Neomorphism

Last updated March 18, 2023

Neomorphism refers to the wet metamorphic process in which diagenetic alterations systematically transform minerals into either polymorphs or crystalline structures that are structurally identical to the rock(s) from which they developed.^[1]

Coined by the late Robert L. Folk, neomorphism encompasses the functions of both recrystallization and inversion, which are geological processes that deal primarily with rock reformation. The neomorphic process, as it relates to geology and petrography, is one of the many major processes that sustain both carbonate minerals and limestone. Neomorphism is largely held accountable for the metastability of aragonite and magnesium-rich calcite, and when conditions permit, neomorphic reactions and interactions can result in texture loss and/or feature deformation of affected rock formations.^[2]

Types of neomorphism

Recrystallization

The term "recrystallization" broadly refers to the many metamorphic processes that change the size and/or shape of crystal formations and preserve the chemical composition and mineralogy of the original mineral. Because recrystallization accounts for the majority of all visible changes produced by neomorphism, the terms "neomorphism" and "recrystallization" implicitly allude to each other and can therefore be used interchangeably under most circumstances. In petrology, there are two forms of recrystallization: recrystallization by inversion and recrystallization by replacement.

Inversion

Inversion is a complex form of neomorphism in which the recrystallization process transforms polymorphs into different polymorphs. Polymorphs, to be clear, are minerals that differ from one another in their crystalline structures but are otherwise composed of identical quantities and types of elements. As with any change in mineral structure, the alteration of polymorphs occurs most often in environments characterized by certain optimal temperatures and pressure levels. Optimal temperature and pressure levels vary in accordance to the type of mineral(s) under consideration.

Specifically, an increase in temperature incites an increase in atomic vibrations, which instigates atoms to distance themselves from each other. The excited atoms continue expanding until the increase in temperature can no longer provide the energy necessary for further expansion. Affected crystals and/or minerals are forced to adapt to the aforementioned atomic changes by expanding their skeletal structures, which results in visible changes of the aforementioned crystals and minerals. All the while, pressure continuously compresses the altered crystals and minerals into dense structures; the final product is a collection of chemically-identical crystals that differs structurally and visibly from its predecessor.^[3]

Perhaps the most pervasive example of inversion occurs on carbon. The inversion of carbon, depending on the temperature and pressure of the environment, results in one of two very distinct polymorphs: Under low temperature and low pressure, recrystallization by inversion will result in coal, and under high pressure and high temperature, recrystallization by inversion will result in diamond. Both coal and diamond are derived from carbon and are chemically identical, but they differ remarkably from each other in terms of physical appearance.^[3]

Replacement

Replacement is a complex form of neomorphism in which the recrystallization process involves the dissolution of one mineral and the almost immediate "precipitation" of another in its place; the resultant mineral differs from its predecessor in terms of its chemical composition. Replacement occurs without any substantial changes in volume between the original and the reformed minerals, and the process is often characterized as being either fabric-destructive or fabric-preserving, which refer to texture loss and texture retention, respectively. The replacement of fossils with chert, for example, is often fabric-preserving, while the replacement of aragonite and calcite with dolomite is fabric-destructive. On a side note, this particular process (the replacement of aragonite and calcite with dolomite) is the most common form of recrystallization by replacement. Being similar to wet polymorphic transformations, recrystallization by replacement occurs on a variety of minerals, including chert, pyrite, hematite, apatite, anhydrite, and dolomite, among others.^[1]

Neomorphic processes

Coalescive Neomorphism

Neomorphism is considered coalescive when the recrystallization process involves either the formation of larger crystals in the place, and at the expense, of smaller crystal formations or the formation of smaller crystals within preexisting formations of crystals. Two types of coalescive neomorphism exist in petrology: aggrading neomorphism and degrading neomorphism.^[4]

Aggrading neomorphism

Neomorphism is considered aggrading when recrystallization results in an any increase in crystal size. The crystal mosaics of the original mineral or crystal formation(s) often experience deterioration in the process and are eventually replaced with either crude crystalline mosaics or polymorphs. Both the resultant crystalline mosaics and/or polymorphs are chemically identical—with a few minor exceptions due to certain relatively minute chemical alterations that occur during the reaction processes—to the minerals from which the aggraded crystals developed.^[4]

One common form of aggrading neomorphism is called porphyroid neomorphism. Porphyroid neomorphism occurs when a small number of large crystals form in the area of static groundmasses, which are—as the name implies—areas of the ground that are characterized by relatively insignificant and unsubstantial metamorphic changes.^[1] Apart from the aforementioned, porphyroid neomorphism is characterized by the destruction of original micritic matrixes.^[5]

Degrading neomorphism

Neomorphism is considered degrading when the recrystallization process is accompanied by a net decrease in the size of any affected crystal formation(s). Degrading neomorphism is a form of coalescive neomorphism in which new crystals form from within preexisting crystals. This form of neomorphism is relatively uncommon and typically only occurs under stressed conditions and on minerals that have been left relatively unaffected by metamorphism.^[4]

Related Research Articles

<span class="mw-page-title-main">Limestone</span> Sedimentary rocks made of calcium carbonate

Limestone is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of CaCO₃. Limestone forms when these minerals precipitate out of water containing dissolved calcium. This can take place through both biological and nonbiological processes, though biological processes, such as the accumulation of corals and shells in the sea, have likely been more important for the last 540 million years. Limestone often contains fossils which provide scientists with information on ancient environments and on the evolution of life.

In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.

Sandstone is a clastic sedimentary rock composed mainly of sand-sized silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.

<span class="mw-page-title-main">Metamorphic rock</span> Rock that was subjected to heat and pressure

Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than 150 to 200 °C and, often, elevated pressure of 100 megapascals (1,000 bar) or more, causing profound physical or chemical changes. During this process, the rock remains mostly in the solid state, but gradually recrystallizes to a new texture or mineral composition. The protolith may be an igneous, sedimentary, or existing metamorphic rock.

Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO₃). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratch hardness comparison. Large calcite crystals are used in optical equipment, and limestone composed mostly of calcite has numerous uses.

Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, CaCO₃. It is formed by biological and physical processes, including precipitation from marine and freshwater environments.

Metamorphism is the transformation of existing rock to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of 150 to 200 °C, and often also at elevated pressure or in the presence of chemically active fluids, but the rock remains mostly solid during the transformation. Metamorphism is distinct from weathering or diagenesis, which are changes that take place at or just beneath Earth's surface.

Quartzite is a hard, non-foliated metamorphic rock which was originally pure quartz sandstone. Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts. Pure quartzite is usually white to grey, though quartzites often occur in various shades of pink and red due to varying amounts of hematite. Other colors, such as yellow, green, blue and orange, are due to other minerals.

Ooids are small, spheroidal, "coated" (layered) sedimentary grains, usually composed of calcium carbonate, but sometimes made up of iron- or phosphate-based minerals. Ooids usually form on the sea floor, most commonly in shallow tropical seas. After being buried under additional sediment, these ooid grains can be cemented together to form a sedimentary rock called an oolite. Oolites usually consist of calcium carbonate; these belong to the limestone rock family. Pisoids are similar to ooids, but are larger than 2 mm in diameter, often considerably larger, as with the pisoids in the hot springs at Carlsbad in the Czech Republic.

<span class="mw-page-title-main">Magnesite</span> Type of mineral

Magnesite is a mineral with the chemical formula MgCO^₃. Iron, manganese, cobalt, and nickel may occur as admixtures, but only in small amounts.

Petrography is a branch of petrology that focuses on detailed descriptions of rocks. Someone who studies petrography is called a petrographer. The mineral content and the textural relationships within the rock are described in detail. The classification of rocks is based on the information acquired during the petrographic analysis. Petrographic descriptions start with the field notes at the outcrop and include macroscopic description of hand specimens. The most important petrographer's tool is the petrographic microscope. The detailed analysis of minerals by optical mineralogy in thin section and the micro-texture and structure are critical to understanding the origin of the rock.

Dolomite (also known as dolomite rock, dolostone or dolomitic rock) is a sedimentary carbonate rock that contains a high percentage of the mineral dolomite, CaMg(CO₃)₂. It occurs widely, often in association with limestone and evaporites, though it is less abundant than limestone and rare in Cenozoic rock beds (beds less than about 66 million years in age). The first geologist to distinguish dolomite rock from limestone was Belsazar Hacquet in 1778.

Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical composition. The minerals which compose the rocks are dissolved and new mineral formations are deposited in their place. Dissolution and deposition occur simultaneously and the rock remains solid.

<span class="mw-page-title-main">Hornfels</span>

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, characteristic of metamorphism at high temperature but without accompanying deformation. 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.

<span class="mw-page-title-main">Carbonate rock</span> Class of sedimentary rock

Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite (different crystal forms of CaCO₃), and dolomite rock (also known as dolostone), which is composed of mineral dolomite (CaMg(CO₃)₂).

Zeolite facies describes the mineral assemblage resulting from the pressure and temperature conditions of low-grade metamorphism.

<span class="mw-page-title-main">Clastic rock</span> Sedimentary rocks made of mineral or rock fragments

Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus, chunks, and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic to refer to sedimentary rocks and particles in sediment transport, whether in suspension or as bed load, and in sediment deposits.

A metamorphic facies is a set of mineral assemblages in metamorphic rocks formed under similar pressures and temperatures. The assemblage is typical of what is formed in conditions corresponding to an area on the two dimensional graph of temperature vs. pressure. Rocks which contain certain minerals can therefore be linked to certain tectonic settings, times and places in the geological history of the area. The boundaries between facies are wide because they are gradational and approximate. The area on the graph corresponding to rock formation at the lowest values of temperature and pressure is the range of formation of sedimentary rocks, as opposed to metamorphic rocks, in a process called diagenesis.

<span class="mw-page-title-main">Symplectite</span>

A symplectite is a material texture: a micrometre-scale or submicrometre-scale intergrowth of two or more crystals. Symplectites form from the breakdown of unstable phases, and may be composed of minerals, ceramics, or metals. Fundamentally, their formation is the result of slow grain-boundary diffusion relative to interface propagation rate.

Cone-in-cone structures are secondary sedimentary structures that form in association with deeper burial and diagenesis. They consist of concentric inter-bedded cones of calcite or more rarely gypsum, siderite or pyrite. Although several mechanisms may be responsible for the formation of cone-in-cone structures, displacive crystal mechanism is preferred. It accounts for the most uniform and consistent explanation of growth and why cone-in-cone can occur with such variable composition.

References

1 2 3 Sam Boggs (March 2009). "Diagenesis of carbonate rocks". Petrology of Sedimentary Rocks. ISBN 9780521897167.
↑ University of Petroleum and Minerals (2007). Carbonate Geology (PDF).
1 2 Prof. Stephen A. Nelson. Minerals. Archived from the original on 2014-03-01. Retrieved 2015-04-08.
1 2 3 Peter Scholle and Dana Ulmer-Scholle (July 8, 2010). "Petrography of Carbonate Rocks". A Color Guide to the Petrography of Carbonate Rocks: Grains, Textures, Cements, and Porosites. ISBN 9780891813583.
↑ Erik Flugel (October 2006). "Alteration and Recrystalization". Microfacies of Carbonate Rocks: Analysis, Interpretation and Application. ISBN 9783642037962.