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Andrew Alden

A Sighting of Stilpnomelane

By August 20, 2013

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stilpnomelaneA year ago I paid a visit to some wonderful blueschist-facies rocks at the Laytonville quarry. Rocks like these have minerals rarely seen elsewhere lurking among the common ones. A band of pinkish gneiss contained these arresting rosettes of the iron-bearing mineral stilpnomelane (stilp-NO-melane). It's a phyllosilicate mineral related to the micas, but with extra iron in both divalent and trivalent states.

This was an outcrop that, in accordance with my standard field practice, I left alone. But there was plenty of broken rock lying around from which I collected a piece of highly schistose rock with stilpnomelane layers shining like wrinkled dinosaur skin. Every field trip with a new mineral is an especially good one, but geologizing is like fishing—the worst day doing it beats the best day of working.
Stilpnomelane — Geology Guide photo

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