Petrography

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petrography

[pə′träg·rə·fē]
(geology)
The branch of geology that deals with the description and systematic classification of rocks, especially by means of microscopic examination.

Petrography

 

the study of rocks, which is concerned with their mineralogical and chemical composition, structure, texture, mode of occurrence, laws of distribution, origin, and change in the earth’s crust and on its surface. There is a tendency to separate the general science of rocks into two parts: petrography, which is primarily descriptive, and petrology, which analyzes genetic relationships. However, the terms “petrography” and “petrology” are often regarded as synonymous.

Areas of study and methods. Petrography, a science of the geological cycle, is closely associated with mineralogy, geochemistry, volcanology, tectonics, stratigraphy, and the study of mineral resources. The science is divided according to the type of rocks studied into petrography of igneous rocks, petrography of metamorphic rocks, and petrography of sedimentary rocks.

Petrography of igneous rocks studies crystalline rocks formed mainly as a result of the solidification and crystallization of magma. Magmatic differentiation during cooling in the earth’s crust and the assimilation of the country rocks into the magma led to the origin of compositionally diverse types of igneous rocks and their association with mineral resources. Igneous rocks are studied to determine their material composition, their relationship with surrounding rocks, and the physical and chemical conditions of magma solidification.

The study of metamorphic rocks is concerned with rocks that have changed, without fracturing or melting, their mineral and chemical composition under the influence of new physical and chemical conditions (seeMETAMORPHISM). The rocks are classified according to the nature of the alteration into different metamorphic facies, whose mineral composition is determined mainly by the pressure and temperature of the environment.

There are some rocks that occupy an intermediate position. For example, some metamorphic rocks undergo partial melting during formation (seePALINGENESIS), and metamorphic processes play an important role in the formation of some igneous rocks. Other rocks (volcanogenic sedimentary rocks, pyroclastic rocks) are transitional between sedimentary and igneous rocks. Although they are composed of magmatic material, their formation and modes of occurrence are typical of sedimentary rocks.

Special research methods are used for studying the composition and structure of rocks. Of primary importance are optical crystallographic methods, which make possible the study of finegrained mineral aggregates. These methods require the use of polarizing microscopes and other instruments. X-ray examination and spectrum analysis are widely used, making it possible to determine the admixture elements present in rocks in negligible quantities. The chemical composition of the minerals in rocks is determined by electron microprobes without removing the minerals from the rocks. The composition of rocks is also studied by chemical analysis. Physical studies of rocks and the minerals composing them are used for determining a number of physical constants, such as density, hardness, thermal expansion, compressibility, seismic velocities, viscosity, and electrical and magnetic properties. Since roughly 1950, computer-based mathematical methods have been used increasingly. Mathematical statistics are used to evaluate the reliability of chemical or spectrum analyses, to establish rational rock classifications, to determine prospecting criteria for different types of mineral resources, and to scale chemical analyses. Thus, the study of rocks involves various types of research, beginning with field observations (geological surveying, coring, or mining excavations). The correlation of geological and petrographic materials on a regional basis makes it possible to explain the role of various types of rocks in the formation and development of the earth’s crust.

The following divisions of petrography are distinguished according to the properties under study and the methods used: petrochemistry, petrophysics, petrotectonics (structural petrography), physicochemical and experimental petrography, industrial petrography, and astropetrography.

Petrochemistry is concerned with the entire complex of chemical interrelationships in individual rocks and in their natural combinations.

The development of geological-engineering and geophysical research fostered the study of the physical properties of rocks. Thus appeared petrophysics, which establishes the relationship of the physical properties of rocks with their composition, structure, and formation.

Petrotectonics studies the relationships between the geometric regularities of rock microstructures and the movements or deformations within them to explain the active forces and stresses. It is based on microstructural (petrostructural) analysis, which is directed toward establishing the predominant dimensional orientation of the planar and linear components of rock structure.

Physicochemical petrography reveals, on the basis of the general laws of thermodynamics, the relationships between the chemical and mineral compositions of rocks on the one hand and the general conditions of their formation on the other.

Experimental petrography is concerned with creating models of the natural formation of rocks (constituents of their minerals and mineral associations).

A special direction in petrography is industrial petrography, the principles of which were set forth by the Soviet geologist D. S. Beliankin. With the aid of petrographic methods, industrial petrography studies the mineral composition of industrial products (slags, procelain, cement, glass, ceramics, artificial stone). It is thereby of great help in silicate production and metallurgy. Utilizing the experience of technology in the production of stone products, industrial petrography helps in the interpretation of many rock-forming processes.

Astropetrography, which developed in the 1970’s, studies meteorites, lunar rocks, and rocks from various planets.

Historical sketch. The bases of petrography were established in the mid-19th century. Until this time, problems of petrography were resolved in part by mineralogy and general geology. All rocks were divided according to their origin into sedimentary, igneous, and metamorphic rocks. The birth of petrography as a science dates to the middle of the 19th century, when H. C. Sorby demonstrated the possibility of studying the mineral composition of rocks in thin sections under a microscope. Later, the polarizing microscope was introduced into petrographic research, followed by improved methods of optical crystallography (German petrographers K. H. Rosenbusch and F. Zirkel, French petrographers F. Fouqué and A. Michel-Lévy, the

Soviet petrographer A. P. Karpinskii, and the American petrog-rapher E. Larsen). E. S. Fedorov developed the method of studying the optical constants of minerals in thin sections with the aid of a universal stage. Techniques for determining the composition of minerals according to their crystal optical properties, which are now the basis for the study of rock matter, were proposed (the Russians Fedorov and V. V. Nikitin and the American A. Winchell). The Fedorov method gave rise to microstructural analysis (German scientists B. Sander, H. Becker, and W. Schmidt; the Soviet scientist N. A. Eliseev).

At the same time, chemical research methods were improved, which, together with the appearance of abundant descriptive petrographic material, led in the 1920’s and 1930’s to quantitative mineralogical (P. Niggli, B. M. Kupletskii) and chemical (F. Iu. Levinson-Lessing, A. N. Zavaritskii, Rosenbusch, Niggli) classifications of igneous rocks, based on various methods of scaling chemical analyses.

In the late 19th and early 20th centuries, petrographers concentrated their attention mainly on the origin and causes of the diversity of igneous rocks. Hypotheses were proposed on the differentiation of parent magma into partial magma and on the assimilation by magma of enclosing rocks. In the late 19th century, Levinson-Lessing demonstrated that two different (in principle) magmas—acid (granitic) magma and basic (basaltic) magma—serve as the parental source of igneous rocks formed on the earth’s surface. This idea was supported by R. Daly in the 1920’s. In the early 1930’s, N. L. Bowen proposed the highly popular hypothesis that within the earth there existed a single basaltic magma, from which almost all igneous rocks could be formed by fractional crystallization (separation from residual magma owing to the emersion or submersion in it of precipitated or separated crystals). Later, fractional crystallization was observed in nature (A. A. Polkanov, the British scientists L. Wager and G. Brown).

Much attention was given by petrographers to granites occurring in plutonic metamorphosed gneissic and migmatite masses. In the early years of the 20th cenury, J. J. Sederholm showed that these rocks possess a number of features that are difficult to explain if one suggests the intrusion of granitic magma. He pointed out that such granites are not magmatic but were formed as a result of metasomatic granitization or ultrametamorphism under the effect of plutonic emanations. Sederholm’s ideas were popular in the 1940’s and 1950’s (P. Eskola, H. G. Backlund, Iu. A. Kuznetsov, N. G. Sudovikov).

D. S. Korzhinskii, beginning in 1936, set forth the fundamentals of the physicochemical analysis of mineral parageneses. The components of rocks were divided into groups in accordance with the role that they play in mineral formation. Introduced were concepts of the differential mobility of components and of systems with totally mobile components. In such systems, the conditions of chemical equilibrium are determined by special thermodynamic potentials (allochemical equilibrium potentials). These concepts expanded considerably the application of mineral paragenesis analysis to natural processes (seeMINERALOGICAL PHASE RULE). Korzhinskii showed that magmatism in the earth’s crust develops in close interaction with fluids (transmag-matic solutions). He substantiated the large role of magmatic replacement processes in the making of igneous rocks under plutonic conditions and developed the theory of metasomatic zonation. Mineral facies systems of igneous, metamorphic, and metasomatic rocks were developed in the 1960’s and 1970’s on the basis of mineral paragenesis analysis (Soviet geologists V. A. Zharikov and A. A. Marakushev).

Experimental research was of great importance in identifying the origin of various igneous and metamorphic rocks (Levinson-Lessing and A. S. Ginzberg in the early 20th century; the Americans Bowen, O. Tuttle, and R. Goranson in the 1920’s and 1930’s). This research increased tremendously in scope in the 1950’s and 1960’s (the Soviets I. A. Ostrovskii, N. I. Khitarov, V. S. Sobolev, G. L. Pospelov; the Americans D. H. Hamilton, H. Joder, and C. Tilley; the Australians D. Green and A. Ring-wood).

Of particular importance was the study of rock melting (fusion) under the vapor pressure of H2O, CO2, H2, and other volatile components. It was established that the melting point of silicates is sharply lowered in the presence of water. Therefore, under natural conditions, granitic melt can be obtained from parent rocks of varying composition in the presence of water and at relatively low temperatures.

Current research. The possibility of granite formation owing to fusion from the plutonic mantles of the earth again came under discussion in the 1960’s and 1970’s on the basis of new petrological, experimental, and geophysical research (the Soviet scientists

D. S. Shteinberg and P. N. Kropotkin). Many petrographers recognize two types of granites. The first was formed from palin-genic granitic magma of relatively low temperature, which developed during partial melting of the rocks of the earth’s crust when the rocks were saturated with water (seePALINGENESIS). Autochthonous or parautochthonous granites are formed when magma crystallizes in situ. The second type of granite originates from acid melts that form during the transformation (differentiation, contamination by sialic material) of basaltic magma, which comes from the upper mantle or lower parts of the earth’s crust. Such acid melts are characterized by high temperature and are capable of reaching the earth’s surface, forming not only intrusive but also extrusive granites.

Much petrographic research is devoted to the problem of magmatic formations, which unite groups of genetically and structurally interrelated igneous rocks that form stable associations (G. D. Afanas’ev, Iu. A. Kuznetsov). Volcano-plutonic formations have been shown to exist (the Soviet petrographer

E. K. Ustiev). Research is continuing on the relationship of magmatism and tectonics, which was first proposed by H. Stille. A great deal of attention is devoted to the study of the magmatism of the oceans, especially of the midocean ridges, whose origin is associated with the abyssal processes of magma formation (Green and Ringwood). It has been suggested that the ophi-olitic series of geosynclinal regions were formed in the oceanic regions of the geological past (seeOPHIOLITE).

Institutions and publications. Petrographic research is carried out in the USSR by institutes of the Academy of Sciences of the USSR, by administrations and departments of the ministries of geology of the USSR and the Union republics, and by educational institutes. The Interdepartmental Petrographic Committee, which resolves questions related to the origin and classification of rocks, was created in 1952 under the auspices of the Division of Geological and Geographical Sciences of the Academy of Sciences of the USSR. Problems of petrography are discussed at the All-Union Petrographic Conferences, which have been held every four or five years since 1953, and at regional petrographic conferences. In addition, the most important problems of petrography are dealt with at sessions of the International Geological Congress.

Articles on petrography are published in a number of periodicals. In the USSR such publications include the geolgical series Doklady (Reports) and Izvestiia (Proceedings) of the Academy of Sciences of the USSR, Zapiski Vsesoiuznogo mineralogicheskogo obshchestva (Notes of the All-Union Mineralogical Society), and the journal Sovetskaia geologiia (Soviet Geology). Problems of petrography are discussed in the Journal of Petrology (Oxford, from 1960).

REFERENCES

Bowen, N. L. Evoliutsiia izverzhennykh porod. Moscow-Leningrad-Novosibirsk, 1934. (Translated from English.)
Rosenbusch, H. Opisatel’naia petrografiia. Moscow-Groznyi-Novosi-birsk, 1934. (Translated from German.)
Levinson-Lessing, F. Iu. Izbr. trudy, vol, 4. Petrografiia. Moscow, 1955.
Eliseev, N. A. Metamorfizm. Moscow, 1963.
Kuznetsov, Iu. A. Glavnye tipy magmaticheskikh formatsii. Moscow, 1964.
Zavaritskii, A. N. Vvedenie v petrokhimiiu izverzhennykh gornykh porod, 2nd ed. Moscow-Leningrad, 1950.
Zavaritskii, A. N. Izverzhennye gornye porody. Moscow, 1961.
Lukin, L. I., V. F. Chernyshev, and I. P. Kushnarev. Mikrostrukturnyi analiz. Moscow, 1965.
Petrologiia verkhnei mantii. Moscow, 1968. (Translated from English.)
Winkler, H. Genezis metamorficheskikh porod. Moscow, 1969. (Translated from German.)
Wager, L., and G. Brown. Rassloennye izverzhennye porody. Moscow, 1970. (Translated from English.)
Solov’ev, S. P. Khimizm magmaticheskikh gornykh porod i nekotorye voprosy petrokhimii. Leningrad, 1970.
Petrov, V. P. Magma i genezis magmaticheskikh gornykh porod. Moscow, 1972.
Korzhinskii, D.S. Teoreticheskie osnovy analiza paragenezisov mineralov. Moscow, 1973.
Perchuk, L. L. Termodinamicheskii rezhim glubinnogo petrogeneza. Moscow, 1973.
Sander, B. Einführung in die Gefügekunde der geologischen Körper, parts 1-2. Vienna-Innsbruck, 1948-50.