Abstract

Lithiophorite and cryptomelane are present in samples taken from five different epigenetic occurrences in which the manganese oxides occur as open space fillings and replacements in a slumped and brecciated chert residuum of Fort Payne Formation from which all of the original carbonate fraction has been dissolved. Texturally the oxides occur in diffusely banded aggregates, tightly laminated nodules, and massive porous mixtures with goethite. Lithiophorite is texturally diverse, ranging from cryptocrystalline to large radially disposed crystals. Cryptomelane occurs as very fine-grained to cryptocrystalline masses, bands or sheaf-like aggregates.Both lithiophorite and cryptomelane vary greatly in their content of cobalt and nickel with lithiophorite containing as much as 4.5% Co and 1% Ni and cryptomelane as much as 0.6% Co and 2% Ni. Fine-grained to cryptocrystalline varieties of lithiophorite and cryptomelane generally contain greater amounts of Co and Ni than do their more coarse-grained equivalents. In sections containing both minerals, lithiophorite is rich in cobalt and relatively poor in nickel whereas cryptomelane is rich in nickel and relatively poor in cobalt. If cryptomelane is absent, lithiophorite commonly contains appreciable amounts of both elements.It is proposed that the manganese oxides formed during the weathering and solution removal of the carbonate fraction of the Fort Payne Formation-concomitent with slumping and fracturing of the chert layers. Manganese oxides, possibly in the form of gels, began to nucleate in open spaces and Co and Ni cations were adsorbed into the growing aggregates. Crystallization, with at least a partial lattice site substitution of Co (super +3) for Mn (super +3) in the resulting lithiophorite, produced the very fine-grained textural varieties of both lithiophorite and cryptomelane in which the Co and Ni content is highest. The coarsely crystalline lithiophorite which contains less Co and Ni may represent precipitation and crystallization from solution without an early gel stage which would have served as an accumulating means for foreign cations, or may be the result of the crystallization of gels in which the conditions of formation allowed a more complete expulsion of Co and Ni. Although at the present time it cannot be proven, the most feasible immediate source for the cobalt and nickel now present in the manganese oxides is the limestone fraction of the Fort Payne Formation.

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