Phase transition induced by solid solution: The Ca-Mg substitution in richteritic amphiboles

Eleven compositions along the join Na(NaMg)Mg5Si8O22(OH)2-Na(NaCa)Mg5Si8O22(OH)2 (“magnesiorichterite”-richterite) have been synthesized at T = 800–850 °C and PH2O = 0.35–0.5 GPa. The run products have been characterized by electron probe microanalysis (EPMA), synchrotron and conventional X-ray powder diffraction (XRPD), Fourier transformed infrared (FTIR) spectroscopy, and selected area electron diffraction (SAED-TEM). Nominally, the chemical variation along the join can be expressed as BMgxBCa1–x with 0 ≤ x ≤ 1. A combination of EPMA and FTIR data in the OH-stretching region show that a complete solid solution is obtained under the conditions used. Nevertheless,
a slight deviation from the nominal compositions involving a limited loss of Na at A and B sites, balanced by an increase of Ca at the B site, is present. Several indications of a displacive and coelastic P21/m → C2/m transformation induced by the Ca-Mg chemical substitution are observed. The phase transition occurs at B-site composition (Xc) close to B(Na1Mg0.7Ca0.3). C2/m samples with a Ca content of 0.34, 0.45, and 0.54 apfu show a significant strain tail related to local compositional inhomogeneities. This residual strain disappears as the amount of BCa significantly increases with respect to that of BMg. The transformation behavior observed here mirrors that of pyroxenes along the join diopside (CaMgSi2O6)-enstatite (Mg2Si2O6). The cell parameters of amphiboles with CMg5, TSi8, and W(OH)2 and variable A- and B-site populations follow almost linear and continuous trends, indicative of small amounts of spontaneous strain accompanying these monoclinic phase transitions and the absence of significant miscibility gaps among different amphibole groups when quenched from higher temperatures of crystallization.