The room-temperature bulk modulus of synthetic end-member clinochlore, Mg5AlSi3AlO10(OD)8, has been determined in the pressure range 0 to 6.5 GPa using neutron powder diffraction: K0 = 75.4(2.7) GPa, K0′ = 4; K0 = 72.3(2.4) GPa, K0′ = 5.4(1.0). A structural comparison is made with the related mineral brucite, Mg(OH)2. Clinochlore is a much stiffer structure than brucite (K0 = 41 – 47 GPa). Both minerals have very similar in-plane compressions of their polyhedral sheets and so their very different bulk moduli arise from different compressibilities normal to the structural layering. Rietveld 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 octahedral sheets of the brucite-like and 2:1 layers are uncompressed to 4.7 GPa. Increasing pressure strengthens the H3(D3)…O3 hydrogen bond, which contracts linearly from 1.88(2) Å at 0 GPa to 1.77(2) Å at 4.7 GPa, possibly with a concomitant lengthening of the O6-H3(D3) hydroxyl bond from 1.05(2) Å to 1.10(2) Å over the same pressure range.

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