Mineralogical Transformations during Weathering of Lignite Overburden in East Texas

A. L. Senkayi, J. B. Dixon, L. R. Hossner and B. E. Viani1
Department of Soil & Crop Sciences, Texas Agricultural Experiment Station, Texas A&M University, College Station, Texas 77843
1 Present address: Department of Agronomy, Purdue University, West Lafayette, Indiana 47907.

Abstract: Mineralogical weathering sequences in sediments overlying lignite beds were investigated in a core (27 m deep) from Calvert, Robertson County, northeast Texas. Weathering trends were evaluated based on the properties and relative distributions of both the expandable and non-expandable minerals. The sulfide minerals in these sediments are the most susceptible to weathering and were only observed in the unaltered (reduced) zone below 7 m. Oxidation of the sulfides has resulted in the formation of jarosite and gypsum in the upper 7 m of the core (oxidized zone). The oxidized zone is further characterized by reddish brown colors (high chroma), a greater quantity of dithionite-extractable iron, and absence of chlorite. Although the major clay minerals in these sediments (smectite, kaolinite, mica) are largely detrital, weathering has resulted in an increase in the content of kaolinite and a decrease in the content of mica towards the surface. The mica appears to have altered to a high-charge smectite characterized by basal spacings of 32Å after intercalation with octadecylammonium cations. The high-charge smectite is most abundant in the soil horizons at the top of the core and gradually decreases with depth. A low-charge smectite is the most abundant species in the unweathered parent sediments and increases with depth.

Key Words: Lignite • Oxidation • Reclamation • Smectite • Soil • Weathering

Clays and Clay Minerals; February 1983 v. 31; no. 1; p. 49-56; DOI: 10.1346/CCMN.1983.0310108
© 1983, The Clay Minerals Society
Clay Minerals Society (www.clays.org)