Geochemical and Mineralogical Maps, with Interpretation, for Soils of the Conterminous United States

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Lanthanum (La) is a metallic element used in the manufacture of carbon arc lights, used in the motion picture industry, and is a major component of the alloy mischmetal, which is used in the manufacture of flints for cigarette lighters. Lanthanum oxide is used in the manufacture of special optical glasses because it improves the optical properties and alkali resistance of the glass.

Lanthanum is 1 of 15 rare earth elements (REEs), many of which are not particularly rare. Most REEs have similar geochemical properties, and the distribution of La in soils is very similar to the distribution of cerium (Ce), another REE, and yttrium (Y), which shares many properties with REEs. The abundance of La in the Earth's upper continental crust is estimated to be 31 milligrams per kilogram (mg/kg) (Rudnick and Gao, 2003). Rare earth elements, including La, are present in low concentrations in many common minerals, such as feldspar and pyroxene, moderate concentrations in clay minerals, and very high concentrations in accessory minerals, including phosphate minerals (such as monazite and apatite), allanite (REE–enriched epidote), and zircon. These accessory minerals typically occur in many rock types, with somewhat higher concentrations in felsic igneous rocks (such as granite, rhyolite, and pegmatite), alkalic igneous rocks, phosphate–rich sedimentary rocks, as well as metamorphic equivalents of these rock types. Granite has an average La concentration of approximately 50 mg/kg. Shale has an average La concentration of approximately 40 mg/kg, with La in clays or accessory minerals. Some La–bearing accessory minerals are resistant to weathering, and are present in unconsolidated materials, such as alluvial, eolian, and glacial deposits. Some REE–bearing minerals, including monazite, xenotime, allanite, and zircon are common detrital heavy minerals that, when released from their parent rock by weathering, can accumulate by density sorting in fluvial, lacustrine, or oceanic coastal shoreline settings.

The distribution of mineral resource deposits with REEs (including La) 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 REE–bearing minerals are available through the USGS National Minerals Information Center (NMIC) website.

In our data, La has a median concentration of 26.1 mg/kg in the soil C horizon, and 25.7 mg/kg in the soil A horizon and in the top 0- to 5-cm layer (see the summary statistics [open in new window]). Only 13 samples from the total suite of soil samples have La concentrations less than the 0.5 mg/kg lower limit of determination (LLD). As suggested by the similar median concentrations, the La map patterns are generally consistent among the three sample types.

High La concentrations occur in soils developed on felsic rocks (granite and rhyolite), intermediate igneous rocks (alkali basalt or syenite, or sedimentary rocks and recent alluvial and eolian deposits sourced from similar rock types. Examples of such areas include:

• Parts of the Central Rocky Mountains and the Southern Rocky Mountains (USDA, 2006), Colorado, Wyoming, Montana, and Idaho (residuum of a variety of rock types, including granite);
• Parts of the Owyhee High Plateau (USDA, 2006), Nevada, Idaho, and Oregon (granite and felsic volcanics);
• Southern Nevada Basin and Range (USDA, 2006), Nevada and California (felsic volcanic rocks and related alluvium);
• Mojave Desert and Lower Colorado Desert (USDA, 2006), California and Nevada (granitic rocks and syenite);
• Trans–Pecos volcanics (Schruben and others, 1997), Texas (rhyolite, alkalic volcanic rocks, and related alluvium); and
• Llano Basin, Texas (Precambrian granitic rocks).

Elevated soil La concentrations in the Kentucky Bluegrass area (USDA, 2006) occur where soils developed on weathered phosphatic, apatite–bearing limestone, similar in geologic age (Ordovician) to phosphatic limestone in the Nashville Basin (USDA, 2006) in Tennessee. Scattered elevated soil La concentrations through the Southern Blue Ridge and Southern Piedmont (USDA, 2006) are attributed to occurrences of REE–bearing minerals in metamorphic and granitic rocks. A number of sites below the 'fall line' (the physiographic boundary between older crystalline rocks of the Piedmont (Fenneman and Johnson, 1946) and fluvial and marine sediments of the Gulf and Atlantic Coastal Plain (Fenneman and Johnson, 1946) have high La concentrations (as well as Ce and Y). These soils developed on sands that likely contain detrital REE–bearing monazite or zircon.

In general, soils developed on recent glacial deposits have moderate to low La concentrations. In many areas of the Upper Midwest, the southern glacial limit (Soller and others, 2012) marks a change in soil La concentrations. Soil La concentrations in the vast loess sheets south of the southern glacial limit are higher than La concentrations in soils developed in carbonate till north of the ice extent. This is likely related to the presence of wind–deposited silt–sized feldspar or a small, but persistent, quantity of REE–rich accessory minerals such as zircon.

Other large areas of the conterminous United States with low concentrations of La in soils typically lack many of the minerals that contain REE. These areas include:

• Pacific Northwest and northern California (mafic rocks);
• Colorado Plateau (USDA, 2006) (quartz–rich sandstone and eolian deposits);
• Nebraska Sand Hills (USDA, 2006) (unconsolidated quartz– and plagioclase–rich sand dunes and sand sheets);
• Parts of the Southern High Plains (USDA, 2006) in eastern New Mexico and western Texas (quartz–rich eolian sands and alluvial sediments); and
• Gulf and Atlantic Coastal Plain (Fenneman and Johnson, 1946) (quartz–rich sedimentary rocks and unconsolidated sediments).

The Gulf and Atlantic Coastal Plain is bisected by the Southern Mississippi River Alluvium and the Southern Mississippi Valley Loess (USDA, 2006). Alluvial sediments were deposited in the Mississippi River valley as the river flooded in recent geologic time. When these sediments dried, winds picked up the fine material and deposited it in thick loess sheets, mainly along the east side of the river valley. The youngest loess sheets are about 10,000 years old. A pattern of higher La concentrations in soils developed on these young sediments reflects long–range transport of La–bearing material from the upper part of the Mississippi River drainage basin.

Statistics - 0 TO 5 CM

Histogram

Boxplot

Empirical cumulative distribution function

Statistics - A Horizon

Histogram

Boxplot

Empirical cumulative distribution function

Statistics - C Horizon

Histogram

Boxplot

Empirical cumulative distribution function