The role of the oxo-component on the compressibility of amphibole was studied by means of high-pressure in situ single-crystal X-ray diffraction on two natural kaersutite megacrysts (samples DL5 and FR12) from alkaline basalts. The oxo-component varies significantly (1.1 and 1.9 apfu in DL5 and FR12, respectively), whereas the cation composition is very similar, apart from the Fe3+/(Fe2++Fe3+), which is 0.33 in DL5 and ~1 in FR12. The larger oxo-component of FR12 is attributed to the Fe2+ + OH− = Fe3+ + O2− + ½H2 substitution.
Unit-cell parameters were collected at different pressures up to about 8 GPa. Structural refinements of both samples were performed with data collected at different P up to 6 GPa. Fitting the P-V data to a third-order Birch Murnaghan EoS yielded the following parameters: K0 = 94(1) GPa, K′ = 6.3(4), and V0 = 903.6(2) Å3 for FR12 and K0 = 91(2) GPa, K′ = 6.2(4), and V0 = 914.1(2) Å3 for DL5. The axial moduli of the two amphibole samples were: K0a = 86(3) GPa, K′a = 7(1), and a0 = 9.815(2) Å; K0b = 115(3) GPa, K′b = 4.8(8), and b0 = 18.012(2) Å; K0c = 112(5) GPa, K″c = 7(1), and c0 = 5.300(1) Å for sample FR12 and K0a = 85(3) GPa, K′a = 5(1), and a0 = 9.8660(9) Å; K0b = 113(2), K′b = 4.4(6), and b0 = 18.0548(6) Å; K0c = 107(3) GPa, K′c = 7(1), and c0 = 5.3185(5) Å for sample DL5. This suggests that the compressibility of kaersutite decreases with increasing oxo-component.
Structural refinements show that the polyhedral compressibility follows the order A = M4 > M2 > M3 > M1 for DL5 and A = M4 > M2 > M1 > M3 for FR12. The most evident geometrical effect induced by P is the decrease in the bending of the double tetrahedral chain, when adjacent I-beams are pushed against each other. This effect is largest for DL5, which has a larger concavity of the A site, (O7-O7′ changes from 3.03 to 2.82 Å) compared to the one of FR12, (O7-O7′ changes from 2.92 to 2.79 Å). This mechanism is confirmed by the evolution of T1-O7-T1 angle (from 135.4° to 132.5° in FR12 and from 136.6° to 132.2° in DL5).