Paleogeology

Paleogeology, Paleoclimate, in relation to Evolution of Life on Earth

crystallization

Crystallization is the process by which minerals form an ordered state (crystal). In chemistry, crystallization occurs from solution, and in geology from molten rock. Crystals have a regularly repeating internal structure in which constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions, and hence display external plane faces.

Recrystallization involves an internal reorganization through partial melting (metamorphism), interaction with percolating chemical solutions (metasomatism), or through a diagenetic process by which unstable minerals in buried sediment are transformed into more stable minerals. Thus, recrystallization can involve a reorganization of elements of the original minerals in a rock in response to changes in temperature, and/or pressure, and/or the activity of pore fluids. Recrystallization can result in an increase in the degree of crystallinity, or crystal perfection or both.

Diagenetic alterations take place in deposited sediments, during and after lithification, and occur at relatively low temperatures and pressures. There exists a gradational rather than distinct boundary between diagenesis and metamorphism, which occurs under conditions of higher temperature and pressure. Diagenesis alters a rock's original mineralogy and texture, and can also contribute to fossilization of buried skeletal material by replacing the collagen of bones with minerals.

The crystal structure of most rock minerals depends upon the variety of arrangements available to tetravalent silicon, which can bond to various anions or be substituted by small cations, such as aluminum. Silicon can adopt triangular, tetrahedral, cubic, octahedral, and dodecahedral configurations upon bonding to other atoms. In the silicates, silica tetrahedra and octahedra can arrange/rearrange as:
● simple tetrahedra – nesosilicates [SiO4]4-olivine
● double tetrahedra – sorosilicates [Si2O7]6-epidote
● rings – cyclosilicates [SinO3n]2n-tourmaline
● chains – inosilicates [SinO3n]2n-pyroxene
● double chains – inosilicates [Si4nO11n]6n-amphibole
● sheets – phyllosilicates [Si2nO5n]2n-micas and clays
● 3D frameworks (lattices) – tectosilicates [AlxSiyO2(x+y)]x−quartz, feldspars, zeolites
● complex intermediates between the above configurations

[Jmol kaolinite, porphyrillite]

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