ALUMINUM SULFATE AND PHOSPHATE MINERALS PRODUCED BY ACID WEATHERING OF CRETACEOUS PYRITIFEROUS SHALES IN NEBRASKA, USA

The weathering of Upper Cretaceous pyritiferous shales and mudstones in eastern and south-central Nebraska has recently been recognized as a mechanism by which comparatively unusual mineral species (e.g., hydrated Al and Fe sulfate salts), acidity, and sulfate-enriched groundwater are generated in specific geographic, stratigraphic, and environmental settings. The recent discovery of two new localities in the state sheds additional light on acid weathering and the production of aluminous minerals in a subhumid, mid-latitude, continental setting.

At a site in Cedar County Nebraska, the uppermost 6-16 cm of the Carlile Shale, immediately below the contact with the Niobrara Formation, are weathered from dark gray, fissile, pyritiferous shale to a strong brown, earthy alteration product containing abundant gypsum crystals. Approximately 20% of this weathered material consists of 5-40 mm, white, powdery masses of aluminite (Al₂[SO₄][OH]₄ • 7[H₂O]). Jarosite (KFe₃ [SO₄][OH]₆) appears below altered zone as fracture coatings in relatively unweathered shale. At a second site in Harlan County, Nebraska, weathering through at least 3 m of the lower Pierre Shale has produced similar powdery masses of aluminite, basaluminite (Al₄[[SO₄][OH]₁₀ • 5[H₂O] ), and alunite (KAl₃ [SO₄]₂[OH]₆). Jarosite also appears as fracture fillings and horizontal seams (in association with gypsum); vashegyite (Al₆[PO₄]₅[OH]₃ • 23[H₂O]) appears as sparse nodules within a specific horizon in the weathered zone. Weathering products at the two sites produce pHs of 3.4-4.7 in 1:1 suspensions in DI water.

Acid weathering of clay minerals during pyrite oxidation in both the host shales and in bentonite seams (Harlan County) has produced aluminous minerals. Both sites appear to have been associated with ongoing, or at least pre-recent-drought, groundwater seepage at contacts marked by major lithostratigraphic and geohydrologic changes. Acidophilic bacteria may have accelerated pyrite oxidation. The observations made in this study are applicable to the long-term, regional weathering of widespread pyritiferous shales and its effects on groundwater chemistry and evolution.