Goethite unit cell

Goethite (named after J.W. von Goethe) is isostructural with diaspore and with various other metal oxides (Tab. 2.1). It has an orthorhombic unit cell with a = 0.9956 nm, h = 0.30215 nm, c = 0.4608 nm (Goldsztaub, 1932 Hoppe, 1941 Forsyth... 

Fig. 2.4 Structure of goethite. a) Hexagonal close packed anion arrangement and distribution of cations in the octahedral interstices projection on (010) with the cation pairs indicated and the unit cell outlined, b) Projection on (001) with the unit cell and the octahedral arrangementindi-cated. c) Projection down [001]. Dashed circles represent Fe in the next lower layer, d) Arrangement of octahedral double chains. H atoms also shown, e) Ball-and-stick model with unit cell out-...

Lepidocrocite (Greek lepidos = scale, flake and krokoeis = saffron-coloured) is iso-structural with boehmite (Tab. 2.1). Unlike goethite and akaganeite which have a turmel structure, lepidocrocite is a layered compound. The orthorhombic unit cell contains four formula units and has the edge lengths, a = 1.2520(6) nm h = 0.3873(2) nm and c = 0.3071(6) nm (Ewing, 1935 Oles et al., 1970 Christensen Norlund-Christensen, 1978). 

This mineral appears to consist of intergrowths of goethite and rutile with a unit cell with a = 0.2868 nm and c = 0.4607 nm (Grey et al., 1983). 

A common method of synthesizing M-substituted oxides, particularly goethite and hematite is to add base to mixed M-Fe salt solutions to precipitate M-associated ferrihydrite. Most ions do not change their oxidation state, but incorporation of Mn and Co in goethite is preceded by oxidation of these ions to the trivalent state (Giovanoli Cornell, 1992). An indication of whether isomorphous substitution has occurred can be obtained from changes in the unit cell dimensions of the Fe oxides... 

Fig. 3.3 Relationship between the edge length b of the unit cell and the AI-for-Fe substitution of synthetic goethites.

Fig. 3.6 Relationship between the unit cell edge length b of synthetic goethites and the level of structurally incorporated Al ", Mn ", Cr ", Co " and V " (data from Schulze Schwertmann, 1984 Stiers Schwertmann, 1985 Schwertmann et al., 1989 Gerth, 1990 and Schwertmann Pfab, 1994, respectively).

Fig. 3.7 Top Relationship between the unit cell edge length a of synthetic goethites and various structurally incorporated metals. Bottom Rate of change of a per mol of substituted metal (= slope of the upper curves) vs. ionic radius of the respective metal cations (Gerth, 1990, with permission).

Fig. 4.5 High resolution electron micrograph of synthetic goethite crystals cut perpendicular to the needle axis [010]. The crystals are bounded by 101 faces. The large crystal contains faults and some of the smaller, fault-free crystals show ca. 1 nm lattice fringes corresponding to the c -parameter of the unit cell (0.9956 nm) (Schwertmann, 1984, with permission, courtesy H. Vali, Schwertmann Cornell, 2000, with permission).

Fig. 10.2 Surface hydroxyl configuration on the goethite 001, 101, 100 and 210 faces. Distances of O and Fe ions to the projection plane are indicated next to the corresponding row of ions. Rows of singly, doubly, and triply coordinated O ions are indicated as S, D, and T, respectively. Solid line rectangles represent the two-dimensional (surface) unit cell. Dotted-line rectangles show contiguous singly coordinated hydroxyls (Barron and Torrent, 1996, with permission).

Tab. 16.3 Linear correlation between the unit-cell parameter b and the chemically determined Al substitution of goethites from soils, sands and lake ores...

Anal. Chem. 316 616-620 Gerth, J. (1990) Unit-cell dimensions of pure and trace metal-associated goethites. Geo-chim. Cosmochim. Acta 54 363-371 Ghanem, S.A. Miklelson, D.S. (1998) Sorption of zinc on iron hydrous oxide. Sod Sd. 146 11-21... 

Schulze, D.G. (1984) The influence of aluminum on iron oxides VIII. Unit-cell dimension of Al-substituted goethites and estimation of A1 from them. Clays Clay Min. 32 36-44... 

Gerth, J. (1990) Unit-cell dimensions of pure and trace metal-associated goethites. Geochim. Cosmochim. Acta 54 363-371. 

Schwertmann, U. and Carlson, L. (1994) Aluminum influence on iron oxide XVII. Unit cell parameters and aluminum substitution of natural goethites. Soil Sci. Soc. J. 58 256-261. 

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