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Book: Acid Sulfate Weathering
Published by: Soil Science Society of America

 

This chapter in ACID SULFATE WEATHERING

  1.  p. 169-191
    SSSA Special Publication 10.
    Acid Sulfate Weathering

    J.A. Kittrick, D.S. Fanning and L.R. Hossner (ed.)

    ISBN: 978-0-89118-905-3

     

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doi:10.2136/sssaspecpub10.c10

Mineralogical Properties of Lignite Overburden as they Relate to Mine Spoil Reclamation1

  1. J. B. Dixon,
  2. L. R. Hossner,
  3. A. L. Senkayi and
  4. K. Egashira2

Abstract

The mineralogical properties of lignite overburdens differ significantly between the oxidized zone and the reduced zone. Reclamation of lignite mine spoil generally involves earthy materials that overlie the lignite. Hard rock types are exceptional. These overburden materials are sometimes capped by good soils that need to be reinstalled as illustrated by the loessial soils of West Germany. On the other hand reclamation may utilize mixed overburdens as is being done in the coastal plain deposits of eastern Texas where the soils may have poor physical and chemical properties and topsoiling is not practiced. In East Texas fresh overburden materials are left on the surface thus exposing carbonates, sulfides, and labile silicates to immediate weathering. The thickness of the weathered zone is about 5 to 20 m depending on the texture of the material and the depth to impermeable strata. Clayey or lignite strata impede penetration of air and water and thus the weathering front.

Iron sulfides and Fe-chlorite are generally absent from the oxidized zone. Labile iron compounds have weathered to iron oxides. Chlorite usually occurs in the unoxidized overburdens analyzed in eastern Texas. Pyrite is present in these deposits but occurs in local concentrations in varying amounts. Feldspars appear to have partially weathered out of the upper oxidized zone based on preliminary data. Feldspars occur mostly in the silt and sand fractions. Mica of the muscovite type is present in modest amounts throughout the overburden section. It is relatively resistant to weathering and persists to some extent to the soil surface. Smectite and kaolinite are abundant in the clay fraction throughout the overburden section except near the soil surface where much of the smectite has been removed. These conclusions are based on data from only a few locations in East Texas and require reevaluation throughout the lignite belt to account for possible variation along the strike of the beds and under different climatic conditions.

Carbonates occur infrequently in materials that overlie Wilcox lignite of East Texas. Thus they are not a reliable indicator of weathering. Calcite has been identified in a few layers. Siderite (FeCO3) is important in local concentrations and weathers to goethite coated rocks that persist in exposed land surfaces.

Jarosite and gypsum are the most common sulfate minerals associated with lignite overburden of East Texas. Both minerals form on the exterior of overburden cores when they are allowed to oxidize and dry in storage. These localized fresh efflorescences suggest the rapid weathering of pyrite. Also, gypsum forms as a widespread white powder on dry shale surfaces suggesting that it formed as the interstitial solution evaporated in the absence of localized pyrite. Melanterite and szmolnokite also form on samples of lignite overburden from East Texas.

The discussions of mineralogical properties of lignite overburden presented here and the experience with reclaiming lignite overburdens in the United States and abroad reflect a favorable outlook for the continuation of such practices. However, it must be recognized that the extent of these experiences has been limited to a few locations and a short interval of time. In the East Texas lignite mining area many of the native upland soils, presumably formed from similar parent material to lignite overburdens, are infertile and difficult to manage. Thus continued investigations are needed of the overburden composition at different locations. Appropriate revegetation practices and long term management practices are needed to assure successful reclamation and return of the land to agricultural use.

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Copyright © 1982. Copyright 1982 by the Soil Science Society of America, Inc., 5585 Guilford Rd., Madison, WI 53711 USA