Holmquistite (Pnma) vs. clinoholmquistite (C2/m): composition and symmetry in BLi amphiboles

A systematic investigation of BLi amphiboles has been done based on complete (EMP+SIMS) in situ chemical analysis and single-crystal structure refinement. The main results are the following: 1.Holotype clinoholmquistite, ideally A? BLi2 C(Mg2Al2Li) TSi8 O22 X(OH)2, turned out to be holotype fluoro-sodic-pedrizite, ideally ANa BLi2 C(Mg2Al2Li) TSi8 O22 XF2. The previous misleading assignment was due to the presence of lithian tremolite in the amphibole separate analysed by wet-chemical methods. Therefore, clinoholmquistite has never been found in Nature; also it could not be synthesised in the LNFMAFSH system at T ranging between 450 °C and 900 °C, P between 0.1 and 1.5 GPa, and fO2 between NNO-0.5 and NNO+3 (Iezzi et al. 2004). 2.On the contrary, ferro-, ferri-, and ferri-ferro- clinoholmquistites have been found in Nature; also, they can be rather easily synthesised under hydrothermal conditions. They generally show extensive solid solution with the corresponding terms of the sodic-pedrizite series, so that Li is often partitioned between the B- and C- group sites (Oberti et al. 2003). 3.Holmquistite samples can be found in many localities all around the world, where they crystallise at the contact between lithium-rich pegmatites and country rocks. Their compositional variations are quite small, and the only abundant substituent is Fe (Fe2+ and Fe2+ always sum to ~1.2 apfu). In holmquistites, Li is always ordered at the B-group sites, and the A site is always empty. 4.Holotype "ferroholmquistite" (IMA-CNMMN approval still pending) has been detected and described in the Greenbushes pegmatite, Western Australia. The maximun Fe/Mg ratio found in holmquistites is however 1.3. No ferri- end-member could be found. 5.The new crystal-chemical data provided by this work can be interpreted in terms of a structural constraint which precludes incorporation of larger cations in orthorhombic holmquistites. This constraint is related to the relative dimensions of the strip of octahedra and of the double-chains of tetrahedra. The presence of larger cations determines crystallisation of the monoclinic polytype. Iezzi, G., Della Ventura, G., Cámara, F., Oberti, R., and Robert, J.-L. (2004) Li-bearing amphiboles: synthesis, stability and composition of clinoholmquistites. 32nd IGC Congress, Florence, Atti. Oberti, R., Cámara, F., Ottolini, L., and Caballero, J.M. (2003) Lithium in amphiboles: detection, quantification and incorporation mechanisms in the compositional space bridging sodic and BLi amphibole. European Journal of Mineralogy, 15, 309-319.