Micrit

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Filigree structures, such as the individual branches of the flag of a bird's feather , can only be handed down in fossil form in very fine-grained rock. The feather shown here comes from Archeopteryx and has been handed down in the Solnhofen limestone , an archetypal micrite (mudstone) from the Upper Jurassic ( Tithonium ) of southern Germany.

Micrite describes solidified lime sludge, i.e. the proportion of carbonate grains with grain sizes <0.063 mm, which often form the matrix of a limestone . The crystal faces of the micrite grains are not formed and they are not even microscopically soluble in the matrix. Micrite and fine sparite therefore do not shine or glitter in the handpiece and only coarse sparite has a crystalline glitter. Structureless limestones can be macroscopically so fine-grained that they appear so dense and homogeneous and act as a matt mass. This rock is then referred to as calculus .

On the basis of the limestone classification by Folk (1962), limestones are classified as micritic if their base material consists predominantly of matrix and not of cement . The rock designation of a micritic limestone depends on the type of coarser carbonate components that are surrounded by the matrix: those that predominantly contain clasts are called intramicrites . Oomicrites , pelmicrites, and oncomicrites . In addition to micrit, they mainly contain ooids , peloids or onkoids . In Biomikriten dominate biogenic components such as shells . The Rügen writing chalk , which is mainly formed from coccoliths , is also considered a biomicritical sediment .

literature

  • Roland Vinx: Rock determination in the field. 2nd edition 2008. Springer-Verlag. Berlin Heidelberg. ISBN 978-3-8274-1925-5
  • Folk, RL, 1959, Practical petrographic classification of limestones: American Association of Petroleum Geologists Bulletin, v. 43, pp. 1-38.
  • Folk, RL, 1962, Spectral subdivision of limestone types, in Ham, WE, ed., Classification of Carbonate Rocks-A Symposium: American Association of Petroleum Geologists Memoir 1, pp. 62-84

Web link

Individual proof

  1. ^ Vinx: Terrain determination. P. 308.