Buchit

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Buchit in the microscope
Contact Basalt-Buntsandstein vom Wildenstein b. Büdingen (thin section, LPL)
Under crossed polarizing filters (XPL): glassy parts in the former sandstone remain dark

Buchit is the name for a rock that has emerged from pyrometamorphosis or combustion metamorphosis and contains more than 20% glass. The name honors the German geologist Leopold von Buch , who among other things gained importance through his research on volcanism.

Historical development of the name

The term was used in the literature for the first time by the German geologist August von Klipstein , who however only referred to "sandstones transformed by volcanic action", thus choosing a much narrower definition. As a type locality , he named the Wildenstein near Büdingen in the Wetteraukreis in the southwestern foreland of the Vogelsberg . At this location, the rock was created by the heat effect of the originally melted rock of a basalt chimney on the surrounding red sandstone (pyrometamorphosis). In the more recent literature, the Hessian geologist Heinrich Möhl is often incorrectly mentioned as the first person to describe it. He described the corresponding "annealed sandstones" from the Otzberg in the Odenwald and published the first thin section of a book of these rocks .

This definition was subsequently expanded to include other sedimentary rocks besides sandstone (such as mudstone or phyllite), which had undergone pyrometamorphic influence.

The current definition comes from the recommendations of the International Union of Geological Sciences , which on the one hand expanded the term by no longer focusing on pyrometamorphosis as a formation mechanism, but can now also refer to products of combustion metamorphosis. In the past, these were often given special names (such as porcelain or porcelain jasper ), which should no longer be used according to these recommendations. In addition, according to the current definition, artificial products can also be designated with this term if they meet the criteria. On the other hand, a previously unused limitation was introduced by specifying the content of the glass component. A corresponding rock which contains less than 20% glass is referred to as fritted rock according to these recommendations ; the term "fritted" for macroscopic description can also be found in Klipstein.

In the past, the term was often used very inconsistently. Correspondingly, older literature references should be approached with caution.

Education and occurrence

Buchites are typical rocks of the metamorphic sanidinite facies , i. that is, that they were formed at high temperatures (above 800 ° C) but at normal pressure or only slightly elevated pressures. Such conditions prevail, for example, at the direct contact of a volcanic vent with the adjacent rock, but also in the adjacent rock of burning coal seams. This secondary rock must also contain enough quartz or silicate minerals to be able to form a glass phase; pure limestones are not able to do this. At the same time, the temperature must be high enough for a sufficient period of time to form a corresponding proportion of melt (glass). This is often only the case in the immediate vicinity of the heat source, for example directly at the contact or in loose rocks that are carried along in the magma. The occurrences are therefore often quite small. Locations already mentioned by Möhl are, for example, the Blaue Kuppe near Eschwege, the Bühl near Kassel, the Beilstein in Spessart or the Stoppelsberg in the Rhön . Corresponding rocks can also be found in the volcanic areas of the Eifel , mostly as xenolites in the local volcanic rocks .

Appearance in the field and response

Column-shaped separation of red sandstone from Wildenstein near Büdingen, caused by thermal stress.

Due to their high glass content, Buchites often have a shell-like break in the handpiece. Depending on the original rock (protolith), they can also have very different looks: Both slag-like and dense rock has been described. The colors are not uniform and range from bleached white to red to black, the latter mostly with a higher glass content. Column- like formations (similar to columnar basalt , but mostly with a smaller diameter of the columns) can sometimes be found directly at the contact .

Since the distinction between fritted rocks requires a quantitative determination of the glass content, which is hardly possible without an examination in thin sections , a reliable approach in the field is problematic.

Metamorphic rocks with a high glass content can also arise through other mechanisms, such as dislocation metamorphism ( pseudotachylite ) or impact metamorphism ( suevite , impactite , tektite ). Reliable differentiation here requires the geological environment to be taken into account and can be difficult in the handpiece.

Mineral inventory

In addition to unchanged constituents of the original rock (in sandstone often round fused quartz grains) and the glass required by definition, the rock often contains typical high-temperature mineral phases such as spinels , cordierite , mullite , tridymite and magnetite . The crystals of these newly formed phases are often very small. Here, too, the specific composition depends on the nature of the original rock.

Individual evidence

  1. a b D. Fettes, J. Desmons (Ed.): Metamorphic Rocks. University Press, Cambridge, 2011, p. 136
  2. A. v. Klipstein: About vulcanized sandstones in the Vogelsgebirge . In: Hertha magazine for geography, ethnology and national studies . tape 10 . Stuttgart 1827, p. 354-368 .
  3. ^ H. Möhl: The southwestern foothills of the Vogelsgebirge. In: Report of the Offenbach Association for Natural History. No. 14, 1873, pp. 51-101
  4. SI Tomkeieff: Dictionary of Petrology. Wiley, Chichester, 1983, p. 79
  5. R. Vinx: Rock determination in the field . 3. Edition. Spectrum, Heidelberg 2011, ISBN 978-3-8274-2748-9 , p. 378 .
  6. R. Vinx: Rock determination in the field . 3. Edition. Spectrum, Heidelberg 2011, ISBN 978-3-8274-2748-9 , p. 358 .
  7. S. Koritning: The "Blue Dome " near Eschwege. In: The opening. Special volume 28, 1978, pp. 237-247
  8. H. Locker: Real Gemstones? Volcanic glasses from the Eifel . In: Lapis . No. 9 , 2009, p. 13-19 .
  9. a b R. Vinx: Rock determination in the field . 3. Edition. Spectrum, Heidelberg 2011, p. 429-434 .
  10. D. Fettes, J. Desmons (Ed.): Metamorphic Rocks. University Press, Cambridge, 2011, p. 189