CHLORINE SUBSTITUTION INTO SYNTHETIC FERRO-PARGASITE

Halogen incorporation into hydrous minerals may reveal the nature of the ambient halogen-bearing fluid and, in turn, the geological processes giving rise to the fluid. Calcium amphiboles are common hosts for chlorine in terrestrial rocks and meteorites, but there is little experimental information available on the dependence of chlorine concentration on such chemical variables as the Fe, Al, K, Na, and Ti contents. In this study we explore the incorporation of Cl into ferro-pargasite, ideally (NaCa₂(Fe²⁺₄Al)(Al₂Si₆)O₂₂(OH,Cl)₂ equilibrated with a range of brine compositions consisting of either NaCl or FeCl₂.

Starting mixtures were prepared by first mixing together reagent-grade SiO₂, Al₂O₃, Na₂CO₃, and CaCO₃, decarbonating this mixture at 900°C for 15 min in air, and then adding Fe₂O₃ and finely powdered metallic Fe in proportions equivalent to FeO. For some mixtures, Cl was introduced as FeCl₂ along with Fe₂O₃ and Fe. All mixtures were treated in sealed AgPd-alloy capsules. Most were treated with ~10 wt% NaCl brine stock solutions (0.1-5 molal NaCl), or in hypersaline solutions made by adding NaCl and H₂O directly to the capsule prior to sealing. Absorbed moisture was the only source of water in FeCl₂–bearing mixtures. Syntheses were done at 600-700°C and 0.2-0.42 GPa for durations of 125-672 h in a reduced oxygen atmosphere (wuestite/magnetite < fO₂< Co/CoO).

The Cl content of ferro-pargasite increased linearly to 0.05 Cl anions (per 23 oxygens) at about 1.5 m NaCl, beyond which the Cl content was essentially constant, even when synthesized in hypersaline solutions up to 25 m NaCl (mole fraction of Cl, X_Cl, = 0.3). Ferro-pargasite has higher Cl contents, up to 0.4 Cl anions (1.4 wt% Cl), when synthesized in the presence of an FeCl₂ brine (X_Cl = 0.34-0.44) rather than NaCl. From this study it can be concluded: (1) end-member ferro-pargasite is insensitive to the Cl content of the ambient fluid from 1.5 – 25 m NaCl with the coexisting amphibole having ~0.05 Cl anions; (2) the maximum Cl contents were obtained using FeCl₂ rather than NaCl but are only about 30% of the values reported in nature for ferro-pargasite (eg Kullerud & Erambert, 1999, GCA); and (3) crystal-chemical factors (probably K content) become more important than the brine concentration in forming the highest Cl-contented amphiboles observed in nature.