Akaganeite crystal structure

Post, J. E. Buchwald, V. F. (1991) Crystal structure refinement of akaganeite. 

Schematic crystal structures of Fe(tll) oxides and oxyhydroxides (a) Hematite (a-FcjOs) (13) Goethite (a-FeO(OH)) (c) Akaganeite (/ -FeO(OH)) (d) Lepidocrocite (y-FeO(OH)). From Flynn (79).

Precipitation of ferric chloride in solution (at an iron concentration higher than 4 x 10" mol 1 ) by the addition of a base or thermohydrolysis leads to the oxyhydroxide /3-FeOOH (akaganeite) [58-6la]. Its structure is a parent of goethite. Double chains of octahedra are present in both structures, but they connect differently (Figure 5.16). /3-FeOOH has much wider channels containing variable amounts of chloride ions [58,62]. Unlike basic salts where polydentate ions are usually incorporated in the crystal structure, they are not part of the structure here, and can be exchanged with other ions [63-65]. 

Somatoids are often twinned on the (322) plane to give star-shaped or x-shaped twins (Eig. 4.15 a). Incorporation of low levels of Si in the structure promotes twinning with 0.04 mol mol Si, akaganeite was almost 100% twinned (Cornell, 1992). These crystals have a visibly roughened surface. Increasing citrate concentration during forced hydrolysis at 100 °C and pH 1 reduced the length of the somatoids from... 

The crystals of akaganeite are not microporous. Micropores observed by TEM are considered to be due to irradiation in the electron beam (Galbrait et al., 1979 Naono et al., 1982). Open ended, cylindrical, interparticular micropores have been reported these arose as a result of alignment of the rod-like crystals into parallel arrays (Paterson and Tait, 1977). Akaganeite does possess a potential structural microporosity arising from the presence of 0.21-0.24 nm across tunnels in the structure. At room... 

Akaganeite cannot be prepared at pHs above 5 because the OH ion is far more competitive than the chloride ion for structural sites. Akaganeite displays two morphologies somatoids or cigar shaped crystals and much smaller rod-like crystals. Samples of somatoids frequently contain a proportion of twinned crystals, whereas the rod-like crystals are never twinned. 

The speciation and structure of Fe-Si systems have also been examined between pH 3 and 10 at n > 3 and Si/Fe = 0 to 4 [80-82]. Based upon XRD results, the phases that are formed are generally amorphous with only small amounts of poorly crystallized akaganeite detected at pH 3. 

---