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Origin and Classification of Coastal Plain Kaolins, Southeastern USA, and the Role of Groundwater and Microbial Action
- Vernon J. Hurst, Sam M. Pickering, Jr.
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- Journal:
- Clays and Clay Minerals / Volume 45 / Issue 2 / April 1997
- Published online by Cambridge University Press:
- 28 February 2024, pp. 274-285
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Along the inner Coastal Plain, kaolinite-metahalloysite-rich, neritic muds of Cretaceous-Eocene age have undergone intense postdepositional alteration in the recharge area of the regional groundwater system. Weathering processes have had the following profound effects on the original sediments: 1) strong compositional and textural modification of both clay and non-clay minerals; 2) whitening of the originally darker sediments by partial removal of organic matter, Fe and Mn; and 3) recrystallization of kaolinite and metahalloysite, most conspicuous where there are coarse stacks and vermiforms. Where the combination of initial sediment composition and alteration intensity was most favorable, these changes have produced important deposits of commercial quality, which now sustain the world's largest kaolin production district. The earliest change was partial sequestration of iron as sulfide and concurrent destruction of some organic matter, mediated by sulfate-reducing bacteria. Subsequent weathering resulted in gradual leaching of alkalies, alkaline earths, iron and silica, and attendant nucleation and growth of minerals compatible with the compositional changes. The existence of several closely spaced erosional unconformities, separated by neritic sediments, is proof that weathering conditions commonly changed at a given site, in response to changes in thickness or lithology of the overlying rocks. Dsyoxic → ← oxic reversals modified both the rate and kind of alteration. (“Dysoxic” refers to molecular oxygen concentration too low to be toxic to anaerobes or cause abiotic oxidation; less extreme than “anoxic”.) Kaolins were produced partly by slower dysoxic weathering in saturated groundwater zones but mainly by more rapid oxic weathering in unsaturated zones, where bauxites also locally formed. Gradual transformation of some sediments to kaolin rarely began and ended in the same epoch. At several places most of the kaolinization (see “Definitions”) took place during Recent time, tens of millions of years after deposition of the sediments. Since the kaolins resulted from postdepositional alteration rather than sedimentary processes, they are better referred to as “Coastal Plain” rather than “sedimentary” kaolins.
Fe-Cycle Bacteria from Industrial Clays Mined in Georgia, USA
- Evgenya S. Shelobolina, Sam M. Pickering, Jr, Derek R. Lovley
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- Journal:
- Clays and Clay Minerals / Volume 53 / Issue 6 / December 2005
- Published online by Cambridge University Press:
- 01 January 2024, pp. 580-586
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Dark Fe oxides and sulfides are major discoloring impurities in mined commercial white kaolin clay. In order to evaluate the potential influence of Fe-cycle bacteria on Fe cycling during post-depositional clay-weathering alteration, Fe(III)-reducing and/or Fe(II)-oxidizing microorganisms were examined in open-pit, subsurface mine samples from kaolin lenses and smectite formations collected from sites in central Georgia. Samples of varying age were examined, including late Eocene smectite overburden, hard kaolin of Middle Eocene age, soft gray kaolin from the late Paleocene, and soft tan kaolin of late Cretaceous age. These clays contained 0.06–5.33% organic carbon, which included various potential organic electron donors for bacterial metabolism: formate (1.1–30.6 mmol/kg), acetate (0–40.5 mmol/kg), lactate (0–12.1 mmol/kg), pyruvate (0.4–78 mmol/kg), oxalate (0–141.7 mmol/kg), and citrate (0–1.4 mmol/kg). All clay samples studied had small concentrations of ‘bio-available’ Fe(III) (0.5 M HCl-extractable Fe, 0.5–2.8 mmol/kg) compared to total Fe (HF-extractable, 25–171.9 mmol/kg). The highest Fe(III)/[Fe(II)+Fe(III)] ratio and the lowest organic carbon content were in kaolin samples in which Fe(III) reduction was determined to be the dominant terminal electron accepting process by hydrogen analysis. All clay samples showed greater numbers of Fe(II)-oxidizing bacteria (22–22,000 cells/g) than Fe(III)-reducing bacteria (3–410 cells/g) as determined by MPN analysis. The Fe(III)-reducing activity in clays could be stimulated with the addition of 1 mM of the Fe(III) chelator, nitrilotriacetic acid. The addition of nitrate stimulated anaerobic Fe(II) oxidation. These results suggest that anaerobic bacteria involved in both oxidation and reduction of Fe exist in these subsurface clay formations, and might have had an influence on post-depositional weathering reactions.