CRYSTAL CHEMICAL CONTROLS ON AMPHIBOLE CL INCORPORATION

Understanding the mechanism of Cl incorporation in amphiboles is necessary for geologists interested in using an amphibole’s composition to gain insight into the chemistry of ancient aqueous fluids. The compositional constraints on Cl incorporation are only partially understood and very little experimental data exists. The current consensus is that the Fe#, ^AK content, and ^TAl content exert the greatest compositional influence on the ability of amphiboles to incorporate Cl. However, recent research has challenged the importance of ^AK in forming Cl-rich amphiboles. This study reports on the composition and unit cell dimensions of a collection of twenty-six synthetic amphiboles with a wide range of Cl content (0 to ~1.7 atoms per formula unit).

Amphiboles were synthesized from reagent grade materials under various conditions. The temperature and fO₂ were mostly held constant, near 700°C and Ni-NiO respectively. The main experimental variables affected amphibole Cl content, other than the bulk composition, were the pressure and the Cl concentration of the coexisting brine (if present). Pressure conditions ranged from 3 to 20 kbar and FeCl₂ brines were used, ranging from 1 to 100 molal. Several bulk compositions were explored, including those that varied the Fe# and A-site cation, though most amphiboles were synthesized from a bulk composition equivalent to potassic-chloro-hastingsite (KCa₂(Fe₄²⁺,Fe³⁺)(Al₂Si₆)O₂₂(OH,Cl,O)₂). All amphiboles were analyzed by electron microprobe (WDS), X-ray diffraction (Reitveld refinement) and Mössbauer spectroscopy.

Analysis of compositional relations revealed that the substitution of ^CFe for ^CMg and ^CFe for ^CAl encouraged Cl acceptance, with the former having a much greater impact. The influence of the A-site remains ambiguous, as both positive and negative correlations with Cl were observed. The amphibole’s unit cell volume was found to expand linearly as the Cl content increased. Expansion was only observed along the b (0.11 Å/apfu Cl) and c (0.02 Å/ apfu Cl) dimensions, not a, which was most sensitive to ^AK, though oddly not ^ANa. If expansion of the unit cell along b and c is the main requirement for Cl incorporation, the A-site would exert little control over the process. However, the role of increased bonding between the A and O(3) sites as Cl replaces OH may also be influential.