THERMOCHEMICAL DATA FOR POTASSIC-CHLORO-HASTINGSITE AND DETERMINATION OF AQUEOUS CHLORIDE CONTENTS

The interaction between Cl-rich fluids and amphiboles in rocks is an area of study useful for understanding geological processes such as the formation of economic deposits, seawater-crust interaction and metasomatism in rocks. This is of interest because Cl aids in the transport of ore-forming minerals, and this data is useful for tracking these hydrothermal fluids in metamorphic rocks. Currently there is very little thermochemical data on Cl-rich amphiboles. Values for ΔH_f and S° for potassic-chloro-hastingsite (KCa₂(Fe₄Fe³⁺(Al₂Si₆)O₂₂Cl₂)=KCH) have been derived here from the upper-thermal stability curve of a K-Cl-rich amphibole.

Synthetic amphiboles close to KCH (K_0.8Ca_1.9(Fe_3.8Fe³⁺_0.8Al_0.4)(Al_1.6Si_6.4)O₂₂OH_0.4Cl_1.2) were made at pressures of 3-3.5 kb and 500-800°C in gas vessels pressurized with mixed H₂-Ar to achieve an f_O2 near the NiNiO oxygen buffer. Reversal reactions were done using equal mixtures of the amphibole and its breakdown products in Ag₅₀Pd₅₀ capsules, locating the boundary at 2kb/735°, 3kb/773°, 4kb/840°, and 7kb/950°C. The end-member reaction is 4KCH = 4 An + 1 Qtz + 4 Hed + 7 Fay + 2 FeCl₂+ 4 Syl + 1 O₂. Phases were analyzed by electron microprobe. Ideal activity of KCH was found to be 0.169 in amphibole. A volume for KCH of 295.57 cm³/mol was obtained from Rietveld refinements of powder diffraction data for a series of related synthetic amphiboles. C_P (in kJ/mol) of KCH was estimated as 1.15 + 1.04 x 10 ^-4(T,K) – 10643.5 / T² – 6.40/T^1/2. Combining these data with the activities (AX62, Holland, 2019) and data for the other phases in the Holland & Powell (2011, JMG) database, we are able to calculate ΔH° = -10640.6 ± 7 kJ/mol and S° = 764.6 ± 6 J/K·mol for KCH.

These thermochemical data were used to determine the aggregate Cl activities of the metamorphic fluids that formed the high-grade iron formations of the Beartooth Mtns, Montana (Henry & Daigle, 2018, AmMin). Using the reported mineral analyses, we were able to determine activities of KCH in amph (0.0062-0.021), fs (0.3-0.4), alm (0.2-0.3), and hed (0.4-0.5). Combining these with the rxn Fs + KCH = Qtz + Alm + 2 Hed + 0.5 Mt + 0.5 FeCl₂ (aq) + KCl (aq) at 7 kb and 800°C, we can determine the aggregate molality of Cl as 1-9 m, indicating relatively concentrated brines were involved. A clear correlation between the a_{ideal KCH} and the mCl can also be seen in this locality.