THE STRUCTURAL PRESSURE BEHAVIOUR OF PARAGONITE. Comodi P., Zanazzi P.F., Department of Earth Sciences, University of Perugia, Perugia, Italy
Structural and volume compressibility data for paragonite
(Na0.9K0.1Al3Si3O10OH
From the unit-cell data the volume variation with P was best described
by the equation V/VO = 1 - 0.00153(4) P (bulk modulus
= 650(20) kbar). The mean compressibility coefficients of the cell edges were
[[beta]]a = 3.5(1).10-4, [[beta]]b
= 3.6(1).10-4, [[beta]]c =
8.3(3).10-4 kbar-l, with a very anisotropic compressional
pattern: [[beta]]a [[beta]]b:
[[beta]]c = 1:1.03:2.37.
The main features of the deformation mechanism resulting from the structural
refinements at pressures of 0.5, 25.4, 40.5 kbar were:
- the different variation of sheet thickness indicated that the compression of
the c parameter was due entirely to the interlayer thickness ([[Delta]]t)
reduction ([[Delta]]t changed from 3.13 Å at 0.5 kbar to 2.76 Å at
40.5 kbar);
- the compressibility of the octahedra (average octahedral bulk modulus ~ 600
kbar) being larger than that of tetrahedra, the dimensional misfit between the
tetrahedral and octahedral sheet increased with P and therefore the
tetrahedral rotation angle ([[alpha]]) increased from 15deg. at 0.5 kbar to
21.6deg. at 40.5 kbar;
- the basal surface corrugation ([[Delta]]z) of the tetrahedral layer,
due to the different dimensions of M1 and M2 octahedra and to the octahedral
distortion, decreased with P ([[Delta]]z = 0.19 and 0.12 Å
at 0.5 and 40.5 kbar respectively).
By comparing the new data on paragonite with the data of a K-muscovite and a
Na-rich muscovite (Comodi and Zanazzi, 1995) a clear trend toward the decrease
of compressibility when Na substitutes for K atoms in the interlayer cavity can
be pointed out. The results may yield important insights on the thermodynamic
properties of dioctahedral micas along the muscovite-paragonite join.
Comodi P. and Zanazzi P. F. (1995) Phys. Chem. Minerals, 22:170-177.