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| Titel |
Principles of Thermal Expansion in Feldspars |
| VerfasserIn |
Guy Hovis, Aaron Medford, Maricate Conlon, Allison Tether, Anthony Romanoski |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250043324
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| Zusammenfassung |
| Following the recent thermal expansion work of Hovis et al. (1) on AlSi3 feldspars, we have
investigated the thermal expansion of plagioclase, Ba-K, and Ca-K feldspar crystalline
solutions. X-ray powder diffraction data were collected between room temperature and 925
°C on six natural plagioclase specimens ranging in composition from anorthite to oligoclase,
the K-exchanged equivalents of these plagioclase specimens, and five synthetic Ba-K
feldspars with compositions ranging from 25 to 99 mol % BaAl2Si2O8. The resulting thermal
expansion coefficients (α) for volume have been combined with earlier results for
end-member Na- and K-feldspars (2,3). Unlike AlSi3 feldspars, Al2Si2 feldspars,
including anorthite and celsian from the present study plus Sr- and Pb-feldspar from
other workers (4,5), show essentially constant and very limited thermal expansion,
regardless of divalent cation size. In the context of structures where the Lowenstein rule
(6) requires Al and Si to alternate among tetrahedra, the proximity of bridging
Al-O-Si oxygen ions to divalent neighbors (ranging from 0 to 2) produces short
Ca-O (or Ba-O) bonds (7,8) that apparently are the result of local charge-balance
requirements (9). Gibbs et al. (10) suggest that short bonds such as these have a partially
covalent character. This in turn stiffens the structure. Thus, for feldspar series with
coupled substitution the change away from a purely divalent M-site occupant gives the
substituting (less strongly bonded) monovalent cations increasingly greater influence on
thermal expansion. Overall, then, thermal expansion in the feldspar system is well
represented on a plot of α against room-temperature volume, where one sees a
quadrilateral bounded by data for (A) AlSi3 feldspars whose expansion behavior is
controlled largely by the size of the monovalent alkali-site occupant, (B) Al2Si2
feldspars whose expansion is uniformly limited by partially-covalent bonds between
divalent M-site occupants and bridging Al-O-Si oxygens, (C) plagioclase (11,12,13)
and (D) Ba-K feldspars (12) where coupled substitution across the series produces
expansion behavior that rapidly transitions from one control to the other. Generally,
thermal expansion coefficients vary linearly as functions of room-temperature volume
between the relevant end members. Thus, the thermal expansion of any feldspar can be
estimated simply from knowledge of its chemical system and room-temperature
volume.
References cited: (1) Hovis, Morabito, Spooner, Mott, Person, Henderson, Roux & Harlov
(2008) American Mineralogist 93, 1568-1573. (2) Hovis & Graeme-Barber (1997) American
Mineralogist 82, 158-164. (3) Hovis, Brennan, Keohane & Crelling (1999) The Canadian
Mineralogist 37, 701-709. (4) Henderson (1984) Progress in Experimental Petrology,
N.E.R.C. Report 6, 78-83. (5) Benna, Tribaudino & Bruno (1999) American Mineralogist 84,
120-129. (6) Lowenstein (1954) American Mineralogist 39, 92 -96. (7) Megaw, Kempster
& Radosolovich (1962) Acta Crystallographica 15, 1017-1035. (8) Newham &
Megaw (1960) Acta Crystallographica 13, 303–312. (9) Pauling (1929) Journal of the
American Chemical Society 51, 1010-1026. (10) Gibbs, Rosso, Cox & Boisen (2003)
Physics and Chemistry of Minerals 30, 317-320. (11) Grundy & Brown (1974) In The
Feldspars, Eds. MacKenzie & Zussman, 163-173. (12) Hovis, Medford, Conlon,
Tether & Romanoski (in review) American Mineralogist. (13) Tribaudino, Angel,
Camara, Nestola, Pasqual, & Margiolaki (in review) Contributions to Mineralogy and
Petrology. |
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