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American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Baker, Don / Xu, Hongwu / Swainson, Ian


IMPACT FACTOR 2018: 2.631

CiteScore 2018: 2.55

SCImago Journal Rank (SJR) 2018: 1.355
Source Normalized Impact per Paper (SNIP) 2018: 1.103

Online
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1945-3027
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Volume 88, Issue 10

Issues

Single-crystal in situ high-temperature structural investigation of the I1̅ –I2/c phase transition in Ca0.2Sr0.8Al2Si2O8 feldspar

Piera Benna
  • Corresponding author
  • Dipartimento di Scienze Mineralogiche e Petrologiche, Via Valperga Caluso 35, I-10125 Torino, and Istituto di Geoscienze e Georisorse, CNR, Sezione di Torino, Italy
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/ Emiliano Bruno
  • Dipartimento di Scienze Mineralogiche e Petrologiche, Via Valperga Caluso 35, I-10125 Torino, and Istituto di Geoscienze e Georisorse, CNR, Sezione di Torino, Italy
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Published Online: 2015-03-31 | DOI: https://doi.org/10.2138/am-2003-1016

Abstract

Structural modifications induced by the I1̅-I2/c displacive transition in Ca0.2Sr0.8Al2Si2O8 feldspar (An20SrF80) have been investigated in situ by single-crystal X-ray diffraction at 20, 200, 400, 500, and 620 °C. Crystals were synthesized from the melt, cooled slowly to 1300 °C, and then quenched in air. At room temperature (a = 8.361, b = 12.973, c = 14.259 Å, α = 90.79, β = 115.55, γ = 90.62°, V = 1394.9 Å3; space group: I1̅; Qod = 0.88), the polyhedra of the non-tetrahedral cation have different configurations at the Ca/Sr(0) and Ca/Sr(z) sites. In monoclinic Sr feldspar, the Sr-OB and Sr-OD distances are regular, but in triclinic An20SrF80 feldspar the OB(m0) atom in the Ca/Sr(0)-polyhedron and the OD(mz) atom in the Ca/Sr(z)-polyhedron are displaced. The topochemical symmetry of the framework is essentially monoclinic and the average dimensions of the pseudo-symmetrical tetrahedra do not change within the error limits. With increasing temperature, the distances between the Ca/Sr cations and the pseudo-related O atom pairs converge on the values adopted at the I1̅-I2/c transition. At the transition point, the M-polyhedron assumes a regular coordination, similar to that observed in monoclinic Sr feldspar at room temperature. The results obtained indicate Ttr = 520 ± 10 °C, in agreement with the transition temperature obtained from the changes of cell dimensions. The variation of cos2α* with temperature is consistent with the solution to Landau 2-4-6 potential with Tc = 506 ± 7 °C.

About the article

Received: 2002-12-09

Accepted: 2003-05-12

Published Online: 2015-03-31

Published in Print: 2003-10-01


Citation Information: American Mineralogist, Volume 88, Issue 10, Pages 1532–1541, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am-2003-1016.

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© 2015 by Walter de Gruyter Berlin/Boston.

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