Browsing by Subject "kaolinitic clay"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Guidebook 19. Field Trip Guidebook Kaolinitic Clays of the Minnesota River Valley and Southwestern Minnesota
    (Minnesota Geological Survey, 1992) Setterholm, Dale R.; Heine, John J.
    A thick mantle of chemical weathering products covers crystalline bedrock of Archean and Proterozoic age in much of central and western Minnesota. The mantle has a vertical profile typically composed of progressively more weathered material upward from fresh bedrock to an erosion surface now covered either by strata of Late Cretaceous age or by glacial and postglacial deposits of Quaternary age. In a pioneering study of regional relationships, Parham (1970) showed that most upland areas underlain by crystalline bedrock in western Minnesota have weathering profiles averaging 30 m in thickness, though profiles as great as 60 m thick may be present locally. Weathered materials are considerably thinner on steep side slopes and absent in many places along valleys cut into the bedrock surface. Nonetheless, their widespread occurrence has created considerable commercial interest. Much of our knowledge of the processes responsible for the origin of the weathering profile in southwestern Minnesota was established by S.S. Goldich in a 1936 University of Minnesota doctoral dissertation entitled "A study in rock weathering" (Goldich, 1938). Six samples of saprolite from the Redwood Falls-Morton vicinity in the Minnesota River Valley were compared chemically and mineralogically with fresh Archean gneiss, their probable parent material. Using these data, together with data from elsewhere, Goldich established an alteration sequence in which calcium-rich plagioclase was transformed to clay, followed by the dissolution of the potassium-rich feldspar and other mafic minerals. These observations are embodied in the so-called Goldich stability series, which in one form or another is taught to every beginning student in geology.
  • Thumbnail Image
    Item
    Information Circular 27. Minnesota Kaolin Clay Deposits: A Subsurface Study in Selected Areas of Southwestern and East-Central Minnesota
    (Minnesota Geological Survey, 1989) Setterholm, D.R.; Morey, G.B.; Boerboom, T.J.; Lamons, R.C.
    Large deposits of kaolin-the residual products of the weathering of igneous and metamorphic rocks-are common in southwestern and east-central Minnesota. Thin and discontinuous beds of sedimentary kaolin (resulting from the reworking of the residuum) are less common and volumetrically less significant. The weathering occurred prior to deposition of Late Cretaceous sedimentary strata. Because climatic conditions were uniform within the study area, the differences in the thickness and composition of the residuum are attributable to the mineralogic and hydrologic properties of the parent rock (protolith). Goldich (1938) and others have shown that weathering and the products of weathering are strongly controlled by the mineralogic composition of the protolith. Rocks rich in mafic minerals and plagioclase are generally more extensively weathered, and their residuum is richer in kaolinite. This susceptibility to weathering is evident in the mineralogic composition of the residuum and is reflected in the concentration of kaolin near the top, and in the addition of other minerals with increasing depth. The weathering process is also dependent on the movement of water and other fluids; therefore, permeability is a second important control on the production of kaolin. In igneous and metamorphic rocks, fractures in the form of joints or faults are the primary paths of fluid movement. Weathering follows these paths, and the resulting kaolin deposits mimic the shape and orientation of the structural features. Preservation is the final factor controlling the distribution of kaolin clays. The upper part of the residuum contains the largest amount of clay, but it also is the most vulnerable to erosion. Glacial erosion has had a profound effect on the distribution and thickness of kaolin clay deposits in the state. The thickest deposits are found where Late Cretaceous sediments overlie the kaolin and indicate total preservation of the weathered products.