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Two-, three- and four-feldspar assemblages with hyalophane and celsian

¹ Department of Geological Sciences, University of Michigan, Ann Arbor MI 48109–1063 USA
² YES College Preparatory School, 353 Crenshaw Road, Houston TX 77034 USA
³ Dipartimento di Scienze della Terra, Università di Siena, Via Laterina, 8, Italy
⁴ Departmento de Geologia, Facultad de Ciencias del Mar, Universidad de Cadiz, Campus Rio San Pedro, 11510 Puerto Real, Cadiz, Spain
⁵ Laboratory for Geochemical Research, Hungarian Academy of Sciences, H-1112 Budapest, Budaoersi ut 45, Hungary
⁶ National Semiconductor, 234 Oxford Street, South Portland ME 04101 USA

^** e-mail: essene{at}umich.edu

The occurrences of natural coexisting feldspars including hyalophane and also celsian delineate two-, three- and possibly four-phase fields in the system BaAl₂Si₂O₈—CaAl₂Si₂O₈—NaAlSi₃O₈—KAlSi₃O₈. Hyalophane occurs with albite and microcline in a very low grade (anchizonal to epizonal) metasedimentary association from the Uppony Mountains, Hungary, and in Grenville marbles from Ontario. Analyses show very little Ba in albite and only limited Na in hyalophane. One marble from the garnet zone has albite (Ab_95–98), oligoclase (An₁₇Sl₃Ab₈₀), hyalophane (Cn₆₅Sl₃An₂Ab₉Or₂₄) and celsian (Cn₉₂Sl₃An₁Ab₂Or₂). The albite and oligoclase are complexly intergrown and may indicate unmixing during cooling. A marble in the sillimanite zone contains albite (Ab_95–98Or_2–3), oligoclase (An₁₇An₂Sl₃Ab₃₃Or₂₄), hyalophane (Cn₆₅Sl₃An₂Ab₉Or₂₄), and an inclusion of celsian (Cn₆₇An₁Sl₂Ab₂Or₉₁) in an albite. Sanidine from the Peshtigo monzonite in Wisconsin unmixed to a symplectic perthite with barian microcline (Cn_8–11An₂Ab_8–13Or_75–80) and oligoclase (CnlAn₁₈Ab₇₉Or₂). The former compositions of the ternary igneous feldspars (Cn₃An₉Ab₄₆Or₄₂, Cn₁An₁₈Ab₆₉Or₁₂) were obtained by reintegration.

The Na content of hyalophane equilibrated with albite is correlated with metamorphic grade. Hyalophane has 5 ± 2 mol % Ab in very low-grade associations, 10 ± 3 mol % Ab in the greenschist facies, 16 ± 2 mol % Ab in the low to middle amphibolite facies, and 30 mol % Ab in the upper amphibolite to granulite facies even when not buffered with albite. The limited Na content of celsian equilibrated with albite in the greenschist facies is in striking disagreement with the narrow solvi obtained from unreversed experiments on the join BaAl₂Si₂O₈₆-NaAlSi₃O₈.

Up to 8 four-feldspar and 24 three-feldspar assemblages may be stable in the system BaAl₂Si₂O₈-CaAl₂Si₂O₈-NaAlSi₃O₈-KAlSi₃O₈. In contrast, the repeatedly observed and variably located discontinuities within zoned hyalophane grains may represent changes in the environment during mineral growth rather than internal miscibility gaps. Given its miscibility gaps with microcline and celsian, the name hyalophane is justified for intermediate feldspars near the Cn-Or join.

Key-words: hyalophane, celsian, solvus, three feldspars, four feldspars, BaAl₂Si₂O₈—NaAlSi₃O₈—KAlSi₃O₈, BaAl₂Si₂O₈—CaAl₂Si₂O₈—NaAlSi₃O₈—KAlSi₃O₈.

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