Pyrolusite is a manganese oxide mineral, MnO2. It is the most common of the dozens of known manganese oxide minerals. They usually have a silvery-black luster. Like uranium, manganese is soluble in acids and insoluble in bases. Surface water and streams (pH = ~5 to 7) tend to dissolve manganese, but in ocean water (pH = ~7.5 to 8.4), it precipitates out. Analysis is often needed to accurately identify what species of Mn-oxide is present. This specimen is labeled as pyrolusite, but I can't confirm that.
Locality: unrecorded/undisclosed site in Nuevo León State, northeastern Mexico
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