Serpentine represents a group of magnesium (Mg) silicate minerals with similar chemistry and crystal structure and a general chemical formula of (Mg,Fe)₃(Si₂O₅)(OH)₄. Serpentine is formed by the hydrothermal alteration of ferromagnesian minerals such as olivine and pyroxene in ultramafic and mafic igneous and metamorphic rocks. In some areas of altered ultramafic rocks, serpentine minerals form large masses. Such areas, including part of northern California and small areas in the Piedmont (Fenneman and Johnson, 1946) along the East Coast, can have 'serpentine' soils, with unusual mineralogy (including serpentine or talc, another Mg silicate mineral) and chemistry (such as high Mg, chromium (Cr), and nickel (Ni), and low calcium (Ca) and potassium (K)). These soils support plant species tolerant of the extreme soil conditions. Serpentine easily weathers to talc and smectite, a clay mineral with a basal spacing of 14 Å (1 Å (angstrom) = 10^–10 meters (m)). Serpentine minerals are rare in soils because of their limited geologic occurrence and weathering susceptibility.

In our data, serpentine is present at only nine sites in the soil C horizon, with a maximum concentration of 26.7 weight percent (wt. %). Serpentine is present at 20 sites in the soil A horizon, with a maximum concentration of 17.9 wt. % (see the summary statistics [open in new window]). Because so few samples have serpentine concentrations above the detection limit (0.2 wt. %), the spatial distribution of serpentine is shown as a proportional symbol map rather than an interpolated and smoothed color surface map.

Most occurrences of serpentine in both the soil A and C horizons are in areas of California where soils developed on weathered ultramafic and mafic rocks. There also are other scattered occurrences in both the western and eastern United States where soil developed on ultramafic or mafic rocks. Serpentine is not present in a 'serpentine' soil sampled in Maryland, recognized because of its geochemistry; the intense weathering in this region has apparently altered all serpentine to talc.