TEM study of the transformation of augite to sodic pyroxene in eclogitized ferrogabbro

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Authors:Otten, M. T.; Buseck, P. R.
Author Affiliations:Primary:
Ariz. State Univ., Dep. Geol. Chem., Tempe, AZ 85287, United States
Volume Title:Contributions to Mineralogy and Petrology
Source:Contributions to Mineralogy and Petrology, 96(4), p.529-538. Publisher: Springer International, Heidelberg-New York, International. ISSN: 0010-7999
Publication Date:1987
Note:In English. 31 refs.; illus. incl. 11 anal., 1 table, charts
Summary:TEM and analytical electron microscopy were used to study relicts of augite that occur in various stages of transformation to sodic pyroxene. The augite relicts are characterized by a hatching produced by two sets of former '001' exsolution lamellae that possess high disolocation densities and were altered completely to sodic pyroxene, even where the augite matrix is still fresh. With further alteration, the sodic pyroxene in these lamellae recrystallized and grew into the augite matrix, resulting in irregular lamellae consisting of subgrains of low dislocation densities. Needles and thin (100) lamellae of sodic pyroxene developed on the '001' lamellae. Alteration of the augite matrix proceeded by growth of areas with defects. All sodic pyroxenes in these microstructures have the same orientation as the precursor augite, indicating a topotactic reaction mechanism. The reactions occurred at roughly constant Si and mainly involved replacement of Ca and Mg by Na and Al. Dislocations may have played a prominent role in the transformation by acting as diffusion pathways and by migrating into untransformed augite, leaving sodic pyroxene in their wake. At the augite grain boundaries, discrete grains of sodic pyroxene formed without any fixed orientation relation with the augite, consistent with a non-topotactic reaction. The predominance of the topotactic reaction inside the augite over the non- topotactic grain-boundary reaction is attributed to the scarcity of fluid during eclogite metamorphism. [P.Br.]
Subjects:Alkali metals; Augite; Chain silicates; Chemical composition; Clinopyroxene; Diorites; Eclogite facies; Facies; Ferrodiorite; Gabbros; Geochemistry; Igneous rocks; Jadeite; Metals; Metamorphic rocks; Microstructure; Mineralogy; Minerals; Petrology; Phase transitions; Plutonic rocks; Pyroxene group; Silicates; Sodium; TEM data; Transmission method
Abstract Numbers:88M/0988
Record ID:1988041492
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom, Reference includes data from Geoline, Bundesanstalt fur Geowissenschaften und Rohstoffe
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