Towards a model for HT behaviour of (orthorhombic and monoclinic) amphiboles

Research over the last 30 years has significantly changed our knowledge of the amphibole supergroup, most particularly in recognizing new compositional spaces, including Li- and oxo-amphiboles. Systematic studies and the use of databases have provided detailed crystal-chemical models that have helped in deciphering the relations between cation ordering and structure. From this analysis it has been possible to evaluate the petrological significance of homovalent and heterovalent cation ordering. This detailed knowledge has also enabled a more accurate, comprehensive and robust scheme for amphibole classification and nomenclature to be constructed (Hawthorne et al. 2012), which takes into account all the chemical variability. In contrast, up to 2007 few data were available on the high-temperature behaviour of amphiboles, and especially of more geologically relevant compositions. These studies were hampered by the fact that thermal annealing generally induces in amphiboles three simultaneous processes: thermal expansion, cation disordering, and deprotonation. In order to unravel this complex behaviour, we have undertaken a program of systematic work that is still in progress. Thermal expansion was studied first in the simplest compositions, for which cation disordering and deprotonation processes are excluded. The study was extended to more complex compositions, which are of major relevance to petrology. Measurement of unit-cell parameters was combined with single-crystal structure refinement done at temperatures chosen to follow the irreversible deprotonation process, which occurs via ordering of Fe2+ at the M(1) and M(3) sites and its oxidation to Fe3+, followed by data collection on cooling to investigate the deprotonated phase. All the available crystal-chemical knowledge was used to interpret changes (site-scattering, site and chain geometry, displacement parameters). Based upon these recent studies we present our first conclusions on the main chemical parameters and structural constraints affecting the complex HT behaviour of orthorhombic (anthophyllite, gedrite, ferro-holmquistite) and monoclinic (pargasite, potassic-pargasite, kaersutite, richterite, ferro-richterite) amphiboles and their partially deprotonated or "oxo" counterparts. The importance of symmetry, the nature of the B cations, the amounts of A(K,Na), TAl, CFe and WO2-, and the consequent constraints in the relative expansion of the ribbon of octahedra and the double-chains of tetrahedral are discussed