CHLORINE-POTASSIUM RELATIONS IN HASTINGSITIC AMPHIBOLES

To metamorphic petrologists like Peter Robinson, amphiboles are “translucent garbage cans” that can provide useful information about its history and formation conditions. A pertinent example of these useful chemical properties is an amphibole’s Cl-content, which can be related to ancient hydrothermal fluids. Chlorine incorporation into amphiboles is not only affected by the activity of Cl in a fluid, but the crystal chemistry of the amphibole as well.

The importance of potassium in forming Cl-rich amphiboles has long been documented in multiple studies of naturally occurring amphiboles. It has been generally accepted that Cl and K are positively correlated. We report here results on synthetic K-Cl hastingsite (K Ca₂ (Fe₄²⁺, Fe³⁺) (Al₂ Si₆) O₂₂ (OH, Cl)₂) that complicate this relationship.

Amphiboles were synthesized from reagent grade materials at a series of conditions designed to yield amphiboles with a wide variety of Cl-content. The syntheses can be divided into two categories: “dry”, at 1-20 kbar and 600-700°C, and hydrothermal, in the presence of initial FeCl₂ brines from 0 - 250 molal (0-97wt%) at 3 kbar and 700°C. The synthetic amphiboles were analyzed by electron microprobe (WDS).

Analysis showed a negative correlation at Cl contents <0.4 apfu Cl and a positive correlation between K and Cl at amphibole Cl contents > 0.4 apfu Cl. A plot of Cl vs K for these amphiboles resembles an asymmetric V, with the lowest (0 apfu) and highest (~1.7 apfu) Cl amphiboles both having near ~1 apfu K. Moving toward the middle of the plot, amphibole K-content decreases until it reaches a minimum at ~0.5 K.

There is a negative correlation between K and ^CAl + ^CFe³⁺ indicating that the exchange vector ^AK^CFe²⁺ ↔ Avac_-1(^CAl,^CFe³⁺)_-1 may be operating. There were also noteworthy trends in the unit cell dimensions among these amphiboles, with the a dimension increasing with K and the c dimension increasing with Cl. Interestingly, the increase in the c dimension as Cl content increases from 0 – 0.4 apfu is counterbalanced by the decrease in the a dimension as K content decreases, allowing the unit cell volume to remain nearly constant (941ų) up until the threshold Cl content of 0.4 is reached, after which, volume increases with Cl (to a maximum of 961ų) and K is positively correlated with Cl. Direct analysis of Fe³⁺ is needed to confirm this exchange vector.