Psilomelane

Psilomelane (sigh-LOW-melane) is a catchall name for hard, black manganese oxides that form crusts like this in various geologic settings. Psilomelane has no precise chemical formula, being a mix of different compounds, but it's approximately (H₂O)₂Mn₅O₁₀. It has a Mohs hardness around 6, a blackish streak, and commonly a botryoidal habit as shown along the bottom of this photo. It also adopts a dendritic habit, making up the fossil-like forms called dendrites.

This specimen is from the Marin Headlands north of San Francisco, where deep-sea chert is widely exposed. (Because the locality is in the National Park system, I left it where I found it.) It is likely that this former seafloor had at least a sprinkling of manganese nodules on it. If those compounds were mobilized during these rocks' travels in the ancient California subduction zone, this crust would be the result.

Manganese oxides are also a major ingredient in desert varnish.

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