An Application of Multicomponent Solution Theory to Jadeitic Pyroxenes
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Abstract:
The pressure-temperature conditions of formation of jadeite-bearing pyroxenes can be calculated from data on the reactions: NaAlSix3O8 + NaAlSiO4 = 2 NaAlSi2O6, NaAlSi3O8 = NaAlSi2O6 + SiO2, where feldspar, nepheline, and pyroxene occur as complex solid solutions. If it is assumed that (a) nepheline can be treated as a binary "sub-regular" solution, (b) pyroxene can be treated as a multicomponent "sub-regular" solution, and (c) the activity coefficient of albite in plagioclase coexisting with jadeitic pyroxene is 1.0, then experimental data yield a relationship between the compositions of coexisting minerals and their temperatures of equilibration. For nepheline-bearing assemblages this relation is: , where T is the temperature in °K, P the pressure in bars, , , and are, respectively, the mol fractions of jadeite, diopside, and hedenbergite in pyroxene, and the mol fractions of other (unspecified) components in pyroxene, the mol fraction of albite in plagioclase, and the mol fraction of KAlSiO4 in nepheline. The corresponding expression for quartz-bearing assemblages: , gives pressure-temperature conditions of equilibration for natural omphacitic pyroxenes in agreement with those estimated by other means. Jadeite-rich pyroxenes can form under amphibolite facies metamorphic conditions in nepheline-bearing assemblages, as demonstrated by metamorphosed alkaline rocks from central Labrador. The model predicts limits on the composition of omphacitic pyroxenes which agree with observation.Keywords:
Nepheline
Pyroxene
Nepheline syenite
Diopside
Grossular
Alkali feldspar
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Nepheline
Nepheline syenite
Alkali feldspar
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Abstract The change in chemical composition trend of magmatic nepheline through magma evolution has been characterized from the alkaline series of the Messum complex in which nepheline occurs in a succession of different mineral parageneses from mafic-rich (theralites) to strongly evolved felsic-rich rock types (nepheline syenites). The nepheline compositions are dependent on those of coexisting feldspar(s). They record an evolution parallel to that of the melt schematized according to experimental phase diagrams, from initially Ca-rich compositions in equilibrium with calcic plagioclase towards increasingly Ca-poor, Na-rich and Si-rich compositions. The K contents show a maximum that corresponds to the appearance of alkali feldspar in the parageneses. This evolution is qualitatively preserved in spite of the low- T Na/K re-equilibration typical of plutonic nephelines. Although a slight increase in the silica content of nepheline is consistent with the experimentally defined magmatic trend, several high-silica nephelines from the Messum rocks as well as from other reported occurrences, cannot be reconciled with the experimental data. The nepheline solid-solution model available suggests that such ‘abnormal’ compositions might be related to different crystallization mechanisms between natural nephelines and some synthetic analogues.
Nepheline
Nepheline syenite
Felsic
Alkali feldspar
Peralkaline rock
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The Raman spectra of nine 1‐atm glass samples in the system nepheline‐diopside are presented. Nepheline glass consists of a fully polymerized network structure with largely six‐membered rings of Q 4 tetrahedra. The structure of diopside glass contains Q 0 , Q 1 Q 2 , and Q 3 units. Intermediate compositions with up to 18 mol% nepheline have Q 0 , Q 1 , Q 2 , Q 3 , and Q 4 structural units. Nepheline‐rich compositions contain two discrete molecular structures with different polymerization modes (Q 2 and Q 4 units). This tendency to form molecular clusters is facilitated by the inability of Ca and Mg to compete with Na on nonframework sites in structural units having only bridged oxygens.
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Diopside
Nepheline syenite
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Abstract The nepheline syenites of Koraput, Orissa, in eastern India, are described for the first time with mineralogical details. The major rock type is a hypersolvus nepheline syenite. The mafic minerals are iron-rich biotite and subordinate alkali ferrohastingsite, the latter coexisting with titanomagnetite. The crystallization history of the magma has been interpreted in the light of chemical data on the rock and coexisting nepheline and alkali feldspar.
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Nepheline syenite
Alkali feldspar
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The Kaleybar nepheline syenite intrusion forms the largest silica undersaturated alkaline exposure in northwestern Iran. It consists of various rock types ranging from nepheline syenite to nepheline diorite that were emplaced during Eocene-Oligocene times, corresponding to the Alpine orogeny. The essential rock-forming minerals in nepheline syenite are plagioclase, K-feldspar, nepheline and amphibole. Clinopyroxene is the dominant phase in nepheline diorites. Titanian garnet occurs as an uncommon accessory phase forming reddish to deep brown individual grains. Chemically it is intermediate between Ti-andradite (67 to 78 mole %) and grossular (21 to 33 mole %) with TiO2 contents ranging from 1.5 to 5.0 wt %. Stoichiometry and R-mode factor analysis on garnet chemistry show that the dominant exchange vectors are Si-Ti and Al-Fe substitutions in the tetrahedral and octahedral crystal sites, respectively. A magmatic origin of the investigated Ti-garnet is suggested on the basis of mineralogical criteria and chemical properties.
Nepheline syenite
Nepheline
Grossular
Andradite
Diorite
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Cites and discusses analyses of nepheline and associated alkali feldspars from nepheline syenites and chemically equivalent rocks which demonstrate that the composition of the nepheline phase is related not only to the chemical environment but also to temperature of crystallization (or recrystallization) of the host rock.
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Nepheline syenite
Alkali feldspar
Recrystallization (geology)
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The purpose of this work is a theoretical consideration of equilibrium conditions in the system nepheline-alkali feldspar-plagioclase-alkali chloride aqueous solution based on the thermodynamic treatment of experimental data (Orville, 1963; Iiyama et al., 1963; Debron et al., 1961) for particular systems. The excess thermodynamic functions (ZE, SE, HE, VE) of solid solutions were calculated for it. These data enable us to derive the diagrams of isotherms for the distribution of sodium components between nepheline and alkali feldspar; between plagioclase and alkali feldspar; and among plagioclase, nepheline, and alkali feldspar for the temperature range 400–1000° C. In order to check the calculated diagrams we analysed the minerals from the binary and ternary assemblages of the systems nepheline$alkali feldspar$plagioclase from various igneous massifs of nepheline syenites of the world. Temperatures obtained from different geothermometers were found to coincide satisfactorily.
Nepheline
Alkali feldspar
Nepheline syenite
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Citations (34)