Neomorphism refers to the wet

metamorphic 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 and, often, elevated pressure of or more, causi ...

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. Coined by the late

Robert L. Folk The name Robert is an ancient Germanic given name, from Proto-Germanic "fame" and "bright" (''Hrōþiberhtaz''). Compare Old Dutch ''Robrecht'' and Old High German ''Hrodebert'' (a compound of '' Hruod'' ( non, Hróðr) "fame, glory, ho ...

, ''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 Geology () is a branch of natural science concerned with Earth and other Astronomical object, astronomical objects, the features or rock (geology), rocks of which it is composed, and the processes by which they change over time. Modern geology ...

and

petrography 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 class ...

, is one of the many major processes that sustain both carbonate minerals and

limestone Limestone ( calcium carbonate ) 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 . Limestone forms w ...

. Neomorphism is largely held accountable for the

metastability In chemistry and physics, metastability denotes an intermediate energetic state within a dynamical system other than the system's state of least energy. A ball resting in a hollow on a slope is a simple example of metastability. If the ball i ...

aragonite Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including pre ...

and magnesium-rich

calcite 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 scratc ...

, and when conditions permit, neomorphic reactions and interactions can result in texture loss and/or feature deformation of affected rock formations.

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 Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical) properties of minerals and mineralized artifacts. Specific studies within mineralogy include the proce ...

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 Petrology () is the branch of geology that studies rocks and the conditions under which they form. Petrology has three subdivisions: igneous, metamorphic, and sedimentary petrology. Igneous and metamorphic petrology are commonly taught together ...

, 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 Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

and

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...

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. Perhaps the most pervasive example of inversion occurs on

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...

. 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 Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when ...

, and under high pressure and high temperature, recrystallization by inversion will result in

diamond Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the chemically stable form of carbon at room temperature and pressure, b ...

. Both coal and diamond are derived from carbon and are chemically identical, but they differ remarkably from each other in terms of physical appearance.

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

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

s with

chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a ...

, for example, is often fabric-preserving, while the replacement of

and

with

dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...

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 The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Iron, FeSulfur, S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic Luster (mineralogy), lust ...

hematite Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...

apatite Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most common ...

anhydrite Anhydrite, or anhydrous calcium sulfate, is a mineral with the chemical formula CaSO4. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. It is not isomorphous with the ...

, and dolomite, among others.

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

: aggrading neomorphism and degrading neomorphism.

Aggrading neomorphism

Neomorphism is considered ''aggrading'' when recrystallization results in an any increase in crystal size. The crystal mosaics of the original

mineral 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.John P. Rafferty, ed. (2 ...

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macro ...

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. 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. Apart from the aforementioned, porphyroid neomorphism is characterized by the destruction of original micritic matrixes.

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 A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

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 Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of ch ...