Abstract. Neutron diffraction data suitable for Rietveld refinements were collected on a powder sample of synthetic Mg(OH)2 by the Polaris time-of-flight spectrometer (ISIS spallation source, U.K.) at 10 -4, 7.8(3) and 10.9(6) GPa. The Paris-Edinburgh high-pressure cell with WC anvils was used. Pressure calibration and equation-ofstate results were attained by separate runs with an NaC1 internal standard. Interpolation of p(V) data by the fourth-order Birch-Murnagban e.o.s, yields K0=41(2 ) GPa, K0=4(2 ) and Ko'= 1.1(9) GPa -1. The bulk modulus obtained is smaller than previously reported results. Rietveld refinements (Rprof= 1.45% and 2.02% at 10 .4 and 10.9 GPa) show that H lies on the threefold axis (1/3, 2/3, z) up to 10.9 GPa, where a model with H disordered in (x, 2x, z) can be refined. In the latter case, a hydrogen bond with O-H=0.902 (7), H..O =2.026(8) A and

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Abstract. Calcite and aragonite have been modeled using rigid-ion, two-body Born-type potentials, supplemented by O-C-O angular terms inside the CO3 groups. A shell model has also been developed for calcite. Atomic charges, repulsive parameters and force constants have been optimized to reproduce the equilibrium crystal structures, the elastic constants and the Raman and infrared vibrational frequencies. The rigid-ion potential RIM (atomic charges: Zo = -0.995 e, Zc = 0.985 e, Zc, = 2.0 e) fitted to calcite properties is able to account for those of aragonite as well. Experimental unit-cell edges, elastic constants, internal and lattice frequencies are reproduced with average relative errors of 2.1, 5.5, 2.4, 15.1% for calcite and of 0.2, 19.4, 2.5, 11.8 % for aragonite, respectively. The RIM potential is suitable for thermodynamic and phase diagram simulations in the CaCO3 system, and is discussed and compared to other potentials.

A powder sampie of 2MI muscovite, (Ko.90N3Qm)(A b .63Feo.23Mgo.l 6Tio.03)(SiJ.20Alo.80)OlO(OH)2, was studied by time-of-flight neutron diffraction at 1 bar and at 2(± 0.1) GPa. Rietveld refinements of the crystal structure were performed, varying all atomic coordinates in the room-pressure case (profile Rw = 0.013, Bragg intensities Rw = 0.040) and only the z coordinates of basal 0 atoms plus xyz of 0 and H of the OH group at 2 GPa (profile Rw = 0.035, Bragg intensities Rw = 0.181). In both cases the lattice constants were refined: a = 5.2108(4), b = 9.0399(8), c = 20.021(2) Ä, ß = 95.76(1)" (1 bar), a = 5.187(2), b = 8.995(4), c = 19.502(4) A, ß = 95.78(2)" (2 GPa). The unit-cell compression shows deviations from linear elastic behaviour. Inter-layer K-O bonds are compressed along z more than the c lattice constant. The O-H group forms an angle of 85(1)" with c • and establishes three very loose contacts (H .. O from 2.62 to 2.67 A) with neighbouring 0 atoms. This environment does not change appreciably at 2 GPa.

The ground state properties of fluorite (CaF2) have been studied using CRYSTAL, an ab initio periodic Hartree-Fock program. Twenty-two and thirteen atomic orbitals (repmented as contracted Gaussian-type functions) are used for the calcium and fluorine atoms, respectiwly. The binding energy (BE), the equilibrium lattice parameter (e), theelasticconstants (C,,) and thecmtralzonephononhpquencies ~( I R )and v(Rama0) have been evaluated, and a g d ageenent obtsined with experiment (for instance the error i s t2.0, +1.7, -0.5% for BE, Q and C11, respectively). The calculated Cu elastic constant reduces from 48 to 44 GPa (experimental value: 37 GPa) when the Euorine atoms are allowed to displace under strain. indicating the importance of inner deformation for shear elasticity. Electron density maps, density of states and band struetun plots are reported which confirm the f d y ionic n a t m of fluorite.