March, 1998                                          SCSB#391

In the southern region, the presence of smectite in identifiable quantities is given significant weight in classification. That is, smectite influences many properties of soils. The highest category of this soil mineral assemblage classification has two classes, 1:1 dominated and 2:1 dominated assemblages. The 1:1 dominated assemblages must not have more than 10% smectite (no smectite detected by routine x-ray methods). Assemblages dominated by 2:1 minerals without identifiable smectite are separated. Definitions and a key to soil mineral assemblage classes are given in Table1.
 
In the 2:1 class, the presence of free calcium carbonates is considered an important differentiating characteristic. Free calcite in soils controls mineral stability, nutrient availability, and other soil reactions. Because of the important role of free calcium carbonate in soils , it is considered differentiating class criterion at low concentration.
 
In 1:1 classes, levels of free iron oxyhydroxides and gibbsite are diagnostic. In addition, weatherable minerals in the coarse fraction are used to form classes.
 
When forming classes, hard-and-fast quantitative limits are not followed. Quantitative soil mineralogy data are not generally available and are not normally obtained in routine analysis. The only approach possible is to use existing data. Currently, most mineralogy data are qualitative and only relatively quantitative. Class criteria and limits reflect the data used to form them.

The classes used for naming mineral map units for the South Region could not represent all significant mineral assemblages in the region. There are some classes of illitic, high iron oxide, and various types of interstratified and interlayered clay mineral systems. These mixed systems include interlayered smectite, mica, vermiculite, dioctahedral chlorite, hydroxy-Al interlayers (partial and full) and kaolinite. No satisfactory assemblage differentia were defined for these mineral assemblages, and aerial distributions are not known.

Mineral class definitions reflect assemblages of minerals and do not correspond to family mineralogy class definitions in Soil Taxonomy (Table 2). The nomenclature selected emphasizes the "assemblage" nature of soil clays. Monosiallitic (1:1 dominated) and bisiallitic (2:1 dominated) classes each have a central concept, the "orthic" subclass. Other subclasses are defined by the way they differ from the central concept.