Figure 3. Global terrestrial distribution of clay-size mineral groups as a percent of the soil clay fraction (% clay), excluding areas of ice sheet, glaciers, shifting desert sands and dunes, and wetlands (histosols).

(a, b) Gibbsite, (c, d) kaolinite, (e, f) illite/mica, (g, h) smectite, (i, j) vermiculite, (k, l) chlorite, (m, n) Fe oxide, (o, p) quartz, (q, r) non-crystalline, and (s, t) others. Data for topsoil (a, c, e, g, i, k, m, o, q, s) and subsoil (b, d, f, h, j, l, n, p, r, t) are shown.