The compositional and structural effects of the Tschermak's substitution (Mg,Si)-1 (Al, 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 Al/(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 Al is distributed evenly throughout the sheet to minimize local charge imbalances. As the Al/ (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 Al contents.
|Original language||English (US)|
|Number of pages||17|
|State||Published - 1991|
ASJC Scopus subject areas
- Geochemistry and Petrology