dendrites - hand/display specimen of manganese oxide dendrites
Dendrites are black films of manganese oxides that commonly grow on rock surfaces, on bedding planes and in cracks in rock. They are commonly, but incorrectly, believed to be the mineral pyrolusite. They are typically formed of the manganese oxide romanechite or of a hollandite-group manganese oxide, but not of pyrolusite, despite what you read in textbooks. Dendrites typically appear as branching structures that are misidentified as fossils of ferns.
These unusual dendrites formed on the surfaces of a platy basalt. Water, rich in manganese and trapped at the rock surface, normally allows the manganese oxide film to extend and branch into a fractal-like distribution. Since the area where these were collected is arid, many did not progress far into the branching pseudo-fern-fossil structure, but instead formed spots with incipient branches around each nucleation center, making the surface of the basalt plates occasionally look like the spots on a firehouse dog.
These are good as unusual examples that offer the opportunity for discussion of how dendrites form. The Tertiary basalt that hosts these dendrites is part of a dissected volcanic rim, the Doherty Rim, that rises above Guano Valley in Lake County and forms the edge of a plateau that stretches eastward through southeast Oregon just north of the Nevada line.
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