Electron micro-probe analyses of myrmekite plagioclases and coexisting feldspars

Widenfalk, Lennart

doi:10.1016/s0024-4937(69)80021-6

Cited by 10 publications

(5 citation statements)

References 10 publications

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“…The composition of the plagioclase, close to that of the primary plagioclase in any particular rock (cf. Phillips & Ransom, 1970), indicates that reaction occurred early in the cooling history of the rock, by analogy with the argument of Widenfalk (1969) cited above (beginning of this Section). It is not held that wholesale metasomatism of the rock is responsible for homogeneous x&, because systematic zoning of the primary plagioclase is preserved: pervasive metasomatic reconstitution is unlikely in such circumstances (Shelley, 1964).…”

Section: Interpretation Of the Observationsmentioning

confidence: 67%

“…Type I myrmekites are attributable to exsolution-dominated processes because of the limited extent to which they replace potash feldspar, and in particular because of the normal zoning towards potash feldspar contacts. This feature is an expected result of disequilibrium exsolution during a period of falling temperature; see Widenfalk (1969) for a discussion of this point. Widenfalk's observations on zoning of potash feldspar towards myrmekite contacts, which support an exsolution model for the examples he studied (c/.…”

Section: Interpretation Of the Observationsmentioning

confidence: 83%

“…In this hypothesis the myrmekite forms by exsolution of (Na,Ca)-bearing components present in the initial potash feldspar. It is assumed that the Ca component is 'Schwantke's molecule' Ca(AlSi 3 O 8 ) 2 (Schwantke, 1909) (Widenfalk, 1969):…”

Section: The Schwantke Hypothesismentioning

confidence: 99%

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Myrmekites of Exsolution and Replacement Origins

Ashworth¹

1972

Geol. Mag.

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SummaryMyrmekite in a two-feldspar migmatite suite has two distinct modes of occurrence: (i) marginally embaying potash feldspar, and (ii) in replacement aggregates after that mineral, the other major constituent of the pseudomorphs being a muscovite-quartz intergrowth. Electron-probe data give quartz proportionalities agreeing with the predictions of existing theories. It is shown that these theories, which refer to either exsolution or small-scale metasomatism, share a common foundation of kinetic impediment to AlSi diffusion. In neither case need one assume the presence of ‘Schwantke's molecule’ as a component of potash feldspar. The two myrmekite occurrences here described are interpreted in terms of essentially simultaneous exsolution and hydrative replacement processes operating as regional metamorphic temperatures began to decline from their climactic values. The existence of exsolution myrmekites of ideal quartz proportionality is not in itself evidence for the existence of ‘Schwantke's molecule’.

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Section: Interpretation Of the Observationsmentioning

confidence: 67%

Section: Interpretation Of the Observationsmentioning

confidence: 83%

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Myrmekites of Exsolution and Replacement Origins

Ashworth¹

1972

Geol. Mag.

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“…Cooling and exsolution can account for the textures described in section 4 ( Fig. Phillips, 1964), the involvement of this kind of myrmekite with secondary albite (which commonly forms an outer gradational zone on the myrmekite due to its chemical potential (Widenfalk, 1969) and a prolonged unmixing of the much more abundant Ab component) strongly suggests that both have the same origin (i.e. The steady growth of myrmekite during exsolution onto plagioclase or alkali feldspar (e.g.…”

Section: Discussionmentioning

confidence: 93%

On polygenetic myrmekite

Phillips

1980

Geol. Mag.

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A brief review of Becke's (1908) replacement and Schwantke's (1909) exsolution models for myrmekite genesis and further consideration of the morphology and spatial distribution of myrmekite as outlined by Phillips (1974) lead to the conclusion that both hypotheses have a place in explaining the origin of myrmekite. The Schwantke model is generally best applied, for example, to high-level undeformed massive granitoids and the Becke model to deformed metamorphic rocks. The latter hypothesis and a third model involving interaction between exsolution and replacement (Ashworth, 1972) may be used especially to explain the association of muscovite and myrmekite in rocks whose mineral assemblages have undergone retrograde modification, involving either redistribution of Na, Ca and K by local metasomatic reactions among the mineral grains, or metasomatic change of bulk rock composition.

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“…Barth (1969, pp. v, 14, 28); Carstens (1967); Hubbard (1966Hubbard ( , 1967aHubbard ( , 1967bHubbard ( , 1969; Kennan (1972); Perry (1968, p. 216); Sturt (1970) and Widenfalk (1969Widenfalk ( ,1972. Rejection of the need for metasomatism in myrmekite genesis reduces the four choices devised by Ashworth to one-the Schwantke hypothesis.…”

mentioning

confidence: 99%

Myrmekites of exsolution and replacement origins—a discussion

Phillips

1973

Geol. Mag.

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Electron micro-probe analyses of myrmekite plagioclases and coexisting feldspars

Cited by 10 publications

References 10 publications

Myrmekites of Exsolution and Replacement Origins

Myrmekites of Exsolution and Replacement Origins

On polygenetic myrmekite

Myrmekites of exsolution and replacement origins—a discussion

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