Manganese Oxides in Nature

Our inquiry begins in the desert southwest where we observe petroglyphs carved through a layer of desert varnish to expose lighter colored rock beneath.  The black varnish is enriched in manganese.


Desert Varnish is also found coating boulders in various stages of disintegration.

The varnish appears to be reforming after pieces of the surface rock fall off.


Thunderstorms bring down varnished rock in canyons in the southern end of Death Valley.


A closer view of the alluvial fan.  Several generations of varnish are present.  Older, darker areas are covered by younger landslides.


Close up of the varnish surface as viewed under a low power microscope.  No crystals of manganese oxides can be identified.


Scanning electron microscope image, about 0.1 mm across of the rock - varnish interface.  Still no crystals are recognizable.


This is a thin section (about 30 micrometers thick) of the varnish and adjacent rock.  It is viewed under an optical microscope.



Thin section under crossed polarizers.  This brings out the texture of the host rock where you can see individual feldspar crystals. Isotropic materials, such as the glass microscope slide to which the thin section is attached, appear black under crossed polarizers.


Higher power view under the optical microscope.  Here you can see the layering of the varnish due to clays mixed in with the manganese (and iron) oxides.


Scanning electron microscope image of the contact between a rock and its desert varnish.  The layering in the varnish can be seen due to the development of cracks in the vacuum of the electron microscope.  Most of the varnish is clay minerals.  The manganese oxide is mixed in with the clays.  Our problem is to identify which manganese oxide phase is in the varnish.


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