DESERT VARNISH AND CALICHE COATINGS
Desert varnish or desert patina is a thin and shiny dark brown to black patina that forms on surfaces in arid and semi-arid environments and is mainly composed of clay minerals. The latter comprise more than 70 percent of the varnish, with silica being the most important mineral. Iron and manganese oxides make up the bulk of the remainder and are dispersed evenly throughout the clay layer. Desert varnish is recognized by a lack of texture and its semi-opaque smoothness and luster. Usually on meteorites its thickness is less than 0.25 mm
If the meteorite shows a brown, grey or light coating that is dissolvable in acid, the fusion crust has been replaced by, or enriched with, clay minerals or caliche. Caliche consists of layers of a hardened calcium carbonate deposit that forms through minerals leached from the upper layer of the soil and which is adhering to contacting surfaces.
With individuals there is often no cut surface available to compare matrix and exterior coating. But almost all meteorites that experienced some degree of weathering show damaged surfaces which are recognizable by a rougher texture than the surfaces resulting from ablation. If there is no difference in color between these and the latter surfaces, this is a sign that sand polishing and the formation of desert varnish is already in a progressed state.
And if neither any textural remains nor a compositional difference between coating and interior can be determined, the specimen is likely weathered beyond the substrate and no fusion crust remains here, either. These meteorites often show complete replacement of troilite by iron sulfate and complete dissolution of all other primary metal. The weathering grade of such material would be, at least, W4.
Even if there may still be remains of the substrate of a fusion crust hidden under the highly-oxidized shale or weathering layer, there is no way to call such meteorite “fusion crusted”. If the outer visible coating of a meteorite is a rind of terrestrial weathering products, one should consider exactly this when describing a meteorite.