The compressional behavior of a natural chromian-clinochlore-Ia polytype has been studied by synchrotron powder X-ray diffraction to 8 GPa at 298 K under hydrostatic conditions. A reversible polytypic transformation occurs at 6 GPa from the Ia-4 to a Ib polytype in which all a-type interlayers are converted to b-type by sheet translation across the interlayer, thus removing the high degree of cation superposition between the brucite-like sheet and the adjacent 2:1 layer that is associated with a-type interlayers. The isothermal bulk modulus for the Ia polytype to 5.9 GPa was determined by fitting pressure-volume data to a second-order Birch-Murnaghan equation of state: K0 = 78.7 ± 1.4 GPa. This value is close to that of clinochlore-IIb (K0 = 81 GPa). Elastic moduli for a, b, and d001 obtained from second-order fits to a Birch-Murnaghan equation of state are: Ka0 = 113(5) GPa, Kb0 = 91(2) GPa, and K001,0 = 54(1) GPa. A possible relationship is discussed between the 6 GPa polytypic transformation observed in chromian-clinochlore and the non-polytypic transformation at 9 GPa in clinochlore-IIb reported recently. Both transformations are dominated by contraction of the interlayer, with negligible in-plane compression of the polyhedral sheets. Both may also involve an increase in oxygen close-packing across the interlayer. However, in chromian-clinochlore, the destabilization of a-type interlayers is a further significant structural factor that may cause the transformation to occur at a lower pressure. Both transformations also involve an increase in the compressibility of chlorite. Although the transformation in chromian-clinochlore is influenced by the vertical superpositions of cations across the a-type interlayer, the hydrogen bonding is robust and does not reorganize until a considerable pressure (6 GPa) is reached.