, however, is the kind of fieldwork that George Barrow conducted in Scotland, which includes careful mapping, sample collection, rock description, and structural measurement.

Once the data and samples are returned to the laboratory, other methods of investigation may be employed. Rocks are commonly studied under the binocular microscope or powdered and made into thin sections to be analyzed in polarized light transmitted through the sample beneath the petrographic microscope. The preparation of the standard thin section involves several steps: cutting a small block of the sample so that it is 2.7 millimeters by 4.5 millimeters on one side; polishing one side and glueing it to a glass slide; and cutting and grinding the glued sample to a uniform thickness of 0.03 millimeter. Observations and descriptions of these thin sections are essential to the study of pelitic rocks because they commonly provide information about the interrelationships among the minerals present and clues to the metamorphic history of the sample.

Whole-rock chemical analyses of pelitic rocks are also important. Standard analytical procedures, atomic absorption, and X-ray fluorescencespectroscopy are all methods used to determine the amounts of the elements within these rocks. Chemical analyses of individual minerals also provide important information about pelitic rocks.

Mineral Analyses

Although mineral analyses have traditionally been conducted by the same analytical techniques as whole-rock analyses, problems obtaining material pure enough for analysis have plagued scientists. Since the 1960s, almost all published mineral analyses have been produced by the electron microprobe, which greatly improves the accuracy of the analyses. These whole-rock analyses and individual mineral analyses are commonly plotted together in several ways on triangular graph paper in order to illustrate the kinds of mineral associations that can exist at different metamorphic conditions.

In experimental petrology, metamorphic minerals are grown under a variety of controlled equilibrium conditions to provide geologists with a better understanding of the actual conditions for their formation. Although such studies have greatly enhanced knowledge of naturally occurring metamorphic reactions, the field is not without controversy. The behavior of the reactions of kyanite to sillimanite, andalusite to sillimanite, and kyanite to sillimanite as temperatures increase shows the great complexity of this endeavor. The various studies conducted on these minerals that occur in pelitic schists are not in agreement, so many geologists remain deeply divided about the exact conditions under which these three minerals are stable.

Physical chemistry and thermodynamics provide geologists with tools to predict how minerals in idealized, simplified chemical reactions theoretically behave. Such comparisons are extremely important in making judgments about more complex natural environments.

Economic Uses

The only important pelitic rock type that is useful as such is very low-grade slate that is quarried for flagstone, roofing material, countertops, blackboards, billiard tables, and switchboard panels. Yet pelitic rocks do contain a number of minerals that are extracted for their usefulness.

Staurolite, almandine-garnet, and kyanite are all minerals from pelitic rocks that have some use as gemstones. In addition, staurolite crystals grow together and may form crosses, which are sold as amulets called fairy stones, although most objects sold as “fairy stones” are not genuine staurolite. (The name “staurolite” is derived from the Greek word meaning “cross” because of the mineral’s diagnostic crystal shape.) Garnet, from pelitic schists, also has some use as an abrasive for sandblasting and spark plug cleaning, and is a common ingredient in sandpaper, which is often called “garnet paper.” In addition, sillimanite and kyanite are used as refractory (high-temperature) materials in porcelain and spark plugs.

Graphite is another mineral that may occur in or be associated with pelitic schists and forms from carbon-rich sediments. Deposits near Turin, Italy, occur in micaceous phyllites, schists, and gneisses. Graphite has a variety of uses, including the production of refractory crucibles for the making of bronze, brass, and steel. It is used with petroleum products as a lubricant and blended with fine clay in the “leads” of pencils. It is also used for electrotype and in steel, batteries, generator brushes, and electrodes.

Another mineral, pyrophyllite, is found in very low-grade aluminum-rich pelitic rocks and has properties and uses similar to those of talc. Pyrophyllite is used in paints, paper, ceramics, and insecticides, and as an absorbent powder. When heated, it can expand and create worm-like textures, leading to the term “vermiculite.” Vermiculite is often used as a potting soil medium. A special fine-grained variety of pyrophyllite called agalmatolite is prized by the Chinese for the carving of small objects.

Principal Terms

equilibrium: a situation in which a mineral is stable at a given set of temperature-pressure conditions

index mineral: an individual mineral that has formed under a limited or very distinct range of temperature and pressure conditions

isograd: a line on a geologic map that marks the first appearance of a single mineral or mineral assemblage in metamorphic rocks

metamorphic zone: areas of rock affected by the same limited range of temperature and pressure conditions, commonly identified by the presence of a key individual mineral or group of minerals

mica: a platy silicate mineral (one silicon atom surrounded by four oxygen atoms) that readily splits in one plane

mineral: a naturally occurring chemical compound that has an orderly internal arrangement of atoms and a definite chemical formula

mudrock: a collective term for sedimentary rocks composed of fine-grained products derived from the physical breakdown of preexisting rocks (shales, for example), which break along distinct planes, and mudstones, which do not

pelitic: an adjective for mudrocks and the metamorphic rocks derived from them

progressive metamorphism: mineralogical and textural changes that take place as temperature and pressure increase

sedimentary rock: a rock formed from the physical breakdown of preexisting rock material or from the precipitation—chemically or biologically—of minerals

texture: the size, shape, and arrangement of crystals or particles in a rock

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