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The alkali feldspars of the Tatoosh Pluton in Mount Rainier National Park

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Author(s): Thomas L. Wright

Document Type:
Publisher: The American Mineralogist
Published Year: 1964
Volume: 49
Pages: 715 to 735
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Twenty five orthoclase microperthites from a shallow intrusion of granodiorite-quartz monzonite have been studied chemically, optically and by xray diffraction. The data presented are: bulk composition; composition and obliquity of the potassium-rich phase; α* and γ* for the sodium-rich phase; 2V for both potassium-rich and sodium-rich phases. 2V of each phase is correlated with the composition of that phase on a 2V-composition diagram. Plotting 2V of perthites opposite the bulk composition should be abandoned.

The potassic phases of the perthites are structurally intermediate between orthoclase and maximum microclinel average 2Vx and obliquity of this phase increase with decreasrng Ab content. The sodic phases are intermediate betrveen high and low albite; 2Vx increases with the size of the exsolution lamellae in individual crystals and γ* measured for the sodic phase is higher (closer to low albite), in specimens showing more exsolution. Thus both phases of the perthites exhibit a continuous inversion from a high to low structural state during exsolution.

The related processes of exsolution and inversion are discussed in terms of two models of the alkali feldspar solvus. Both models indicate that exsolution, if carried to completion, will result in a perthite consisting of maximum microcline and low albite.

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Suggested Citations:
In Text Citation:
Wright (1964) or (Wright, 1964)

References Citation:
Wright, T.L., 1964, The alkali feldspars of the Tatoosh Pluton in Mount Rainier National Park: The American Mineralogist, Vol. 49, pp. 715-735.