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 constrains lithium occurrence in amphiboles to the following compositions and exchange vectors: (1) BLi is incorporated according to M4'Li M4Na-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 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 opportunity to characterise the M4'Li <-> M4Na and M3Li <-> M3Fe2+ solid solutions, as well as model crystal-chemical mechanisms and understand their dependence on intensive parameters.