Substitution of [6,4]Al in phlogopite: Mica characterization, unit-cell variation, 27Al and 29Si MAS-NMR spectroscopy, and Al-Si distribution in the tetrahedral sheet

Susan Circone, Alexandra Navrotsky, R. James Kirkpatrick, Colin M. Graham

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43 Scopus citations

Abstract

The compositional and structural effects of the Tschermak's substitution ([6]Mg,[4]Si)-1 ([6]Al, [4]Al) in phlogopite were investigated for nine synthetic magnesium aluminum phlogopite compositions. The observed synthetic mica compositions lie along the phlogopite-castonite join, ranging between 0.00 ≤ Xeast ≤ 0.92, based on the formula K(Mg3-xAlx)(Si 3-xAl1+xO10OH2 and extending well beyond the upper limit of Xeast = 0.62 proposed by Hewitt and Wones (1975). High resolution 27Al and 29Si MAS-NMR spectra also indicate a continuous increase in the [6,4]Al content of the micas. The [4]Al/[4](Al + Si) ratios determined from electron microprobe analyses on fine-grained aggregates of the micas are in excellent agreement with the 29Si NMR spectroscopie results. Ther-mogravimetric analyses indicate that the micas contain stoichiometric H2O contents plus varying amounts of adsorbed H2O. The onset of dehydroxylation is not significantly affected by increasing [6,4]Al content. Refinements of powder X-ray diffraction data indicate that the a and b unit-cell parameters decrease linearly with increasing [6,4]Al content, corresponding to a 1.3% reduction in the lateral dimensions of the 2:1 layers from Xeast = 0.00-0.92. The increasing misfit between adjacent tetrahedral and octahedral sheets is partially compensated by tetrahedral rotation. The calculated tetrahedral rotation angle increases linearly with increasing [6,4]Al content, significantly decreasing the size of the interlayer cavity. The unit-cell volume decreases linearly with increasing 6,4]Al content, and the volume of mixing is zero for the phlogopite-eastonitc solid solution. The extent of Al-Si ordering in the tetrahedral sheets of the synthetic micas has been determined by computer modeling of the 29Si NMR. spectra. The Al-Si distributions are short-range ordered, constrained primarily by the avoidance of adjacent Al tetrahedra. The computer simulations are further improved if the [4]Al is distributed evenly throughout the sheet to minimize local charge imbalances. As the [4]Al/[4] (Al + Si) ratio increases from 0.24 to 0.47, the Al-Si distribution becomes increasingly ordered, converging with the long-range ordered distribution of strictly alternating Al and Si tetrahedra at high [4]Al contents.

Original languageEnglish (US)
Pages (from-to)1485-1501
Number of pages17
JournalAmerican Mineralogist
Volume76
Issue number9-10
StatePublished - 1991
Externally publishedYes

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

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