Pargasite at high pressure and temperature

The P–T phase stability field, the thermoelastic behavior and the P-induced deformation mechanisms at the atomic scale of pargasite crystals, from the “phlogopite peridotite unit” of the Finero mafic–ultramafic complex (Ivrea-Verbano Formation, Italy), have been investigated by a series of in situ experiments: (a) at high pressure (up to 20.1 GPa), by single-crystal synchrotron X-ray diffraction with a diamond anvil cell, (b) at high temperature (up to 823 K), by powder synchrotron X-ray diffraction using a hot air blower device, and (c) at simultaneous HP–HT conditions, by single-crystal synchrotron X-ray diffraction with a resistive-heated diamond anvil cell (P max = 16.5 GPa, T max = 1200 K). No phase transition has been observed within the P–T range investigated. At ambient T, the refined compressional parameters, calculated by fitting a second-order Birch–Murnaghan Equation of State (BM-EoS), are: V 0 = 915.2(8) A3 and K P0,T0 = 95(2) GPa (β P0,T0 = 0.0121(2) GPa−1) for the unit-cell volume; a 0 = 9.909(4) A and K(a) P0,T0 = 76(2) GPa for the a-axis; b 0 = 18.066(7) A and K(b) P0,T0 = 111(2) GPa for the b-axis; c 0 = 5.299(5) A and K(c) P0,T0 = 122(12) GPa for the c-axis [K(c) P0,T0 ~ K(b) P0,T0 > K(a) P0,T0]. The high-pressure structure refinements (at ambient T) show a moderate contraction of the TO4 double chain and a decrease of its bending in response to the hydrostatic compression, along with a pronounced compressibility of the A- and M(4)-polyhedra [K P0, T0(A) = 38(2) GPa, K P0, T0(M4) = 79(5) GPa] if compared to the M(1)-, M(2)-, M(3)-octahedra [K P0, T0(M1,2,3) ≤ 120 GPa] and to the rigid tetrahedra [K P0, T0(T1,T2) ~ 300 GPa]. The thermal behavior, at ambient pressure up to 823 K, was modelled with Berman’s formalism, which gives: V 0 = 909.1(2) A3, α0 = 2.7(2)·10−5 K−1 and α1 = 1.4(6)·10−9 K−2 [with α0(a) = 0.47(6)·10−5 K−1, α0(b) = 1.07(4)·10−5 K−1, and α0(c) = 0.97(7)·10−5 K−1]. The petrological implications for the experimental findings of this study are discussed.