EXPERIMENTAL STUDY OF CHLORINE ACCEPTANCE IN THE HASTINGSITE-MAGNESIOHASTINGSITE JOIN

Hastingsite has been observed to be a high-chlorine amphibole, where some of the highest wt% are found in martian meteorites (McCubbin et al. 2013). However, the agent required to adequately provide chlorine to the amphibole, as well as the favorable host amphibole composition for high Cl content are not well understood. It is important to learn how these two factors relate and if they can be applied to other halogen-bearing amphiboles. This study focuses on the correlation of the Fe# and the acceptance of Cl along the hastingsite-magnesiohastingsite join, NaCa₂(Fe²⁺Fe³⁺)(Al₂Si₆)O₂₂(OH, Cl)₂- NaCa₂(Mg₄Fe³⁺)(Al₂Si₆)O₂₂(OH, Cl)₂.

Samples were made from chemical reagents and then synthesized at 750-850ºC and ~0.4 GPa for 3-4 days, where the fO₂ is within the stability field of Co, or below log(FMQ)-2. The samples were prepared in Ag-Pd capsules, where the Cl was introduced as iron(II)chloride. Although these samples were nominally anhydrous, some moisture was absorbed producing FeCl₂ brines ranging from mole fraction Cl, X_Cl = 0.47 to 0.59. Phases were identified using X-ray diffraction, followed by chemical analysis by the electron microprobe.

The results indicate a positive correlation between Cl anions ranging from 0.04-0.2 Cl (apfu) and the Fe# increasing from 0.0-1.0 (mole fraction). There is a pronounced maximum at Fe# of 0.64 +/- 0.07; Cl = 0.64 apfu (2.2 +/- 0.2 wt% Cl). The end-member, pure hastingsite shows a Cl content of 0.21 Cl apfu. These results are consistent with McCormick and McDonald's (1999) low-K hastingsite, which has a Cl content maximum of ~0.2 Cl apfu.

While a Cl content up to 0.64 apfu has been observed here, the maximum Cl content seen in nature and meteoritic samples has not yet been achieved. There are several other studies that observe high-chlorine hastingsite, where the Cl content reaches 0.84 apfu (Vanko, 1986), 1.37 apfu (Mazdab, 2003), and 1.75 apfu (McCubbin et al. 2013). It is hypothesized that the amphibole structure requires the presence of other cations, like K or Ti, to allow more Cl into the lattice structure.