The mineralogy of Pennsylvanian underclays and associated rocks collected from the Appalachian, Illinois, and Mid-Continent basins was determined to see how it varied regionally, stratigraphically, and within underclay profiles.

Underclays are composed principally of kaolinite of a poorly crystallized type, illite, mixed-layer illite-montmorillonite, quartz, and in many places a 14 Å component ranging from vermiculite to chlorite. Mixed-layer material is the only clay mineral common to all underclays. Although the mineralogy of shales from above coals in most cases does not differ greatly from that of underclays, most of the shales contain less kaolinite and mixed-layer clay minerals and more and better-crystallized 14 Å clay material than do the underclays. Shales, slates, sandstones, and calcareous rocks from below coals are mineralogically even more like associated underclays than are the shales from above coals.

Underclays from the central parts of the geosynclinal basins differ from those of the shelf in the same way shales in general differ from their associated underclays. They contain less kaolinite and more 14 A clay, both of which are usually slightly better crystallized, and also contain less mixed-layer clay. Most underclays from parts of the column in which marine beds are common contain abundant mixed-layer montmorillonite. Stratigraphically, lower Pennsylvanian underclays are more kaolinitie than those of the upper Pennsylvanian; the change came earliest in the Mid-Continent Basin and occurs at progressively higher horizons toward the east where the decrease in kaolinite content is less marked.

Field observations show that underclays were formed before deposition of coal-forming material began and therefore cannot be the residual soils on which the coal-forming flora grew. The mineral distribution within underclay profiles also indicates that underclays are not residual soils. Yet, the mineralogy of underclays in general does resemble that of modern soils in several ways. The excess kaolinite and mixed-layer clay is thought to result from soil formation in the source area, and regional and stratigraphic variations in the mineralogy of underclays result from different lengths of time and types of weathering in the various source areas. Slickensides and lack of bedding in underclays are attributed to flocculation of the clay combined with slipping when the hydrous mass was compacted.

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