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Solid solution and unmixing in the feldspar system, albite (NaAlSi (sub 3) O (sub 8) )-celsian (BaAl (sub 2) Si (sub 2) O (sub 8) )

K. Viswanathan and O. Harneit
Solid solution and unmixing in the feldspar system, albite (NaAlSi (sub 3) O (sub 8) )-celsian (BaAl (sub 2) Si (sub 2) O (sub 8) )
European Journal of Mineralogy (April 1989) 1 (2): 239-248


Hydrothermally synthesized (Na,Ba)-feldspars show limited miscibility on the albite-rich side below approx 875 degrees C, while complete solid solutions were observed from approx Cn (sub 30) to Cn (sub 100) ; Precise lattice parameters of these solid solutions are presented for the first time and their variations with composition discussed. The volume-composition curve is S- shaped and the two-trend variation shown by the b and c parameters seems to confirm the dissimilarities in the structures of the albite- and celsian-rich members of the series. The miscibility gap was found to extend from approx Ab (sub 95) Cn (sub 5) to approx Ab (sub 70) Cn (sub 30) . Experiments to reproduce the gap at 700 degrees C and 1 kbar by using a mixture of gels, Ab (sub 100) and Cn (sub 100) , with a bulk composition lying inside the solvus showed an unexpected behaviour. This consisted of three stages: 1) two feldspars, approx Cn (sub 2) and approx Cn (sub 60) were observed after two hours; 2) after 21 days a homogeneous feldspar with the bulk composition appeared instead; 3) after 84 days two feldspars, approx Cn (sub 3) and Cn (sub 25-30) had formed. This sequence of observations can be understood from a consideration of hypothetical curves of Gibbs energies of mixing of (Na,Ba)-feldspars. Probably the very fine grain size of the crystals favours rapid chemical diffusion and reaction; the large surface area leads to the formation of intermediate metastable phases until finally the equilibrium assemblage is attained.

ISSN: 0935-1221
Serial Title: European Journal of Mineralogy
Serial Volume: 1
Serial Issue: 2
Title: Solid solution and unmixing in the feldspar system, albite (NaAlSi (sub 3) O (sub 8) )-celsian (BaAl (sub 2) Si (sub 2) O (sub 8) )
Affiliation: Tech. Univ. Braunschweig, Mineral. Inst., D-3300 Brunswick, Federal Republic of Germany
Pages: 239-248
Published: 198904
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 23
Accession Number: 1993-015424
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: 2 tables
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United KingdomTwickenhamUKUnited Kingdom. Reference includes data from Geoline, Bundesanstalt fur Geowissenschaften und Rohstoffe, Hanover, Germany
Update Code: 1993
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