Structural thermal behavior of paragonite and its dehydroxylate: a high-temperature single-crystal study

The lattice constants of paragonite-2M(1), NaAl2(AlSi3)O-10(OH)(2), were determined to 800 degrees C by the single-crystal diffraction method. Mean thermal expansion coefficients, in the range 25-600 degrees C, were: alpha(a) = 1.51(8;) x 10(-5), alpha(b) = 1.94(6) x 10(-5), alpha(c) = 2.15(7) x 10(-5)degrees C-1, and alpha(v) = 5.9(2) x 10(-5)degrees C-1. At T higher than 600 degrees C, cell parameters showed a change in expansion rate due to a dehydroxylation process. The structural refinements of natural paragonite, carried out at 25, 210, 450 and 600 degrees C, before dehydroxylation, showed that the larger thermal expansion along the c parameter was mainly due to interlayer thickness dilatation. In the 25-600 degrees C range, Si,Al tetrahedra remained quite unchanged, whereas the other polyhedra expanded linearly with expansion rate proportional to their volume. The polyhedron around the interlayer cation Na became more regular with temperature. Tetrahedral rotation angle alpha changed from 16.2 to 12.9 degrees. The structure of the new phase, nominally NaAl2 (AlSi3)O-11, obtained as a consequence of dehydroxylation, had a cell volume 4.2% larger than that of paragonite. It was refined at room temperature and its expansion coefficients determined in the range 25-800 degrees C. The most significant structural difference from paragonite was the presence of Al in fivefold coordination, according to a distorted trigonal bipyramid. Results confirm the structural effects of the dehydration mechanism of micas and dioctahedral 2:1 layer silicates. By combining thermal expansion and compressibility data, the following approximate equation of state in the PTV space was obtained for paragonite: V/V-0 = 1 + 5.9(2) x 10(-5) T(degrees C) - 0.00153(4) P(kbar).