Soil Survey Laboratory Mineralogy Analysis Capabilities  

this emerald is a member of the beryl family found in pegmatites throughout the world

The Soil Survey Laboratory (SSL) at the National Soil Survey Center (NSSC) performs many soil property analyses, which can be grouped as chemical analyses (pH, CEC, trace element composition, etc.), physical analyses, (particle size distribution, bulk density, water content, etc.), and mineralogical analyses (clay species (kind of clay) and amounts, and kinds of minerals in the silt and fine sand fractions). Most of the laboratories associated with the National Cooperative Soil Survey are equipped to perform the chemical analyses and particle size distribution analysis. Few are equipped for mineralogical analysis or for bulk density and water release analysis by the clod method used at the SSL.

The primary mineralogical analysis for coarser-textured soils is optical grain counts. For this analysis, the most abundant particle size fraction in the coarse silt to fine sand range is separated and placed on a microscope slide. A petrographic microscope is used to view, identify, and count a minimum of 300 individual mineral grains. The number of grains of each mineral species is reported. The data are grouped as resistant and weatherable minerals to determine the relative weathering potential of the soil material.

The relative weathering potential is considered for taxonomic classification and for interpretations of the potential of the soil to supply plant growth nutrients. The mineralogical classification is a component of the soil taxonomic classification. The data are used for other interpretations (high amounts of mica contribute to slip failure, for example) and are used to provide information about the soil parent material. The important questions about clay in soils are, “What kinds of clay are present and in what quantities?” The primary method used for clay mineral analysis is X-ray diffraction (XRD). The clay-size fraction is separated from a soil sample and placed on slides by a method that orients the clay particles with reference to one crystal axis. Each kind of clay has a specific crystal structure. The slides are treated several times with different saturating cations and with different heat treatments. The cation and heat treatments cause changes in the clay structure, and the kinds of changes differ from clay to clay. Potassium, for example, will expand the distance between layers of certain clays, but will have no effect on other clays.

The clay samples are then analyzed in an X-ray diffractometer where an X-ray beam is directed into the sample at a specific angle. The angle at which the beam is diffracted (changed in direction) by the clay depends upon the clay mineral and the treatments the clay sample has received. The angle of refraction is measured for each of the treatments and the sizes and presence or absence of intensity peaks in the refracted X-ray beam allows identification of the kind of clay in the sample. Only a rough estimate of the quantity of the clay can be made from the X-ray procedure. If more accurate measurement of the amount of clay is desired, the sample is analyzed by one of two heat treating methods (differential scanning calorimetry (DSC) or thermogravimetric analysis (TGA). These methods measure energy change DSC or weight change TGA as the clay minerals are decomposed by heat. Although these techniques allow much more accurate measurement of clay volume, not all clays respond to heat treatments. However, the heat (thermal) treatment values allow one of the clay minerals determined by XRD to be used as an internal standard, so the estimate of the quantity of the other clays in the sample can be made more accurately.

The clay fraction is the most chemically and physically active fraction of most soils, so even a small amount of clay can substantially influence the properties (and, thus, the interpretations for use) of the soil. The amounts and kinds of clay minerals in soils are important for soil classification and many interpretations for soil use and management. Clay properties strongly influence both fertility and physical properties of most soils. The mineralogical analyses are very time-consuming, so SSL performs those analyses on a subset of the samples analyzed by the laboratory. Statistical methods are used to derive relationships to estimate mineralogical properties for samples that are not analyzed. The image analyzer is another instrument used to provide mineralogical information. A thin section or grain mount is placed in a petrographic microscope to which a digital camera has been attached. A computer captures a calibrated image of the sample and software on the computer can be used to determine grain sizes, pore sizes, clay film thickness, grain shape parameters (such as roundness), the areas of different colors, and many other visual features of the sample. These analyses are quite time-consuming, so they are typically performed only on special projects or when an unusual question arises when analyzing data.
Your contact is Ellis Benham,  NRCS research soil scientist, at 402-437-5132.