Blastesis

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Fruit slate with cordierite porphyroblasts that have grown during contact metamorphosis .

Blastesis (from Greek βλάστησις blástēsis , the germination ') or crystal loblastesis refers to the sprouting of new minerals. In the narrower sense, crystal loblastic structures are structures that arise from recrystallization during metamorphosis , i.e. while the rock is remodeled by pressure and heat, but remains in a solid state. The term "blastesis" was introduced in this sense in 1903 by Friedrich Becke . In a broader sense, the term was later used for the growth of feldspar crystals in granite , i.e. in a plutonitic rock, as well as for the formation of crystals in rocks of any genesis . In the middle of the 20th century, the terms endoblastesis and metablastesis were introduced in order to differentiate the magmatic and metasomatic crystal formation processes from the metamorphic ones. However, the exact reconstruction of the genesis of the crystals is not easily possible in all cases.

Differentiation of crystal loblastic structures

In order to differentiate and classify the grain structure of metamorphic rocks, various prefixes of Latin or Greek origin were added to the word component "blastic". First of all, a distinction is made between uniformly grained ( homeoblastic ) and non- uniformly grained (heteroblastic) structures.

Homeoblastic structure

A distinction is made between the same-grain structures:

  • granoblastic : All crystal loblasts have a granular shape and there is no preferred direction of growth.
  • lepidoblastic : flake-form minerals predominate, which is typical for Phyllite .
  • nematoblastic : The minerals are in a stem- like, long prismatic shape, which is the case with many amphibolites.
  • fibroblastic The minarals are arranged in the form of fibers, which is relatively rare, an example is fiber asbestos .

Heteroblastic structure

A distinction is made between non-uniform grain structures:

  • porphyroblastic : The crystals are significantly larger than the surrounding minerals of the matrix . Usually the minerals of these crystal loblasts belong to one type of mineral. This form is the most important and most common heteroblastic structure. Such a structure is similar to the porphyry structure of magmatites.
  • idioblastic : The minerals have realized an ideal crystal form, which in metamorphic rocks indicates a late growth. The idioblasts are limited by crystallographic growth areas.
  • xenoblastic : This is the opposite of idioblastic, that is, xenoblastic are not limited by crystallographic growth areas.

Individual evidence

  1. Ulrich Sebastian: Geology. A guide for beginners and users. 4th edition, Berlin 2018, ISBN 978-3-662-55322-0 , p. 65.
  2. a b c d R. Variation: Petrology of the Metamorphites. FU Berlin, 2007 ( online ).
  3. a b c Stylianos Augustithis: Atlas of the textural patterns of Ore Minerals and metallogenic Processes. de Gruyter, Berlin / New York, 1995, ISBN 3-11-013639-2 , pp. 64–66 ( Google books )
  4. Friedrich Becke: About mineral inventory and structure of crystalline slate. In: Memoranda of the Imperial Academy of Sciences. Vienna 1913.
  5. Friedrich Drescher-Kaden: The feldspar-quartz reaction structure of granite and gneiss and their genetic significance. Springer-Verlag, Berlin / Göttingen / Heidelberg 1948.
  6. Christof Exner: Mikroklinporphyroblaststen with helicite inclusion trains near Badgastein. In: Festival edition for the 50th anniversary of the Vienna Mineralogical Society (since 1946 Austrian). Pp. 111-130, Springer-Verlag, Vienna 1951 ( Google books ).
  7. a b Siegfried Matthes: Mineralogie. Springer Verlag, Berlin / Heidelberg 1993, ISBN 978-3-540-99507-4 , pp. 381-388 ( Google books ).
  8. a b Freie Universität Berlin: Crystal loblastic structure. In: PETROgraph, learning portal for the earth1 mineral and rock determination internship. Retrieved December 25, 2018.