Hematites from nine soils and one from itabirite rock were concentrated with 5 mol L-1 NaOH, and analyzed by XRD (step scanning 0.02º2θ/20 s), to determine the unit cell parameters. The samples were dissolved by extraction with DCB to determine the Fe (Fe d) and Al (Al d) content. Al substitution was estimated from a0 using established regressions currently in use based on hematites synthesized at 25°C (Al25) and 70°C (Al70). The a0 values ranged from 0.50380 to 0.50200 nm, corresponding to values of zero to 0.125 mol mol-1 Al d. The estimated values by the regression between a0 and Al d deviated from the Al70 regression by -0.003 to + 0.009 mol mol-1 Al, whereas the Al25 regression overestimated the substitution by an average of 0.03 mol mol-1 Al. Thus, for soil hematites, Al substitution is best estimated by the regression based on hematites synthesized at 70°C (Al mol mol-1 = 31.09 - 61.714a0).
hematite; aluminum; XRD; DCB