Maskelynite

from Wikipedia, the free encyclopedia

Maskelynite is a glass with the composition of bytownite , from which it is produced by shock events (collisions, impacts) ( impact metamorphosis ). For a long time, maskelynite was considered a diaplectic glass that was formed directly from plagioclases without melting. However, it lacks some structural features of diaplectic glasses, such as e.g. B. cracks in the starting mineral, so that today it is assumed that high-density plagioclass melts are formed by deterring them.

Under the polarizing microscope , maskelynite appears white with parallel polarizers and black with crossed polarizers because of its amorphous structure. Despite its amorphous structure, maskelynite is considered a variety of bytownite because it retains the crystal form of feldspar and does not flow as a melt like other impact glasses.

It was named after the British mineralogist Mervyn Herbert Nevil Story Maskelyne (grandson of the Royal Astronomer Nevil Maskelyne ). Story Maskelyne was Keeper of Minerals at the British Museum from 1857 to 1880 and described glass-shaped feldspar as one of the first in meteorites .

Maskelynite is found in meteorites, often in the shock veins of chondrites , but also in lunar rocks. The shergottitic Martian meteorites , in which all plagioclase has been converted into maskelynite, contain a particularly large amount . This has to do with the fact that Martian meteorites have to be accelerated to the high escape speed of Mars in order to escape its gravitational pull. For example, the 15 g heavy Martian meteorite Dhofar 378 found on June 17, 2000 in the Omani desert consists of 47% maskelynite, so it must have been exposed to a severe shock event when it was cut off from its mother body.

Maskelynite is formed from anorthite-rich plagioclase at the beginning of the impact metamorphosis at a pressure of up to ~ 29 GPa. When the pressure drops, Tissintit can crystallize at 6–8 GPa and 1350–1000 ° C.

literature

Web links

Individual evidence

  1. ^ M. Chen and A. El Goresy: The nature of "maskelynite" in shocked meteorites: not diaplectic glass but a glass quenched from shock-induced dense melt at high-pressures . In: 62nd Annual Meteoritical Society Meeting . June 1999 (English, lpi.usra.edu [PDF; 15 kB ; accessed on September 30, 2019]).
  2. ↑ Stefan Weiß: The large Lapis mineral directory. All minerals from A - Z and their properties. Status 03/2018 . 7th, completely revised and supplemented edition. Weise, Munich 2018, ISBN 978-3-921656-83-9 .
  3. ↑ MJ Rucksack, ML Whitaker, TD Glotch, JB Parise: Tissintite: An Experimental Investigation into an Impact-Induced, Defective Clinopyroxene . In: Acta Crystallographica . A73, 2017, p. 245 ( journals.iucr.org [PDF; 593 kB ; accessed on September 30, 2019]).
  4. ↑ Melinda J. Rucksack, Matthew L. Whitaker, Timothy D. Glotch, John B. Parise, Steven J. Jaret, Tristan Catalano, and M. Darby Dyar: Making tissintite: Mimicking meteorites in the multi-anvil . In: American Mineralogist . tape 103 , 2018, p. 1516–1519 ( sunysb.edu [PDF; 1.1 MB ; accessed on January 16, 2019]).