Phys Chem Minerals (1997) 24:274–280 C Springer-Verlag 1997 ORIGINAL PAPER P. Comodi  P.F. Zanazzi Pressure dependence of structural parameters of paragonite Received April 10, 1996  Revised, accepted September 27, 1996 Abstract The compressibility and structure of a 2M 1 paragonite with composition [Na 0.88 K 0.10 Ca 0.01 Ba 0.01 ] [Al 1.97 Ti 0.007 Fe 0.01 Mn 0.002 Mg 0.006 ]Si 3.01 Al 0.99 O 10 OH 2 were determined at pressures between 1 bar and 41 kbar, by single crystal X-ray diffraction using a Merrill-Bas- sett diamond anvil cell. Compressibility turned out to be largely anisotropic, linear compressibility coefficients parallel to the unit cell edges being b a =3.5(1)10 -4 , b b =3.6(1)10 -4 , b c =8.3(3)10 -4 kbar -1 (b a : b b  b c =1: 1028: 2.371). The isothermal bulk modulus, calculated as the reciprocal of the mean compressibility of the cell volume, was 650(20) kbar. The main features of the deformation mechanism re- sulting from structural refinements at pressures of 0.5, 25.4, 40.5 kbar were: – variation in sheet thickness, showing that compression of the c parameter was mainly due to the interlayer thick- ness reduction from 3.07 A ˚ at 0.5 kbar to 2.81 A ˚ at 40.5 kbar; – the compressibility of octahedra was greater than that of tetrahedra, the dimensional misfit between tetrahedral and octahedral sheets increased with P, so that tetrahe- dral rotation angel a increased from 15° at 0.5 kbar to 21.6° at 40.5 kbar; – the basal surface corrugation (Dz) of the tetrahedral layer, due to the different dimensions of M1 and M2 octahedra and to the octahedral distortion, decreased with P (Dz=0.19 and 0.12 A ˚ at 0.5 and 40.5 kbar respec- tively). Comparison of the new data on paragonite with those of a K-muscovite and a Na-rich muscovite (Comodi and Zanazzi 1995) revealed a clear trend toward decreasing of compressibility when Na substitutes for K atoms in the interlayer sites. Introduction Paragonite, NaAl 2 Si 3 AlO 10 (OH) 2 , is a dioctahedral mica which is the Na analogue of muscovite. It occurs com- monly in many blueschist and eclogites facies metamor- phic rocks. It also occurs in metapelitic rocks from the greenschist to sillimanite zone of amphibole facies, pri- marily if the rocks are more Al-rich than the typical metapelite (Guidotti 1984). The large asymmetric solvus between Paragonite (Pg) and Muscovite (Ms) has long been known (see Guidotti etal. 1994 for early references). Moreover, there have been many attempts to use the partitioning of Na and K between coexisting Ms ang Pg as a geothermometer (see Blencoe et al. 1994 for a recent discussion thereof). Inas- much as the exact shape of the solvus and how it changes with pressure is not yet well known, thermometric esti- mates are often inconsistent with corresponding temper- atures derived from alternative methods. The molar vol- umes of the terms along Ms-Pg join and how they change with pressure must be known before a more accurate phase diagram can be drawn. The aim of this work is to contribute to the knowledge of the molar volume of paragonite and its variation with P through compressibil- ity measurements on single crystals. Structural refine- ments with high-pressure data can also show the defor- mation mechanism of the Pg structure with P. Comparison of the data presented here with com- pressibility data from a Ms (K-ms) and a Na-rich Ms (Na-ms) (Comodi and Zanazzi 1995), allows evaluation of the role of NaVK substitution in the baric behaviour of white dioctahedral micas. Experimental Because Pg usually occurs as fine-grained aggregates, it was very difficult to find a good single crystal suitable for study on a four- circle automated diffractometer. A good sample of the Pg 2M 1 polytype was found in a specimen from the Western Alps (sample (Al-433 obtained from M. Frey), and kindly provided to us by C.V. Paola Comodi  Pier Francesco Zanazzi ( ✉ ) Dipartimento di Scienze della Terra, Universita ` di Perugia, Piazza Universita `, I-06100 Perugia, Italy Fax: +39-75-5853203; e-mail: Zanazzi@unipg.it