Hornblende ((Ca,Na)_2-3(Mg,Fe,Al)₅(Al,Si)₈O₂₂(OH,F)₂) is a chemically complex mineral in which solid solution of numerous elements results in widely variable composition. It is a member of the amphibole group of minerals. We have used a crystal structure of hornblende in quantitative calculations, but other related amphibole minerals might be included in addition to hornblende. Hornblende forms under high temperatures and occurs in a variety of igneous and metamorphic rocks. It is relatively stable in many weathering regimes, so it can also occur as clastic grains in sedimentary rocks.

Hornblende is not present in most samples and its median concentration is less than the detection limit of 0.2 weight percent (wt. %) for both the soil A and C horizons. But 5 percent of samples contain more than 1.8 wt. % hornblende in the soil C horizon and 1.6 wt. % in the soil A horizon (see the summary statistics [open in new window]).

The distribution of hornblende across the conterminous United States strongly mimics the occurrence of igneous and metamorphic rocks. It is not detectable in many large areas, such as the central United States, where there are few occurrences of hornblende–bearing bedrock. The greatest abundance lies in a belt in California that is underlain by granitic rocks of the Sierra Nevada Mountains (USDA, 2006) and an area in northern California and southern Oregon where bedrock is a complex assemblage of sedimentary, volcanic, and igneous rocks. Hornblende is also widespread in soils of Washington and parts of northern Idaho. In these areas, bedrock contains igneous and metamorphic rocks of the Columbia Plateau (Fenneman and Johnson, 1946), Cascade Range, and Northern Rocky Mountains (USDA, 2006).

A second large area of hornblende–bearing soils is in the Upper Midwest in parts of Minnesota, Wisconsin, and Michigan. The bedrock source is Precambrian crystalline rocks, but hornblende has been disbursed by Pleistocene glacial transport beyond the outcrop area of the Precambrian bedrock. The sharp western and southern boundaries of this area correspond to the outer limits of the Wadena, Rainy, and Superior lobes of glaciation (Grimley, 2000) more closely than to the bedrock patterns. Parts of the northeastern United States also have hornblende–bearing soils in areas of crystalline bedrock, especially in the Adirondack highlands (Fenneman and Johnson, 1946), New York. A smaller area of high concentrations of hornblende is in the Piedmont and Blue Ridge Provinces (Fenneman and Johnson, 1946) of North Carolina, South Carolina, and Tennessee where mafic metamorphic rocks are common in the bedrock.