micrite envelope

A dark-coloured, fine-grained, carbonate exterior found around carbonate skeletal fragments. It is produced by the alteration of the original mineralogy of the fragment to micrite, commonly caused by the boring into the skeletal material of certain algae. In some micrite envelopes the tiny, tube-like borings of the algae are preserved. Some micrite envelopes are formed by coatings of filamentous algae over the grain. If the development of the micrite envelope proceeds to an advanced stage, the primary structure of the skeletal fragment will be totally replaced by micrite, leaving an amorphous peloid, or lump (see intraclast).