The high-temperature P21/m -> C2/m phase transitions in synthetic amphiboles along the richterite-(BMg)-richterite join

The thermal behavior of three amphiboles along the join “Mg-richterite” [MRIC: ANaB(NaMg) CMg5TSi8O22W(OH)2]–richterite [RIC: ANaB(NaCa)CMg5TSi8O22W(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.