THE INFLUENCE OF AL UMINUM ON IRON OXIDES . VIII . UNIT-CELL DIMENSIONS OF AI-SUBSTITUTED GOETHITES A N D ESTIMATION OF A 1 FROM THEM

-The unit-cell dimensions of synthetic, Al-substituted goethites showed that the c dimension is a linear function of A1 substitution in the range 0-33 mole % A1, but that the a dimension is variable over this same range. The b dimension is also linearly related to Al substitution but is slightly more variable than the c dimension for A1 substitutions of 20-33 mole %. The variability of the a dimension is postulated to be the result of structural defects. An improved procedure for estimating Al substitution from x-ray powder diffraction positions requires (1) calculation of the c dimension from the positions of the 1 l0 and 111 diffraction lines using the formula: c = (l/d(11 l) 2 l/d(110)2) -'h, and (2) estimation of A1 substitution from the relationship: mole % A1 = 1730 572.0c. The 95% confidence interval of the estimate is +2.6 mole % A1 when using this procedure, in contrast to _+4.0 mole % A1 when the position of the 111 reflection alone is used. Key Words--Aluminum, Goethite, Iron, Unit-cell dimensions, X-ray powder diffraction. I N T R O D U C T I O N The ionic substitution of a luminum for iron in goethite is well documented and has been shown to occur in goethites from soils (Norrish and Taylor, 1961; Ja-. not et al., 1971; Davey et al., 1975; Nahon et al., 1977; Bigham et al., 1978; Fitzpatrick, 1978; Mendelovici et al., 1979; Torrent et al., 1980; Fitzpatrick and Schwertmann, 1981; K~impf, 1981), oolitic iron ores (Scheiderh6hn, 1964; Schellmann, 1964), and bauxites U6ntis and Solymfir, 1970). Al-substituted goethites can also be easily synthesized in the laboratory (Thiel, 1963; J6nts and Solymtr, 1970; Golden, 1978; Lewis and Schwertmann, 1979a, 1979b; Fey and Dixon, 1981). Goethite is the most ubiquitous of the iron oxide minerals in soils and occurs in almost every type of soil environment (Schwertmann and Taylor, 1977). A1 substitution ranges from zero to about 33 mole %. Fitzpatrick and Schwertmann (1981) showed that the amount of A1 substitution in goethite varies among different soil environments and that A1 substitution may be an indicator of past or present pedogenic conditions. The A13+ ion is slightly smaller that the Fe 3+ ion, 0.53 vs. 0.65 ~ (Shannon and Prewitt, 1969); thus, when A1 substitutes for Fe in the goethite structure, the average size of the unit cell decreases. All other things being equal, the unit-cell size is related to the amount of A1 substitution and is indicated by shifts of the goethite X-ray diffraction lines to smaller d-values. Thiel (1963) studied Al-substituted goethites synPresent address: Department of Agronomy, Purdue University, West Lafayette, Indiana, 47907. thesized under hydrothermal conditions at high pH and found that d(111) and the unit-cell dimensions decreased linearly with A1 substitution. J6nhs and Solymtr (1970) attempted to duplicate the relationships found by Thiel, but on a plot of d(111) vs. mole % A1, their data deviated considerably from those of Thiel. They attributed the deviation to extra A1 present as "amorphous free aluminum hydroxide" which they believed could not be completely washed out of their Samples before analysis, but they gave no additional evidence for the existence of the extra AI. My measurements of d(111) and mole % AI for synthetic, AIsubstituted goethites also deviated from those of Thiel (1963) and J6nfis and Solymfir (1971). For some synthesis series the data appeared to fall on a curve rather than the straight line predicted by Thiel, thereby casting doubt on the validity of the assumption that the unit-ceil dimensions decrease linearly with A1 substitution. Taylor and Schwertmann (1978) reported data which showed d(110) to be more variable than d(111) and d(130) for several Al-substituted goethites synthesized from the Fe 2§ system. Their data suggested that the a dimension was the reason for the variability. The shift of X-ray diffraction lines, particularly the 111 line (Norrish and Taylor, 1961), has commonly been used to estimate A1 substitution in unknown goethites. Regression lines calculated from data for synthetic, Al-substituted goethites, particularly the data from Thiel (1963), are usually used as "s tandard" curves. This procedure could be in error if d(111) is influenced by factors other than A1 substitution. The purpose of this study was: (1) to determine why some goethite diffraction lines vary for samples with the same amount o rAl substitution, and (2) to develop Copyright 9 1984, The Clay Minerals Society 36 Vol. 32, No. 1, 1984 Unit-cell dimensions of Al-goethites 37 Table 1. Methods used to synthesize the goethites.