Gibbsite (Al(OH)₃) is a chemically simple mineral that is the end product of intense chemical weathering in areas of high rainfall and warm temperatures. It is an important component of aluminum (Al) ores where it is sufficiently concentrated in large volumes. Our survey detected gibbsite only rarely. Gibbsite is reported for only 50 sites in the soil A horizon, with a maximum concentration of 12.9 weight percent (wt. %), and for 122 sites in the soil C horizon, with a maximum concentration of 30.4 wt. % (see the summary statistics [open in new window]). Because of the low percentage of detection, gibbsite distribution is shown by proportional symbol maps rather than as an interpolated and smoothed color surface map.

All gibbsite occurrences correlate with high kaolinite concentrations and, thus, very aluminous soils. The principal area with detectable gibbsite is within the belt of kaolinite–rich soils of the southeastern United States. Kaolinite (Al₂Si₂O₅(OH)₄) itself, forms by intense chemical weathering of feldspars and clays, but can be further modified to gibbsite by loss of silica, resulting in the chemically stable aluminum hydroxide. Gibbsite was also detected in some samples in northern California and the coastal region of Oregon and Washington, also in areas of high rainfall and kaolinite–rich soils. Other isolated occurrences are in kaolinite–rich soils of southeastern Oklahoma, western Arkansas, and southern Missouri. In general, gibbsite is more abundant in the soil C horizon than the soil A horizon. At many sites with gibbsite–bearing C–horizon soils, the soil A horizon does not have detectable gibbsite.