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the branch of petrography that studies the distribution of chemical elements in magmatic rocks and in rock-forming minerals. The concept of petrochemistry was introduced by the Soviet scientist A. N. Zavaritskii in 1944. As an independent discipline, petrochemistry arose where the concerns of petrography meet those of chemistry. Petrochemistry studies through laboratory analysis the chemical composition of rocks, which is expressed in terms of the percent weight of oxides. Its methodology includes various systems for interpreting the laboratory analyses so as to reveal regular relationships between chemical composition and mineral composition.
In the USSR, the most commonly used system for interpreting data on magmatic rocks was devised by Zavaritskii. The system places the data on a vector diagram and distinguishes three chemical classes, or series, of rocks. Class I is called the normal series and includes rocks in which the aluminum-oxide content is greater than the sum of sodium- and potassium-oxide contents but less than the sum of calcium-, sodium-, and potassium-oxide contents. Class II, the plumasite series, contains rocks that are supersaturated with aluminum oxide, that is, the aluminum-oxide content exceeds the sum of calcium-, sodium-, and potassium-oxide contents. Rocks in Class III, the agpaite series, are supersaturated with alkalis; that is, the content of sodium and potassium oxides is greater than that of aluminum oxide. Outside the USSR, the interpretative system of the American scientists W. Cross, J. Iddings, L. Pirsson, and H. Washington (called the C.I.P.W. classification) and that of the Swiss geologist P. Niggli are commonly used.
By comparing the chemistry of various classes of rocks, petrochemistry reveals and explains the diversity of rocks, develops chemical classifications, and explains the origin of rocks. The development of petrochemical methods has broadened the applications of the discipline. Petrochemistry is now being used to study differentiation and fractional crystallization and the effects of assimilation on how rocks combine magmatic associations. Furthermore, the discipline is now being applied to clarifying the interrelationship of ore formation and magmatism and to mapping petrographical provinces that are characterized by a known set of metallogenic features.
REFERENCESZavaritskii, A. N. Vvedenie v petrokhimiiu izverzhennykh gornykh porod, 2nd ed. Moscow-Leningrad, 1950.
Levinson-Lessing, F. Iu. Izbr. trudy, vols. 1-2. Moscow-Leningrad, 1949-50.
Solov’ev, S. P. Khimizm magmaticheskikh gornykh porod i nekotorye voprosy petrokhimii. Leningrad, 1970.
S. P. SOLOV’EV