Metamorphic processes, rocks, and mineral deposits

The word “metamorphism,” based on Greek roots, translates as the “process of changing form.” Existing sedimentary or igneous rocks are transformed in the solid state to metamorphic rocks as the temperature and pressure of their environment increase at various depths within the Earth. The numerous transformations that occur are collectively termed metamorphic processes.

Background

Every metamorphic process relates either to the formation of new minerals, called neocrystallization, or to the formation of a new texture in the metamorphic rock. The new texture may simply be an increase in size and change in shape of existing minerals (recrystallization). The new texture may also involve the development of a “foliation,” in which the elongate and platy minerals assume a parallel orientation. These general processes are further divided depending upon the specific chemical and mechanical changes occurring during the metamorphic transformation. Long periods of erosion can expose metamorphic rocks on the surface of the Earth; surface metamorphic rocks are often valuable resources, either because of their new minerals or because of the physical properties that the rocks themselves have as a result of their new textures.

Neocrystallization

New minerals form at the expense of old minerals. As the pressure and temperature increase on an existing igneous or sedimentary rock (called the protolith), the old minerals become unstable and break down into chemical components that recombine to form new minerals. Some of the chemicals, for example, H₂O and CO₂, occur as gases at metamorphic temperatures. These gases mix to form a vapor that exists in the cracks and along the boundaries between the individual grains of the minerals. The gain and loss of gases from the vapor are part of the overall chemical reconstruction that takes place during metamorphism. The vapor inevitably escapes from the rock during the long period of cooling and erosion that exposes such rocks on the Earth’s surface.

The neocrystallization process is usually expressed as a chemical reaction. The minerals of the protolith (existing rock) are the reactants, shown on the left side of the reaction, and the new metamorphic minerals that form are the products, listed on the right side. The reactions often will generate and/or consume chemicals residing in the vapor. The reactions illustrated in the figures accompanying this article are shown in triplicate, first as rock changes, second as mineral changes, and third as chemical recombinations. As an example, refer to the three parts of reaction 1. Reaction (a) is the conversion of the sedimentary rock (protolith) called dolostone, which commonly contains silica as chert nodules, to the metamorphic rock called marble. Reaction (b) is the same reaction with attention focused on the transformation of the minerals and the creation of the metamorphic mineral called tremolite, where the beginning vapor was water and the ending vapor is carbon dioxide. Reaction (c) shows how the individual chemical components have recombined, often changing from the mineral to vapor state during the transformation.

As with any chemical reaction, there are specific temperature and pressure conditions that must exist before the reaction can occur. Each metamorphic mineral of interest forms within a specific temperature and pressure region in the Earth. The exact temperature and pressure conditions under which a metamorphic mineral or group of minerals will form can be determined by laboratory experiments; geologists then deduce that similar conditions must have existed whenever these minerals are found in the geological environment. The geological environment required for the development of a given metamorphic mineral is usually controlled by plate tectonic movements. Explorations for metamorphic resources are targeted to specific tectonic regions that correspond to the proper temperature-pressure environments for their formation.

There are three tectonic environments with specific pressure and temperature conditions that control the location for the development of metamorphic minerals. Burial metamorphism results from a high-pressure and low-temperature environment that occurs where two plates converge and one plate is actively subducted. During the recent geological past, the coastline along Oregon and Northern California experienced this tectonic environment. Contact metamorphism is a high-temperature, low-pressure environment occurring slightly farther inland from the region of burial metamorphism. Contact metamorphism results when magma generated during the subduction of a plate rises into the overriding plate and solidifies as shallow igneous plutons. Contact metamorphism has occurred along the margins of the Sierra Nevadabatholiths of eastern California. The third tectonic environment is regional metamorphism, often called dynothermal metamorphism, which corresponds to moderately high pressures and temperatures. Regional metamorphism is seen after extensive erosion of a contact metamorphism area has exposed deeper regions within the Earth’s crust.

Isochemical Processes

Neocrystallization that occurs without any influx of new chemicals (other than the water and carbon dioxide from the vapor) is called isochemical metamorphism. Isochemical metamorphism produces about a dozen minerals that are considered valuable resources. The isochemical-neocrystallization processes responsible for the formation of some of these minerals are described below, with a brief indication of the tectonic environments that favor their formation.

Serpentine

When serpentine (Mg₃Si₂O₅(OH)₄) is the major mineral formed during the low-temperature, low-pressure metamorphism associated with the beginning of regional metamorphism, the resulting metamorphic rock is called a serpentinite. Polished serpentinites are used widely as a facing stone in both interior and exterior applications. When the serpentinites contain some carbonate minerals they are marketed as “verde antique marble.” Serpentine can occur in any one of three forms. The form called chrysotile is the most common asbestos mineral. Asbestos veins are common in serpentinites, and in many locations in eastern Canada and northern New England serpentinites have been mined for their asbestos.

Serpentine generally forms by metamorphism of ultramafic igneous rocks by one of two reactions. One type of serpentine reaction (see reaction 2) involves a mixed vapor phase of carbon dioxide and water, which produces some carbonate minerals. A second serpentine-forming reaction (see reaction 3) requires that some silica be dissolved in the water vapor.

Talc

Talc (Mg₃Si₄O₁₀(OH)₂) can form large masses of randomly oriented interlocking small flakes to make a rock called soapstone, used extensively for carving and as a source of talcum powder for health and beauty applications. The term “steatite” refers to talc-rich rocks that are used because of talc’s lack of chemical reactivity or its high heat capacity. Talc forms by regional metamorphism at low to moderate temperatures and low to moderate pressures. When the protolith is a sedimentary limestone or dolostone, the reaction for the formation of talc deposits is as shown in reaction 4.

A second common reaction that produces major talc deposits is the continuing metamorphism of a peridotite protolith. Talc forms by this reaction at temperatures slightly above 300° Celsius; however, the temperatures must remain below 700° Celsius to prevent the breakdown of talc.

Graphite

Graphite (a form of carbon, C) is used in a wide variety of applications from lubrication to high-temperature crucibles. Deposits of amorphous graphite form by contact metamorphism of coal beds, whereas deposits of flake graphite form by regional metamorphism of sedimentary rocks with the graphite being disseminated in mica schist and micaceous quartzite. Extensive weathering of these rocks assists in the release of the graphite. The graphite content of such metamorphic ores is usually 5 to 6 percent.

Clinker is a common term used by English miners for the graphite ore created by the contact metamorphism of coal beds. The reaction involves the breakdown of a wide variety of organic molecules. Continued high-temperature metamorphism of coal beds can transform the graphite into a natural coke, which has been mined in Wyoming and Utah.

Kyanite

Kyanite (Al₂SiO₅) and the related minerals andalusite and sillimanite are used in the production of refractory ceramics, such as those used in spark plugs. Kyanite forms from aluminum-rich clay-shale protoliths during regional metamorphism at moderate to high temperatures (see reaction 5).

Wollastonite

Wollastonite (CaSiO₃) is used extensively in the manufacturing of tiles. It forms by high-temperature contact metamorphism of silica-bearing limestones. An example may be found in Willsboro, New York, where the wollastonite mine is in a metamorphosed limestone on the margin of the igneous intrusion that forms the Adirondack Mountains. This type of reaction is shown in example 6. This reaction normally occurs at temperatures around 650° Celsius.

Jadeite

The pure form of the mineral jadeite (NaAlSi₂O₆) is the best quality of all materials called jade. Jade has been a valued material for sculpture and other art-and-craft applications for more than twenty-five centuries. It forms during burial metamorphism of alkali-rich igneous rocks that have been subjected to very high pressures and low temperatures. Such conditions are found in the mountains of the Coast Range in California, where jade has been mined (reaction 7).

Corundum

Corundum (Al₂O₃) is used extensively as an abrasive, and its pure colored variants known as ruby and sapphire are valued as gemstones. Corundum forms during regional metamorphism of aluminum-rich shale protoliths. The progressing metamorphism of the shale makes an intermediate mineral called staurolite, which commonly is sold in mineral shops and displayed in museums as “fairy crosses” because of its well-developed cruciform twining. Corundum forms when the staurolite breaks down at very high temperatures, as shown in reaction 8.

Metasomatism

A special type of metamorphism occurs whenever a major influx of new dissolved chemical components is added to the chemistry of the protolith. A water-rich fluid or vapor is the means of transport for this added chemistry. The process of adding chemistry to the rock through the vapor is called metasomatism. Metasomatism occurs chiefly in regions of contact metamorphism where highly volatile elements such as boron, fluorine, or chlorine are released into a water-rich fluid associated with the igneous pluton. The igneous-based fluid also carries dissolved silicon, aluminum, iron, magnesium, manganese, minor sodium, potassium, and often some tin, copper, tungsten, lead, and zinc. This saline fluid invades the adjacent limestone and reacts with calcium to form pronounced monomineralic zones at the contact between the pluton and the limestone.

The rocks produced by metasomatism are called skarns or tactites, and they are the coarsest grained of all metamorphic rocks. The garnet zone of a skarn may have individual grains of garnet that are as large as 20 centimeters in diameter. Skarns are mined throughout the world. Scheelite (CaWO₄), a major ore of tungsten, is mined from numerous metasomatized contact zones in California, Nevada, Idaho, and British Columbia. Other minerals that are mined from skarns are wollastonite, galena (an ore of lead), sphalerite (an ore of zinc), magnetite (an ore of iron), and chalcopyrite (an ore of copper).

Texture Changes and Recrystallization

During metamorphism changes may occur in the size, the shape, and often the orientation of the mineral grains within the rock. There are at least six different processes related to texture changes; the exact process is dependent upon which of the texture variables are changed and the mechanics of the change.

A change in size and shape of an existing mineral without the formation of any new minerals is a process called recrystallization. Certain sedimentary protoliths may be monomineralic rocks; two common examples are a limestone that is made entirely of the mineral calcite and a silica-cemented sandstone that is made entirely of the mineral quartz. Such single-mineral rocks are unable to promote any form of neocrystallization, and recrystallization is the only result of metamorphism.

Marble

The transformation from a sedimentary limestone to a metamorphic rock called marble often results in more than a thousandfold increase in the size of the calcite grains. The grains in the limestone protolith are commonly round in shape, whereas the grains in the marble interlock like a jigsaw puzzle to give a mosaic texture.

The interlocking texture in marble imparts a high coherence to the rock, yet its calcite mineralogy gives it a low hardness, allowing marble to be easily cut and polished. Pure white marble is used extensively for sculpting to form statues, as in the Lincoln Memorial; for building stone, as in the Greek Parthenon; and for ornamental carvings. Many marbles may contain an impurity that imparts a striking color pattern allowing their use in architecture as facings, tabletops, and flooring. Italy has more marble quarries than any other country. The United States quarries marble from both the Rocky and Appalachian mountain chains, with major quarries in Vermont and Colorado.

Foliation: Slate

A metamorphic rock in which the platy and elongate shaped minerals are parallel in their orientation is said to be foliated. A foliated texture can be seen in the rock by a tendency for the rock to break along parallel planes.

Slate is a foliated metamorphic rock in which the individual mineral flakes are so small that they can be seen only under the highest magnifications of a microscope. The foliation imparts to the slate the ability to break in near perfect planes. Slate is used as flagstones, roofing, floor tiles, hearthstones, and tabletops, especially billiard tables. A few slates are used not because of their foliation but because of their composition. Very clay-rich slates are ground because the smaller pieces will bloat when heated to form a material used as a lightweight aggregate.

Metamorphic Differentiation: Gneiss

At relatively high temperatures a metamorphic process occurs in which minerals segregate. The light-colored minerals such as quartz and feldspar move into zones parallel to the rock’s foliation, leaving behind alternate zones of dark minerals such as biotite and amphibole. Metamorphic differentiations cause a marked dark versus light layering in the rock. Such rock is commonly called gneiss. gneiss is quarried locally in many places as dimension stone.

Anatexis: Migmatites

At the more extreme temperatures for regional metamorphism, partial melting will begin to occur within the light-colored layers of a gneiss. The process of partially melting a rock is called anatexis, and this process begins the transformation from metamorphic to igneous rocks. Migmatite is the name for such a mixed rock. Migmatites occur in regions that have experienced a great amount of erosion to reveal the highest levels of metamorphism. Migmatites are common in the shield regions of the major continents. The shield for the North American continent is exposed in the upper peninsula of Michigan, northern Wisconsin and Minnesota, and throughout most of Canada.

Migmatites are commonly used as monument stone. The contortions of pattern generated by the partial melting make each stone unique and generally quite handsome. Migmatites are quarried in Minnesota, New York, and Michigan and are used as building stone throughout the United States.

Cataclastite

A special texture develops in rocks when the metamorphic pressure involves tectonic forces having a distinctly linear or planar orientation on the rock. Such opposing forces result in shear stress, and they cause mechanical breakage of the mineral grains in the rock. The name “cataclastite” refers to a metamorphic rock that exhibits a sheared texture containing many fragmented and distorted mineral grains that are often cemented together by a calcite matrix. Cataclastites are formed in tectonic regions that are experiencing active crustal movements. Some cataclastites are quarried and polished for use as a decorative “marble.” A famous cataclastite, the “Fantastica di Lasa,” is quarried from the northern Alps in Italy because of its attractive and unique appearance.

Bibliography

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U.S. Geological Survey. Metamorphic Rocks. http://vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Notes/metamorphic‗rocks.html