• journal_title：American Mineralogist
• Contributor：G. Iezzi ; M. Tribaudino ; G. Della Ventura ; I. Margiolaki
• Publisher：Mineralogical Society of America
• Date：2011-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2011.3559
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：96
• issue：2-3
• firstpage：353
• section：Articles

The thermal behavior of three amphiboles along the join “Mg-richterite” [MRIC: ^ANa^B(NaMg) ^CMg₅^TSi₈O₂₂^W(OH)₂]–richterite [RIC: ^ANa^B(NaCa)^CMg₅^TSi₈O₂₂^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), Na_0.97Mg_0.8Ca_0.24 (sample RN2), and Na_0.97 Mg_0.58Ca_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 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 T_c = 462(3) and 378(1) K, and saturation temperature 𝛉_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, 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.