Lithium in amphiboles : detection, quantification, and incorporation mechanisms in the compositional space bridging sodic and BLi-amphiboles

Lithium is an important constituent in amphiboles, where it can be incorporated up to a limit of three atoms per formula unit (apfu). Lithium can partition itself between the B-group sites (where it occurs at the [6+2]-coordinated M4' position) and the C-group sites (where it occurs at the M3 site). Systematic analysis of the available chemical (EMP + SIMS) and structural data constraints lithium occurrence in amphiboles to the following compositions and exchange vectors: (1) BLi is incorporated according to M4'Li M4 Na-1, and no miscibility gap is apparent, despite the difference in the ionic radii; (2) CLi is incorporated according to M3Li M2Fe3+ M3Fe2+-1 M2Fe2+-1; however, a partial bond-strength contribution is provided by Si at the T1 site and by Na or K at the Am site. Amphiboles with CLi > 0.5 apfu (root names: leakeite, kornite, whittakerite and pedrizite) have more than half-occupied A-group sites. Seven new amphibole end-members containing lithium have been discovered in epysienites (dequartzified and albitised granites) from the Pedriza Massif (Central Spain), where lithium incorporation and partitioning is controlled both by the composition of the fluid and the temperature conditions of crystallisation. This occurrence provides an unique oppurtunity to characterise the M4Li M4Na and M3Li M3Fe2+ solid solutions, as well as model crystal-chemical mechanisms and understand their dependence on intensive parameters. An accurate quantification and partitioning of lithium in amphiboles is not trivial, and requires a combination of ion-microprobe analysis and structure refinement. Analysis of the available data provides criteria for calculating reliable H2O and Li2O values, as well as for obtaining reliable unit formulae from routine EMP results. These criteria can then be used to simplify petrological studies in Li-rich environments.