High-pressure behavior of clinochlore

The room-temperature bulk modulus of synthetic end-member clinochlore, Mg5A lSi3AlO10(OD)(8), has been determined in the pressure range 0 to 6.5 GPa us ing neutron powder diffraction: K-o = 75.4(2.7) GPa, K-o' = 4; K-o = 72.3(2 .4) GPa, K-o' = 5.4(1.0). A structural comparison is made with the related mineral brucite, Mg(OH)(2). Clinochlore is a much stiffer structure than br ucite (K-o = 41 - 47 GPa). Both minerals have very similar in-plane compres sions of their polyhedral sheets and so their very different bulk moduli ar ise from different compressibilities normal to the structural layering. Rie tveld refinements of neutron-diffraction data for clinochlore collected at 0, 1.2, 2.5, and 4.7 GPa reveal that compression normal to the layering is equally partitioned between the interlayer and the 2:1 layer; the octahedra l sheets of the brucite-like and 2:1 layers are uncompressed to 4.7 GPa. In creasing pressure strengthens the H3(D3). . . O3 hydrogen bond, which contr acts linearly from 1.88(2) Angstrom at 0 GPa to 1.77(2) Angstrom at 4.7 GPa , possibly with a concomitant lengthening of the O6-H3(D3) hydroxyl bond fr om 1.05(2) Angstrom to 1.10(2) Angstrom over the same pressure range.