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Counterfeit manganese deposits can take various forms.

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Desert varnish is a dark coating on rocks found in arid regions.

The coating is composed dominantly of fine-grained clay minerals.

It looks like a coating of manganese oxide, but contains no manganese at all.

This is a fungal dendritic growth from sandstone from Meteor Crater, Arizona which also looks like a manganese deposit to the casual observer.

The sample was prepared by fracturing a thin slab of a varnish-coated rock.

The exposed surface filling most of the frame is a fracture through the varnish layer.

Here you can see how the entire underside of the rock is coated with the red coating Desert varnish is dominantly clay.

The clay minerals represent the clays found locally in the region where the varnish develops.

Chemical compositions of rock varnish from the Negev Desert of Israel and local settled dust were used to constrain the mechanisms of varnish formation and patterns of Mn enrichment and accumulation in the varnish.

Rock varnish was sampled from coeval, undisturbed prehistoric flint artifacts along a south–north climatic transect (∼30–120 mm/yr of rain).

In the clay layer, black manganese oxide (the mineral birnesite) and red iron oxide (the mineral hematite) add color.

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