Talc is a magnesium (Mg) silicate that commonly forms by the hydrothermal alteration of ferromagnesian minerals (such as olivine, pyroxene, and amphibole) in ultramafic and mafic rocks. An ideal chemical formula for talc is Mg₃(Si₄O₁₀)(OH)₂. Serpentine (a group of Mg silicate minerals characteristic of altered ultramafic and mafic rocks) can also alter to talc, and both minerals can occur together.

The distribution of mineral resource deposits with talc as a commodity (major or minor) in the United States, extracted from the U.S. Geological Survey (USGS) Mineral Resource Data System (MRDS) website, can be seen by hovering the mouse here. Statistics and information on the worldwide supply of, demand for, and flow of talc are available through the USGS National Minerals Information Center (NMIC) website.

In our data, talc is present at only 26 sites, each in the soil A and C horizons, and with few exceptions, typically at the same site in both soil horizons (see the summary statistics [open in new window]). Because so few samples have talc concentrations above the detection limit (0.2 weight percent (wt. %)), the spatial distribution of talc is shown as a proportional symbol map rather than an interpolated and smoothed color surface map.

Talc is relatively stable in soils; thus, its distribution is controlled by the mineralogy of the soil parent material. Many soils with detectable talc also contain the amphibole mineral hornblende, a mineral characteristic of ultramafic and mafic igneous and metamorphic bedrock. In the Siskiyou–Trinity Area (USDA, 2006) in California, as well as in central Maryland, talc is present in 'serpentine soils' that developed on metamorphosed ultramafic rocks. These soils have unusual mineralogy (including serpentine and talc) and chemistry (high Mg and chromium (Cr) and low calcium (Ca) and potassium (K)), and support unique plant species tolerant of the extreme soil conditions. Other scattered occurrences of talc in soils of Washington, Arizona, Montana, and Arkansas have a similar ultramafic or mafic rock geologic origin.