The high-pressure behavior of an Al- and Fe-rich natural orthopyroxene

A single crystal of a natural orthopyroxene with composition [sup.M2][[[Fe.sup.2+.sub.818][Mg.sub.0.156][Ca.sub.0.010] [Mn.sub.0.016]].sup.Ml][[Fe.sup.2+.sub.0.081][Mg.sub.0.0767] [Al.sub.0.84][Fe.sup.3+.sub.0.068]sup.TA][Si][sup.TA] [Si.sub.0.848][Al.sub.0.152][O.sub.6] and space group Pbca(sampleS95) was investigated a thigh pressure by X-ray diffraction using a diamond-anvil cell up to 9.56 GPa. No phase transitions were detected in the pressure range investigated. The unit-cell parameters, a, b, and c, decrease non-linearly with pressure and show an axial compression anisotropy with a ratio [[beta].sub.a]:[[beta].sub.b]: [[beta].sub.c] = 1.00:1.64:1.16. The unit-cell volume decreases non-linearly as well and with a negative variation, by about 6.3% up to 9.56 GPa. The equation of state calculated using high-accuracy volume-pressure data up to 5.5 GPa gave the following coefficients: [V.sub.o] = 846.02(4) [[angstrom].sup.3], [K.sub.T0] = 115.4(6) GPa, K' = 7.7(3). Among the Mgorthopyroxenes investigated at high pressure so far S95 shows the highest bulk modulus. Six complete intensity data were collected at 0, 0.16, 1.72, 3.95, 8.[O.sub.3], and 9.56 GPa. The results confirm previous conclusions regarding the compressional mechanism in orthopyroxenes. At lower pressures, compression is mostly connected to a decrease in volume of the two M-coordination octahedra, accompanied by an increased kink in the B-tetrahedral chain. At higher pressures, compression of the M sites decreases, the kink of the tetrahedral chains stops to change, and reduction in unit-cell volume is accompanied mainly by compression of tetrahedra. This change in compressional trends results in a relatively large K' parameter and a pronounced stiffening of the structure with pressure. The presence oral in the TB tetrahedral site influences the kink of the B tetrahedral chain, which depends on the ratio of sizes of the M2 and TB coordination polyhedra. The increased stiffness of the M polyhedra, caused by the presence of Fe, is the main reason for the high bulk modulus of S95 and its resistance to shortening of the e axis. This explains the limited shortening of the c axis in S95 and the different compressional axial anisotropy with respect to other orthopyroxenes investigated under high pressure. Keywords: Orthopyroxene, high pressure, crystal structure, X-ray diffraction