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Infuence of Micaceous Minerals on Mineralogy Class Placement of Loamy and Sandy Soils

Dep. of Plant Science, Univ. of Delaware, Newark, DE 19717-1303

S. W. Buol

Dep. of Soil Science, North Carolina State Univ., Raleigh, NC 27695-7619

* Corresponding author.

ABSTRACT

In Soil Taxonomy, mineralogy classes of pedons that have a fragmental, sandy, sandy-skeletal, loamy, or loamy-skeletal particle-size class are defined by weight percent minerals in the coarse silt and larger fractions of the control section. However, a footnote to the key to mineralogy classes states that percentages by weight are estimated from grain counts of one or two of the dominant separates from a conventional mechanical analysis. The relationship of grain count number to volumetric percentage depends upon grain shape, size and cleavage, and the counting method used, making weight estimations, or even volumetric estimations impracticable. Revisions in the key to mineralogy classes are suggested because presently the key requires determination of grain size and shape relationships as well as the identification of minerals and their densities where often there is a gradation of mineral compositions and properties. According to the estimates used in this study, a line count of 80 to 85% micaceous minerals is needed to classify a pedon as micaceous; however, field classification of micaceous occurred at around 70% line count micaceous grains. This paper discusses limitations of grain counts for quantitative mineralogical analysis of the coarser fine-earth fractions, presents a case for the revision of the key to mineralogy classes through use of actual mineralogic data, and recommends adoption of the line count method of determining composition for the purpose of mineralogy class placement.

Published as Misc. Pap. 1215 of the Delaware Agric. Exp. Stn.

Received for publication February 15, 1988.