Authigenic Illite and Organic Matter: The Principal Hosts of Vanadium in the Mecca Quarry Shale at Velpen, Indiana

Donald R. Peacor1, Raymond M. Coveney Jr.2 and Zhao Gengmei1
1 Department of Geological Sciences, The University of Michigan, Ann Arbor, Michigan 48109, USA
2 Department of Geosciences, University of Missouri, Kansas City, Missouri 64110-2499, USA
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Abstract: The Mecca Quarry Shale Member from Velpen, Indiana contains abundant vanadium which occurs in solid solution within illite-rich illite-smectite (I-S) having an average content of 1.65 wt. % V, and an overall composition of K0.8(Al2.8Mg0.5Fe0.4V0.3)(Si7.2Al0.8)O20(OH)4, analogous to the V-rich dioctahedral mica, roscoelite. The illite contains more than twice as much V as the associated kerogen. Detrital mica has a composition typical of 2M1 muscovite and contains no vanadium. The V-rich illite has a structure and composition typical of formation during normal prograde diagenesis and probably is widespread in the Mecca Quarry Shale because the bed is enriched in V throughout the Midwest. The smectite-to-illite reaction can not be a result of passive burial metamorphism because the host strata were buried no deeper than ∼0.5 km at Velpen. The formation of illite occurred in unlithified sediments at shallow depths under the influence of pervasive 80–110°C basinal brines, possibly the same fluids that were responsible for the Mississippi Valley-type lead-zinc mineralization common in the Midwest. The presence of two types of K-rich phyllosilicates may be part of the reason for the lack of correlation between bulk V concentrations and the intensities of X-ray diffraction peaks of illite reported by others.

Key Words: Black Shale • Illite • Mecca Quarry • Organic Matter • Vanadium

Clays and Clay Minerals; June 2000 v. 48; no. 3; p. 311-316; DOI: 10.1346/CCMN.2000.0480301
© 2000, The Clay Minerals Society
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