Abstract
Concretions which carry chemical remanent magnetization in sediments in Baja California, Mexico, previously called “hydropsilomelane,” were found to consist of siltstone matrix minerals cemented by a manganese oxide. The X-ray powder pattern, infrared spectra, and chemical analyses indicate that the manganese oxide has the chalcophanite structure. The high concentration of magnesium, presumed to occur in the interlayer position of this mineral, suggests that it is the magnesium analogue of chalcophanite and extends the known range of substitution in chalcophanite-structure minerals.
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