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a process of alteration (hydration) of ultrabasic rocks under the action of thermal aqueous solutions resulting in the replacement of anhydrous magnesium-rich silicates by minerals of the serpentine group. The products of this alteration are referred to as serpentinites. A distinction is made between metamorphic and metasomatic serpentinization. Metamorphic serpentinization, typical of deep ophiolite zones and related to general processes of regional metamorphism, is caused by the introduction of H2O with only minor changes in the content of the other components, which leads to an increase in the original volume of rock. Metasomatic serpentinization is related to the action of highly aggressive solutions, which remove MgO and SiO2 (approximately 30 percent by weight) from the rock while preserving the rock’s original volume. This type of serpentinization is a local process, typical of shallow stratified intrusions of ultrabasic rocks.
The American scientists N. Bowen and O. Tuttle in 1950 demonstrated experimentally the place of serpentinization in a series of hydration processes of ultrabasic rocks under the effect of water vapor, where the lithostatic pressure (Ps was equal to the pressure of the water vapor (PH2O). Serpentinization under these conditions is preceded by the transition of enstatite to talc and olivine, which is typical of complexes of shallow ultrabasic rocks. Moving to deeper zones of metamorphism, the lithostatic pressure (Ps increases relative to the partial pressure of water (PH2O the sequence of hydration reactions changes, and the enstatite in dunites is replaced directly by serpentine. Here, pseudomorphs of serpentine after enstatite (bastites) are formed.
Serpentinization is a multistage process and is accompanied by the formation of modifications of serpentine, namely, lizardite, chrysotile (in early stages), and antigorite (in relation to ensuing deformations and to schist-forming processes of the rock). Serpentinization is a feature made use of in searching for many minerals, for example, asbestos, anthophylite, and talc.
REFERENCESBowen, N., and O. Tuttle. “Sistema MgO—SiO2—H2O.” In the collection Voprosy fizikokhimii ν mineralogii i pelrografii. Moscow, 1950.
Marakushev, A. A. Termodinamika metamorficheskoi gidralatsii mineralov. Moscow, 1968.
A. A. MARAKUSHEV