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⁺ Fe₂³⁺Si₈O₂₂(OH)₂]-ferri-clinoferroholmquistite [□Li₂Fe²⁺₃ Fe₂³⁺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.