THE ROLE OF CHLORIDE TYPE ON THE SYNTHESIS OF CHLORINE-BEARING AMPHIBOLES

Calcium amphiboles with chlorine (Cl) contents of 1.6-1.7 anions per formula unit (apfu) have been the focus of interest for decades owing to the information they can reveal about the fluids that lead to economic deposits, or about the nature of fluids or melts in planetary crusts. In most cases the source of Cl is assumed to be NaCl; however, other salts such as CaCl₂, MgCl₂, and FeCl₂ have been identified as being important constituents of high-temperature hydrothermal or deep-crustal metamorphic fluids. A simple comparison of the efficacy of several different halides in forming Cl-bearing amphibole is presented here.

Synthesis of calcium amphibole from mixtures with the bulk composition of hastingsite (NaCa₂(Fe²⁺₄Fe³⁺)(Al₂Si₆)O₂₂Cl₂ = Hs) was done using starting mixtures with four different chlorides or chloride combinations. These mixtures used CaCl₂, 1:0.5 NaCl:CaCl₂, 1:0.5 NaCl:FeCl₂, or FeCl₂ to provide a full 2.0 Cl apfu along with appropriate proportions of Na₂CO₃, CaCO₃, Al₂O₃, SiO₂, Fe, and Fe₂O₃, the latter two being added after the former reagents were mixed and decarbonated. Care was taken to minimize the highly hygroscopic CaCl₂ from exposure to air while weighing, and each mixture was dried at 160-200°C prior to being sealed (dry) in AgPd₅₀-capsules. Treatment was made at 700°C and 0.43 GPa for 168 h in internally-heated gas vessels using an Ar+H₂ mixture as both the pressure medium and to provide a H₂ fugacity of 125 bars (= -0.5 log(fO₂) below Co-CoO). The products consisted of amphibole along with hedenbergite, plagioclase, fayalite, ± garnet, and a salt. Analysis by electron microprobe showed that each starting mixture produced amphiboles with nearly the same Cl content (average of 0.76 ± 0.12 Cl apfu), with the remaining Cl residing in the coexisting salt. Slightly, but not significantly, higher Cl (0.89 ± 0.14) resulted when only CaCl₂ was the source, and slightly lower (0.61 ± 0.16) when NaCl+CaCl₂ was used.

These results suggest the type of chloride is less important than the total Cl concentration (activity) in the starting mixture. It should be emphasized that these experiments were carried out as dry syntheses and it is not yet known if these results apply to aqueous solutions. These experiments do, however, provide a valuable reference for understanding the role of concentrated brines.