Rinds, Coatings, and Crusts

Many Earth rocks have crusts, but not fusion crusts

The photos below were all sent to me by persons who thought that the coatings were meteorite be fusion crusts.

Meteorite fusion crusts are thin because as soon as the exterior of the meteorite melts, the liquid is sloughed off due to of the high velocity of travel of the meteoroid through the atmosphere. The fusion crust does not build up, except perhaps on the trailing side. Meteorite fusion crusts are rarely greater than 1-2 mm thick.

Terrestrial processes, most notably weathering, chemical deposition, and desert varnish, can produce rinds, coatings, and crusts on rocks that are often mistaken for meteorite fusion crusts. Typically, however, such rinds are thicker than meteorite fusion crusts. Sometimes these coatings are shiny like meteorite fusion crusts. Meteorite fusion crusts are shiny because they are glassy. None of the coatings in the photos below are glassy.

Hematite Coatings

Iron dissolved in water will precipitate onto rocks often forming thick coatings of hematite or iron oxyhydroxides. The coatings may resemble meteorite fusions crusts but are often much thicker. I have not actually seen any of the rocks in the photos, but I think the coatings are all chemical deposits of hematite. Hematite coatings often flake off.

Onion-Skin Weathering

Another process that leads to features that are often mistaken for meteorite fusion crusts is onion-skin weathering or spheroidal weathering.  Again, the layers on the rocks in the photos below are much too thick to be meteorite fusion crusts. Also, they are not glassy. Fusion crusts do not flake off like the exterior layers of rocks in the photos below.

Chemical Weathering

Over thousands of years the exterior of any crystalline rock will interact with water and air to form rinds of a different color.

Industrial Coatings

See ferromanganese.