User:Kbraun94/Neomorphism

Coined by the late Robert L. Folk, the term, “neomorphism,” refers to the wet metamorphic process in which diagenetic alterations systematically transform minerals into either polymorphs or crystalline structures that are chemically identical to the rock(s) from which they developed.^[1] Folk intended the term, "neomorphism," to encompass the functions of both recrystallization and inversion, which are geological concepts 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 carbonates and limestone, and neomorphism is largely accountable for the metastability of aragonite and magnesium calcite. 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

Recrystallization is a term that broadly refers to any change in the size or shape of crystals that is not accompanied by a change in chemical composition or mineralogy. Because recrystallization accounts for the majority of all visible changes produced as a result of neopmorphism, 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, as opposed to crystals, 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 or formation, the alteration of polymorphs occurs most often in environments characterized by optimal temperatures and pressure levels. Optimal temperature and pressure levels vary in accordance to the type of mineral(s).

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, in effect, is a collection of chemically-identical crystals that differs 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; under high pressure and high temperature, recrystallization by inversion will result in diamond. Both coal and diamond are derived from coal and are chemically identical, yet they differ remarkably in 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 places; the resultant mineral differs from its predecessor in terms of its composition. Replacement occurs without any substantial differences in volume between the original and the changed minerals, and the process is 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 with a variety of minerals, including " chert, pyrite, hematite, apartite, 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) 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]

Aggrading neomorphism occurs most frequently in limestone and other carbonate minerals will, more often than not, result in "microsparitic patches, laminae, and/or beds." The recrystallization of a carbonate named micrite calcite into microsparite is an example of aggrading neomorphism. In this process, freshwater flows across micrite calcite and releases several of the magnesium ions that protect the regarded carbonate mineral from external weathering; the process of recrystallization is initiated once this Magnesium ion "cage" is dissolved, resulting in the growth of microspar precursors that, subsequent to the direct influences of cementation and replacement, eventually form into microsparite.^[4]

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 can 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]

Gallery

The following images demonstrate the texture quality that is characteristic of minerals that have undergone neomorphism:

References

^ ^a ^b Sam Boggs (March 2009). "Diagenesis of carbonate rocks". Petrology of Sedimentary Rocks. ISBN 9780521897167.
^ University of Petroleum and Minerals (2007). Carbonate Geology (PDF).
^ ^a ^b Prof. Stephen A. Nelson. Minerals.
^ ^a ^b ^c ^d Erik Flugel (July 8, 2010). "Alteration and Recrystalization". Microfacies of Carbonate Rocks: Analysis, Interpretation and Application.

[2braun1994-1] Sam Boggs (March 2009). "Diagenesis of carbonate rocks". Petrology of Sedimentary Rocks. ISBN 9780521897167.

[2] University of Petroleum and Minerals (2007). Carbonate Geology (PDF).

[3kbraun94-3] Prof. Stephen A. Nelson. Minerals.

[1braun1994-4] Erik Flugel (July 8, 2010). "Alteration and Recrystalization". Microfacies of Carbonate Rocks: Analysis, Interpretation and Application.

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