CHEMICAL CONTROLS ON CHLORINE SUBSTITUTION INTO CALCIUM AMPHIBOLES

In honor of the influential MSA RiM Volumes 9a and 9b on amphiboles edited by David Veblen, it seems appropriate to provide a brief summary on research that has been done the past several years at Binghamton University dealing with crystal-chemical controls on chlorine substitution into calcium amphiboles. This information is essential for extracting the geologically relevant question of the Cl concentration of the ambient brine/melt, and therefore the geological setting, that formed a Cl-rich amphibole.

The approach that has been used here is that of synthesizing amphiboles from the appropriate reagents, as opposed to anion exchanges with already formed amphiboles, as being the more expedient manner to make Cl-bearing amphiboles. Research in our lab over the past several years has focused on the joins pargasite—ferro-pargasite and magnesio-hastingsite—hastingsite using reagent-grade oxides, metallic Fe, and various salts treated in the range of 600-900°C, 0.2-0.5 GPa, and oxygen fugacities that are generally in the range Δlog(fO₂) CoCoO of 0 to -1. These previous studies have (1) confirmed the positive correlation between Fe²⁺ and Cl content, (2) shown that amphiboles formed in NaCl brines below saturation (at elevated P-T conditions) have relatively low Cl contents indicating strong partitioning into an aqueous brine, and (3) shown there is an inverse relationship between Cl and Fe³⁺.

The most recent research has investigated the effect of K by synthesizing hastingsitic amphiboles [(K,Na)Ca₂(Fe₄Fe³⁺)(Al₂Si₆)O₂₂(OH,Cl)₂] with variable K for Na substitution at the A site. These syntheses were done at 700-750 °C, 0.2-0.45 GPa, 111-268 h, and at the same fO₂ conditions as above. Significant amphibole yields were only obtained above 0.4 GPa using FeCl₂ as the source of Cl. These syntheses, which used concentrated FeCl₂ brines (X_Cl = 0.51-0.88) showed a positive correlation with K. Parallel syntheses of OH-equivalent amphibole without Cl produced amphibole but with coexisting annite, indicating the K went into making biotite rather than entering the amphibole. The Cl contents formed in these K-hastingsitic amphiboles were comparable to those found in meta-mafic rocks from various localities and emphasizes the importance of K, as well as Fe²⁺ in permitting Cl substitution into calcium amphiboles.