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

Abstract

Amphiboles were hydrothermally synthesized at 500 °C and 4 kbar in the system Li 2 O-Na 2 OFeO- Fe 2 O 3 -SiO 2 -H 2 O, with nominal compositions along the riebeckite [□Na 2 Fe 3 2+ Fe 2 3+ Si 8 O 22 (OH) 2 ]-ferri-clinoferroholmquistite [□Li 2 Fe 3 2+ Fe 2 3+ 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 B Li 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 B Na 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 A Na (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 Fe 2+ 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.