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The thermal behavior of three amphiboles along the join "Mg-richterite" [MRIC: ANaB (NaMg) CMg5T Si8O22W (OH)2]--richterite [RIC: ANaB (NaCa) CMg5T Si8O22 W(OH)2] was investigated by in situ synchrotron radiation powder diffraction between 90 and 873 K. The studied samples have B-site compositions Na1Mg1 (sample RN1), Na0.97Mg0.8Ca0.24 (sample RN2), and Na0.97 Mg0.58Ca0.45 (sample RN6). The evolution of cell parameters as a function of T shows a discontinuity in the two Mg-richer samples (RN1 and RN2), which is interpreted as evidence of a P21/m → C2/m phase transition, whereas the Ca-richer sample (RN6) shows no evidence of a phase transition. The transition in samples RN1 and RN2 follows a different thermodynamic behavior, being tricritical in end-member "Mg-richterite" (RN1) and second order in the BCa-bearing amphibole RN2. A thermodynamic analysis done according to the Landau formalism and allowing for order parameter saturation, gives Tc = 462(3) and 378(1) K, and saturation temperature θs =116(21) and 141(7) for RN1 and RN2, respectively. Comparison with data from literature shows that the thermal strain of C-centered amphiboles with constant A-, C-, T-, and W-site occupancy equal to Na, Mg5, Si8, and (OH)2, respectively, and a B-site occupied by variable amounts of Li, Na, Mg, and/or Ca, mainly expands about 70° from c toward the a cell-edge onto the 010 plane. Conversely, the spontaneous strain accompanying the thermal transition shows that the maximum expansion is oriented about 25° from c and is coupled with a contraction close to the a cell direction. On the other side, transition induced by solid solution at room-T follows an almost opposite deformation pattern. The present data confirm the hypothesis of a first-order character of the transition induced by the increase of the B-site dimension for increasing BCa contents, similarly to the closely related P21/c → C2/c transition in pyroxenes. [ABSTRACT FROM AUTHOR]

Synthetic NaGaSi2O6-pyroxene, space group C2/c, was investigated at low temperature between 295 to 110 K by X-ray diffraction. The evolution of the unit-cell parameters as a function of temperature does not indicate any structural phase transition down to 110 K. Calculated values for the Debye temperature, θD, and the Grüneisen parameter, γ, are 653(39) and 0.84(8) K, respectively, close to those of other pyroxenes. Five complete intensity data collections were performed at 295, 235, 190, 145, and 110 K to investigate the temperature dependence of the crystal structure. The space group remains C2/c down to 110 K, but anomalies were found between 235 and 190 K for the temperature dependencies of the volume and strain of the M1 polyhedron. This anomalous behavior could be related to variations of the Ga-O2(C1,D1) bond length and the O1(A1,B1)-Ga-O2(C1,D1) bond angle. The C2/c-P1̅ phase transition observed in a previous work for NaTiSi2O6 at about 200 K did not occur for NaGaSi2O6 down to the minimum temperature investigated in this work. This is not due to the different ionic radius at M1 site or to a different tetrahedral chain extension between these two compositions but it is likely due to unfilled t2g orbitals, which do not exist in NaGaSi2O6. [ABSTRACT FROM AUTHOR]

The crystal-structures of seven synthetic pyroxenes along the jadeite--hedenbergite (Jd57Hd43, Jd26Hd74, Jd0Hd100) and jadeite--aegirine (Jd100Ae0, Jd76Ae24, Jd35Ae65, Jd0Ae100) joins were refined using data collected by means of single-crystal X-ray diffraction (space group C2/c, R4σ between 2.2 and 3.4%). The M2 and M1 polyhedral volumes and bond lengths increase with increasing aegirine and hedenbergite content, moreover the Ca for Na substitution along the jadeite.hedenbergite join changes the M2 coordination from 6 + 2 to 4 + 4, with remarkable tilting of the tetrahedral chains. The value of the displacement parameters follows the trend UeqM2 > UeqO2 > UeqO3 > UeqO1 > UeqM1 ≈ UeqT for all samples belonging to the jadeite.aegirine join and for pure hedenbergite; in contrast,, for pyroxenes with intermediate compositions between hedenbergite and jadeite the trend is UeqO1 > UeqO2 > UeqM2 > UeqO3 > UeqM1 ≈ UeqT, with O1 and O2 having anomalously large displacement parameters, probably due to different local structural configuration around the cations with different size and charge. Cation substitution at the M1 site of Na-pyroxenes gives rise to a different structural deformation with respect of the double substitution at both the M1 and M2 sites in (Na,Ca)(M3+,M2+)Si2O6 pyroxenes as the rigid tetrahedral chains try to accommodate both the increasing size of the M1 site and the different coordination requirement of the M2 site. [ABSTRACT FROM AUTHOR]

Synthetic NaGaSi2O6-pyroxene, space group C2/c, was investigated at low temperature between 295 to 110 K by X-ray diffraction. The evolution of the unit-cell parameters as a function of temperature does not indicate any structural phase transition down to 110 K. Calculated values for the Debye temperature, θD, and the Grüneisen parameter, γ, are 653(39) and 0.84(8) K, respectively, close to those of other pyroxenes. Five complete intensity data collections were performed at 295, 235, 190, 145, and 110 K to investigate the temperature dependence of the crystal structure. The space group remains C2/c down to 110 K, but anomalies were found between 235 and 190 K for the temperature dependencies of the volume and strain of the M1 polyhedron. This anomalous behavior could be related to variations of the Ga-O2(C1,D1) bond length and the O1(A1,B1)-Ga-O2(C1,D1) bond angle. The C2/c-P1̅ phase transition observed in a previous work for NaTiSi2O6 at about 200 K did not occur for NaGaSi2O6 down to the minimum temperature investigated in this work. This is not due to the different ionic radius at M1 site or to a different tetrahedral chain extension between these two compositions but it is likely due to unfilled t2g orbitals, which do not exist in NaGaSi2O6. [ABSTRACT FROM AUTHOR]