The high-temperature P2(1)/m -> C2/m phase transitions in synthetic amphiboles along the richterite ((B)Mg)-richterite join

The thermal behavior of three amphiboles along the join "Mg-richterite" [MRIC: (A)Na(B)(NaMg)(C)Mg(5)(T)Si(8)O(22)(W)(OH)(2)]-richterite [RIC: (A)Na(B)(NaCa)(C)Mg(5)(T)Si(8)O(22)(W)(OH)(2)] was investigated by in situ synchrotron radiation powder diffraction between 90 and 873 K. The studied samples have Bsite compositions Na(1)Mg(1), (sample RN 1), Na(0.97)Mg(0.8)Ca(0.24) (sample RN2), and Na(0.97) Mg(0.58)Ca(0.45) (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 P2(1)/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 (B)Ca-bearing amphibole RN2. A thermodynamic analysis done according to the Landau formalism and allowing for order parameter saturation, gives T(c) = 462(3) and 378(1) K, and saturation temperature theta(s) =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, Mg(5), Si(8), and (OH)(2), respectively, and a B-site occupied by variable amounts of Li, Na, Mg, and/or Ca, mainly expands about 70 degrees 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 degrees 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 (B)Ca contents, similarly to the closely related P2(1)/c -> C(2)/c transition in pyroxenes.