METASOMATISM, in petrology a process of alteration of rocks which involves enrichment of the rock by new substances introduced from without change, body). Sub stances originally present are partly or wholly removed in solu tion, the newly introduced material entering as gas or in aqueous solution. Usually the enrichment takes place by definite chemical reactions, but this is not always so ; the conversion of limestone into siliceous chert though recognized as a metasomatic process is not directly governed by any definite chemical reaction. Pre cipitation of material may result from changes in solution ren dering insoluble a substance foreign to the mineral acted upon. Replacement may be partial or complete, with or without preser vation of original structures and textures. Metasomatic processes take place over a wide range of temperatures and pressures and may be accompanied by a volume change ; in general they obey the law of mass action, and in order to effect replacement the introduced solutions must attain a minimum concentration. This minimum concentration of solutions or minimum vapour pressure in the case of gas reactions varies both with different reactions and the temperatures and pressures at which the re placement is effected. Among the common examples of metaso matism may be noted the dolomitization and conversion into siderite of iron oxides of limestones. Phosphatization of lime stones is a further example. These replacements probably take place at low temperatures, as do also the metasomatic changes in salt deposits, such as the conversion of anhydrite into glau berite or polyphalite (see PNEUMATOLYSIS).
Metasomatism is widespread in silicate rocks and is especially characteristic of contact zones of igneous intrusions. These re placements affect, however, not only the intruded rocks but also the crystallized magma from which the emanations are derived.
Metasomatism involving addition of alkalis is represented by such processes as albitization, analcitization and the formation of fel spar in certain contact zones of injected rocks.
The production of tourmaline, topaz and lithra-mica as seen in tourmaline and topaz-hornfelses, greisens, etc., is a metaso matic process involving enrichment of the rocks in boron, fluorine and lithium. Similarly the formation of chloride-marialite (scapo lite) at the expense of felspars, serpentine from olivine, sericiti zation of felspars and felspathoid minerals are common examples of metasomatism occurring around igneous intrusions.
Many ore deposits and the regions in their vicinity show evidences of widespread chemical replacement leading to enrich ment, as in sulphide deposits.
Limestones are particularly prone to metasomatic alteration.
Their conversion to dolomite, siderite and iron oxide has already been referred to. Around igneous contacts they are not infre quently locally changed to andradite garnet or hedenbergite rocks, processes involving great enrichment in iron and silica. These garnet and pyroxene rocks are known as "skarns." Probably iron is introduced in the gaseous form as fluoride or chloride.
Valuable ore deposits are frequently associated with rocks of this character. In many parts of western North America (Ne vada, Arizona, etc.), great deposits of copper, lead and silver ores are worked in crystalline limestones and are often clearly replacement products of the limestones themselves. The constant presence of igneous rocks in their vicinity indicates that they are connected with the introduction of the metals, and the deposits are often of such a kind as to show that post-volcanic discharges or magmatic gases and water were the mineralizing agents.
(C. E. T.)