BNa-BLi solid-solution in A-site-vacant amphiboles : synthesis and cation ordering along the ferri-clinoferroholmquistite-riebeckite join

Amphiboles were hydrothermally synthesized at 500 °C and 4 kbar in the system Li2O-Na2O-[FeO-Fe 2O3-SiO2-H2O, with nominal compositions along the riebeckite [□Na 2Fe32+ Fe2 3+Si8O22(OH)2]-ferri-clinoferroholmquistite [□Li2Fe2+ 3 Fe2 3+Si8O22(OH)2] join, where the exchange vector is NaLi–1 at the B-site. Experimental products were characterized by powder XRD and SEM-EDAX, confirming very high amphibole yield along the join (>95%, plus minor quartz). The XRD patterns can be indexed in C2/m, and the refined cell parameters show linear variation as a function of composition. For the BLi end-member, the IR spectrum shows a single sharp main band centered at 3614 cm−1, which is assigned to the FeFeFe-OH-A□ configuration. With increasing BNa in the mineral, this band broadens and shifts 4 cm−1 to higher frequencies. This effect can be attributed to the change in M4 site occupancy. Minor ANa (partial solid-solution toward arfvedsonite) is also observed with increasing sodium in the system. Mössbauer spectroscopy confirms the cation distribution provided by IR data, and shows that a small, but significant amount of Fe2+ occurs at M4 along the join. Infrared spectroscopy shows that the Li end-member has a very ordered structure, whereas intermediate compositions show local heterogeneities associated with the presence of two different B sites occupied by Na or Li.