Synthesis and characterization of white micas in the join muscovite-aluminoceladonite

Potassic white micas were synthesized in the K2O-MgO-Al2O3-SiO2-H2O system along the pseudobinary join muscovite-aluminoceladonite (mu-Alcel). Composi tion of run products as measured by electron microprobe analysis are in the range mu(89)-Alcel(11) to mu(01)-Alcel(99). Cell parameters were determine d on powder samples by full-profile Rietveld refinement, using both a singl e-polytype and a multi-polytype model. The results of both analysis models are in full agreement, and show that the phengite cell parameters have a di stinct dependence on the celadonite content: the c parameter shows a monoto nic decrease over the full compositional range, whereas the a and b paramet ers both increase in the Alcel(0)-Alcel(60) range but decrease in the Alcel (60)-Alcel(100) range. The monoclinic b angle decreases slightly with incre asing celadonite content. The overall behavior of the cell parameters indic ates a decrease of the ditrigonal distortion of the tetrahedral 6-rings, an d an increased trioctahedral character of the structure at high celadonite compositions. The molar volume along the solid solution join shows a maximu m at about Alcel(30). Molar volume vs. composition can be fitted by a symme tric function for the excess volume yielding a molar volume for end member aluminoceladonite of 13.957 +/- 0.006 J/bar, for muscovite 14.076 +/- 0.004 J/bar, and a symmetric positive deviation from ideal volumes of mixing wit h W = 0.198 +/- 0.025 J/bar, and r(2) = 0.941. The use of an asymmetric exc ess volume function does not significantly improve the fit quality (r(2) = 0.945).