Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via ISI Web of Science (1) Citing Articles via Google Scholar Google Scholar Articles by Konzett, J. Articles by Japel, S. L. Search for Related Content GeoRef GeoRef Citation

High P-T phase relations and stability of a (21)-hydrous clinopyribole in the system K₂O-Na₂O-CaO-MgO-Al₂O₃-SiO₂-H₂O: An experimental study to 18 GPa

Jürgen Konzett^1,* and Stefanie L. Japel^2,

¹ Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road N.W., Washington D.C., 20015-1305, U.S.A.
² Department of Earth and Planetary Sciences, Olin Hall, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, U.S.A.

Phase relations and stability of a mixed-chain (21)-hydrous clinopyribole [(21)-MHP] were investigated using a multi-anvil apparatus in the P-T range 5–18 GPa and 1100–1600 °C in a simplified KNCMASH-system. (21)-MHP is stable in the range 7–16 GPa and 1100–1400 °C, and coexists with clinopyroxene + sodic phase X ± potassic richterite ± aenigmatite-structured phase. Its breakdown products are sodic phase X + melt and Na-rich garnet + aenigmatite-structured phase toward high T (13 GPa/1600 °C) and P (18 GPa/1250 °C). In the KNCMASH-system investigated, the stability fields of potassic richterite and (21)-MHP only overlap between 7 and 10 GPa, and the P-stability of (21)-MHP exceeds that of potassic richterite by at least 3 GPa. The composition of (21)-MHP can be described as a combination of 1 potassic richterite + 2 omphacitic clinopyroxene K(Na₂Ca₂)(Mg₆Al)Si₁₂O₃₄(OH)₂ with variable degrees of Al₂Mg_–1Si_–1-exchange and NaSiCa_–1Al_–1-exchange component dependent upon P and T. At P >10 GPa, both (21)-MHP and coexisting clinopyroxene contain excess Si compared with the ideal clinopyroxene, and (21)-MHP stoichiometries with up to 2.09 Si/6 O atoms and 12.4 Si/33 O atoms + stoichiometric (OH) respectively. This silica excess is attributed to the presence of ^VISi as Na(Mg_0.5Si₀₅)Si₂O₆ component in clinopyroxene and within the pyroxene-like slabs of (21)-MHP. A TEM analysis of (21)-MHP synthesized at 10 GPa/1250 °C shows a regular alternation of single- and double-tetrahedral chains without evidence for stacking disorder. Potential factors responsible for the unusually high P-T stability of (21)-MHP compared to all MHPs known so far are: (1) the fact that the unit-cell volume of (21)-MHP is 1.5% smaller than that of an equivalent mixture of potassic richterite + omphacite; (2) an Na-rich bulk composition that enables the presence of ^VISi-bearing clinopyroxene in solid-solution within the single-chain slabs of (21)-MHP; and (3) the availability of K to completely fill the large A-sites of the (21)-MHP structure. The results of this study demonstrate that mixed-chain hydrous pyriboles represent a new class of high-pressure silicate structures capable of storing water and alkali-elements under upper mantle P-T conditions. However, their stability is restricted to K-Na-rich bulk compositions.