CHLORINE IN METAMORPHIC AMPHIBOLE AND BIOTITE – CONSTRAINTS ON OPTIMAL PETROLOGIC AND CRYSTALLOGRAPHIC ENVIRONMENTS (Invited Presentation)

Chlorine (Cl^-) is an unusual anion because its size is large (1.81Å) relative to other anions such as OH^- (1.38Å) or F^‑ (1.31Å) and, yet, it is known to be a dominant or significant constituent anion in crystallographic sites commonly occupied by OH^-, F^- and/or O^2-. Among the potential Cl-rich minerals, amphibole and biotite are of particular importance because they have a wide stability range in a variety of petrologic environments. Cl-rich amphibole and biotite are found in hydrothermally altered ocean rocks, gold-porphyry deposits, skarns, high-grade marbles, amphibolite/granulite facies mafic rocks and iron formations. A characteristic feature of minerals in these environments is the interaction with Cl-rich aqueous fluids either as an external fluxing fluid or as residual, possibly unmixed, brine-CO₂ fluids.

Another common feature is the Fe-rich composition of Cl-rich amphibole (X_Fe>0.7) and biotite (X_Fe>0.5). Cl-rich amphibole in these petrologic settings is typically potassic hastingsite, sadanagaite, ferri-hornblende or ferro-pargasite. These minerals have significantly larger unit cell volumes than their Cl-free counterparts e.g. Cl-rich potassic hastingsite = 945.6 ų vs. tremolite = 906.6 ų. Incorporation of Cl into the amphibole structure expands the M(1) and M(3) octahedra leading to preferential incorporation of Fe²⁺ over Mg, and the expanded lattice favors substitution of ^viAl and ^A(K+Na). These relations result in the so-called “Cl-Mg avoidance” phenomenon and explains the potassic and aluminous enhancements of these amphiboles. Data from natural settings suggest that there may be Cl-saturation that is crystallographically controlled. Biotite exhibits similar crystallographic expansion such that Cl-incorporation tends to optimize fit of octahedral and tetrahedral layers.

As an example, granulite-facies Archean iron formations from the eastern Beartooth Mountains, Montana, typified by anhydrous assemblages of qtz + mt + opx + grt ± cpx, contain minor prograde metamorphic Cl-rich amphibole and biotite. Amphibole exhibits a strong positive correlation between Cl and X_Fe, ^IVAl and K. Petrologic and mineral chemical data suggest that unmixed CO₂-brine fluids were associated with the granulite-facies iron formation and that high Cl enhances amphibole and biotite stability.