A high-pressure single-crystal X-ray diffraction (XRD) study has been carried out on two natural zoisite samples Ca2Al3−xFexSi3O12OH, one Fe-free (x = 0) and one Fe-rich (x = 0.12). The unit-cell parameters were determined for the Fe-free sample at 18 different pressures up to 7.76 GPa and for the Fe-rich sample at 13 different pressures up to 7.63 GPa. The P(V) data for both of the samples were fitted by a third-order Birch-Murnaghan equation of state (BM3 EoS). The equation of state coefficients are: V0 = 903.39(5) Å3, KT0 = 122.1(7) GPa, and K′0 = 6.8(2) for the Fe-free sample and V0 = 906.95(5) Å3, KT0 = 119.1(7) GPa, and K′0 = 7.3(2) for the Fe-rich sample. This shows that the addition of Fe in to the crystal structure of zoisite leads to a slight softening of the structure.
Both compositions exhibit axial compressibilities βc > βa >> βb, with the compressibilities of the a and b axes of the two samples being indistinguishable. The softening of the bulk modulus of zoisite with Fe content follows from softening of the c-axis of the structure. A high-pressure structural study of the Fe-free sample showed that the main compression mechanisms in the structure are the compression of soft inter-octahedral distance along  and soft intra-octahedral distances along  directions, while along  the main compression occurs because of the compression of stiff intra-octahedral distances. The substitution of Fe on to the M3 octahedral site of the structure leads to an increase of the intra-octahedral distance of the M3 that triggers the rotation of M12 and therefore leads to the softening of the M12 inter-octahedral distances that accounts for the softening of the c-axis of the structure.