Chlorine incorporation in amphiboles synthesized along the magnesio-hastingsite–hastingsite compositional join

Hastingsitic amphiboles can contain high Cl contents in both terrestrial and meteorite samples; however, the solutions or melts providing Cl to the amphibole, as well as the crystal-chemical controls of the host amphibole, are not well understood. This study focuses on the correlation between the Fe# (= Fe²⁺/(Fe²⁺ + Mg)) and the acceptance of Cl for amphiboles synthesized along the magnesio-hastingsite–hastingsite bulk compositional join, NaCa₂(Mg₄Fe³⁺)(Al₂Si₆)O₂₂(OH,Cl)₂–NaCa₂(Fe₄²⁺Fe³⁺)(Al₂Si₆)O₂₂(OH,Cl)₂. Samples were made from mixtures of reagent-grade starting materials prepared along the compositional join at approximately 20 mol% increments and synthesized at 700–850 °C and 0.2–0.4 GPa for 3–4 days, where the oxygen fugacity was broadly constrained to lie below the Co–CoO and above the wüstite–magnetite oxygen buffers. The source of Cl was FeCl₂. Although the amphibole syntheses were nominally anhydrous, some moisture was inevitably absorbed by the hygroscopic FeCl₂ and could have produced brines with mole fractions of Cl (=X_Cl) of 0.58 to 0.91, assuming such brine concentrations are below saturation in FeCl₂. Ferric iron contents of the synthetic amphiboles were determined by Mössbauer spectroscopy. Amphiboles formed along this compositional join were generally deficient in Fe³⁺ near the Mg-rich end and deficient in Na near the Fe-rich end, yielding amphiboles that are classified as pargasite and ferro- or ferro-ferri-hornblende, respectively, rather than hastingsitic amphiboles. An overall positive correlation was observed between Cl content and the Fe#, but with a distinct maximum at Fe# of 0.69 ± 0.04 where the Cl reaches 0.42 ± 0.06 atoms per formula unit (apfu) or 1.5 ± 0.2 wt% Cl, and decreases to 0.20 Cl apfu at Fe# of 1.0. This observation points to a factor other than Fe# as being more important, namely the proportion of Fe²⁺ (=Fe²⁺/(Fe²⁺ Fe³⁺). In comparison to other Cl-rich pargasitic amphiboles in nature, the maximum of 0.42 Cl apfu observed in this study is still about a factor of two to three lower than the maximum observed in natural samples. The implications of this study are that the amphiboles most likely to host the highest Cl contents are those with the highest proportion of Fe²⁺, and conversely the lowest proportion of Fe³⁺, as well as having significant ^TAl and most likely K.