The BNa- BLi exchange in A-site vacant amphiboles: synthesis and cation ordering along the ferri-clinoferroholmquistite – riebeckite join

Amphiboles were hydrothermally synthesized at 500 degreesC and 4 kbar in the system Li2O-Na2O-FeO-Fe2O3-SiO2-H2O, with nominal compositions along the riebeckite [rectangleNa(2)Fe(3)(2+)Fe(2)(3+)Si(8)O(22)(OH)(2)]-ferri-clinoferroholmquistite [rectangleLi(2)Fe(3)(2+)Fe(3)(2+)Si(8)O(22)(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 Li-B end-member, the IR spectrum shows a single sharp main band centered at 3614 cm(-1), which is assigned to the FeFeFe-OH-(Lambda)rectangle configuration. With increasing Na-B 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 Na-A (partial solid-solution toward arfvedsonite) is also observed with increasing sodium in the system. Mossbauer 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.