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2011 |
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Тезисы международной конференции |
Abstracts of International conference |
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Mineralogical features of alkaline rocks with reference to the phase rule Khomyakov A.P.
Institute of mineralogy, geochemistry and crystal chemistry of rare elements Moscow, Russia; noomin@mail.ru
Исходя из закономерностей распределения разновозрастных минеральных комплексов в щелочных пегматитах и других минералогически уникальных объектах автором обосновывается представление о термодинамическом правиле фаз как важнейшей движущей силе эволюционной диверсификации минерального мира.
As is well known, the relation between the numbers of components (N), phases (P), and degrees of freedom (F) of a heterogeneous physicochemical system, derived by J. Gibbs from the basic laws of thermodynamics, is expressed by the phase rule P + F = N + 2, which implies that at arbitrary temperature and pressure the number of coexisting phases in an equilibrium system can not exceed the number of its independent components. This corollary, which makes the phase rule function as the principle of minimization of the number of coexisting minerals in equilibrium (Khomyakov, 1999, 2009), was successfully used by V.M. Goldschmidt (1933) and D.S. Korzhinskii (1973) as a theoretical basis for their graphical methods of the analysis of mineral parageneses. In fact, most igneous and metamorphic rocks of the lithosphere consist of not more than three- or four-phase assemblages of rock-forming minerals; this is typical of mineralogically unique rocks, in particular agpaitic nepheline syenites, which while accounting for a tiny fraction of igneous rocks, surpass any other rock types in mineral diversity. Such are alkaline rocks of the Khibiny-Lovozero complex, whose list of known minerals now totals about 700 species (Khomyakov, 2010). The total number of minerals that can potentially be discovered in this complex far exceeds this figure in our estimate. From the standpoint of physical chemistry, we are dealing here with a megasystem, whose number of phases can be brought into relation with the number of phase-forming components only by breaking it down into a large set of particular systems. At the same time, the main rock types of Khibiny and Lovozero, as well as igneous rocks in general, are characterized by no more than three- and four-phase assemblages of rock-forming minerals. This applies in particular to the rocks of the differentiated complex of the Lovozero massif, which formed by fractional crystallization of agpaitic magma (Kogarko, 1977). The vertical section of this complex consists of interbedded lujavrites, foyaites, and urtites which are 90% composed of nepheline, potassium feldspar, and aegirine, differing mainly in quantitative relationships between these three minerals. This complex is abundant in intensely mineralized hyperagpaitic pegmatite bodies (Khomyakov, 1990, 1995), which formed during the crystallization of residual melts. The best studied of these bodies, the Yubileinaya vein, has been found to contain about 60 different minerals, belonging to 13 classes and subclasses of chemical compounds - oxides, halides, sulfides, arsenides, carbonates, phosphates, phosphatosilicates, aluminosilicates, beryllosilicates, borosilicates, zircono-, titano- and niobosilicates. In addition to the petrogenic elements O, H, Si, Al, Na, K, Ca, Fe, Mg, Mn, and Ti, there are about 20 other elements that form their own minerals in the Yubileinaya vein: Li, Be, Sr, Ba, B, Ln, Th, U, Zr, Nb, P, Zn, Pb, As, Co, F, Cl, S, and C. The formation of mineral assemblages of hyperagpaitic pegmatites is subject to three main stages in the evolution of the acid-base properties of mineralizing solutions, corresponding to the stages of increasing (I), maximum (II), and decreasing (III) alkalinity. Stage I assemblages, which are similar in mineral composition to parent rocks, usually make up the marginal zones of differentiated pegmatites or the entire bodies of simple pegmatites, whereas stage II and III assemblages, which comprise the bulk of mineral species diversity, occur mainly in the central cores of pegmatite bodies. Similar zonal distribution patterns of minerals of different ages in differentiated pegmatites are found in various localities. They clearly indicate that as a result of the phase rule controlled priority transition of petrogenic elements to the crystalline state, most low-abundance elements, which are regarded in paragenetic analysis as trace components that have no effect on the equilibria of rock-forming minerals, are now capable of rapidly accumulating in the residual liquid, reaching phase saturation. This leads to the conclusion that the thermodynamic phase rule plays a fundamental role as a factor determining the universal (inherent in any evolving multicomponent systems) sequence of mineral precipitation: from a few rock-forming minerals (usually constitutionally primitive) to numerous minor and rare minerals (most of them unique in composition and structure), making the phase rule a major driving force in the evolutionary diversification of the mineral kingdom as a whole (Khomyakov, 2011). A.E. Fersman (1940) was the first to notice the important role of the phase rule in the evolution of pegmatite systems. After establishing, in various granite pegmatite bodies, a clear tendency for the number of minerals, averaging close to 20-25, to coincide with the number of elements that form their own lattices, he concluded that this pattern can only be explained by applying Goldschmidt's phase rule, whereby the maximum amount of solid minerals that can exist simultaneously is equal to the number of individual components contained in the minerals. He also proved that pegmatite processes can be regarded as successive changes in equilibria obeying the physical and chemical laws and that the evolution of the pegmatite process reduces to the cooling of the system, the gradual release of volatiles and solid crystalline precipitates, and the movement of some yet unknown physicochemical equilibrium diagram along the crystallization path. These findings are clearly applicable to the analysis of pegmatite processes associated with the differentiation of agpaitic magmas, with the difference that the derivatives of these magmas are typically much more advanced in their evolution compared to their granitic counterparts. This difference between the two rocks is quite understandable, since agpaitic magmas are considered by petrologists (Kogarko, 1977) as residual products of a long evolution of natural silicate melts, originating in the Earth's subcrustal zones, so that their derivatives initially accumulate a wider range of low-abundance elements capable of reaching saturation concentration and precipitating as independent minerals in the final stages of evolution.
References: Fersman A.E. Pegmatites. Vol. I. Granite pegmatites. Мoscow; Leningrad: Akad. Nauk SSSR, 1940. 712 p. (in Russian). Goldschmidt V.M. Mineral association laws from the standpoint of the phase rule // Basic Ideas of Geochemistry. Leningrad: Goskhimtekhizdat, 1933. Issue 1 (in Russian). Khomyakov A.P. Mineralogy of hyperagpaitic alkaline rocks. Moscow: Nauka, 1990. 196 p. (in Russian). Khomyakov A.P. Mineralogy of hyperagpaitic alkaline rocks. Oxford, U.K.: Clarendon Press, 1995. 224 p. Khomyakov A.P. Recent mineralogical discoveries and their physicochemical premises // Physicochemical problems of endogenous geological processes. Intern. symposium dedicated to the 100th anniversary of Acad. D.S. Korzhinskii. Abstracts. Moscow, 1999. P. 20-21 (in Russian). Khomyakov A.P. Phase rule and phase diversity of the mineral kingdom // Physicochemical factors in petro- and ore genesis: New frontiers. Moscow: Center for Information Technologies in Environmental Management, Moscow, 2009. P. 424-427 (in Russian). Khomyakov A.P. Khibiny-Lovozero complex as a mineralogical "Mecca" of Russia, Proc. Scientific and Practical Conference "Unique geological features of the Kola Peninsula: Khibiny." Apatity: K&M, 2010. P. 53-57 (in Russian). Khomyakov A.P. Phase rule as a driving force in the evolutionary diversification of the mineral kingdom // Mineralogical prospects. Proc. International mineralogical seminar. Syktyvkar: Geoprint, 2011. P. 154-156 (in Russian). Kogarko L.N. Problems of the genesis of agpaitic magmas. Moscow: Nauka, 1977. 295 p. (in Russian). Korzhinskii D.S. Theoretical basis of the analysis of the paragenesis of minerals. Moscow: Nauka, 1973. 288 p. (in Russian). |
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