Opacitization

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Opacit in the microscope
Fully opacitized hornblende crystal (thin section, LPL, hornblende analcimite, Vogelsberg): Rhönite is dark brown, augite light brown, plagioclase clear and magnetite black
Completely opaque hornblende crystal (thin section, XPL): Rhönite is dark brown, augite blue, plagioclase white and magnetite black

As Opacitisierung (also: Opazitisierung ) is in the Petrologie a conversion reaction of magmatic formed, iron, magnesium and hydroxide-containing minerals ( biotite , basaltic hornblende ) in a Opacit named, symplektitisches referred mineral mixture. The term “opacite” is not a recognized mineral name, but is often used in microscopic petrography as a descriptive name for the reaction product.

background

If there is a high water vapor partial pressure in a magma , which is often the case at greater depths depending on the composition (due to the lithostatic pressure prevailing there), mineral phases can crystallize out of it, which are bound in their crystal lattice in the form of hydroxide ions (OH - ) Have water (biotite, hornblende). If this magma rises to shallower depths, where there is a lower lithostatic pressure, and if this rise happens so slowly that an equilibrium is possible, these mineral phases react to the now reduced water vapor partial pressure, in that water is released from the hydroxide ions and new, now anhydrous minerals are formed. If such magma reaches the surface of the earth as lava , the original mineral phases can no longer be detected or can only be detected in relics. Sometimes this decay is limited to the outer surfaces of the crystal, so that seams of decay products form. If, on the other hand, the ascent of the magma takes place quickly (often with the eruption of pyroclastics ), there is no time for equilibrium to be established, and the hydroxide-containing minerals are found unchanged in the products of such eruptions ( tephra , ignimbrite ).

mineralogy

Relic hornblende surrounded by Rhönite, Augite and Plagioclase. Thin section, XPL.

The exact composition of the mineral mixture formed during opacitization depends on the composition of the starting minerals as well as the other conditions that prevail in the original magma. Common components of the conversion product are magnetite , clinopyroxene ( augite ), rhoenite , plagioclase , olivine and volcanic glass .

The outline of the original crystal can be retained during the process. When this occurs, the newly formed mineral phases (especially Rhönite) are often oriented along the crystallographic axes of this original crystal.

The phenomenon was already apparent in the early days of petrographic microscopy. For example, a picture of a hornblende crystal with an opaque rim can already be found by Ferdinand Zirkel , and a detailed description of the phenomenon is given in a work on the basalts of the Rhön from 1883, with particular emphasis on the consideration of the mineral that was not yet recognized as an independent mineral at the time Rhönits is laid.

Occurrence

The phenomenon is linked to volcanic rocks and naturally occurs where on the one hand the magma of origin contained enough water to be able to form hydroxide-containing minerals, but on the other hand this water content was insufficient to maintain the partial pressure of water vapor during the ascent in the earth's crust, that the stability range of these minerals is not left. The occurrence of opacitization in basalts or andesites containing hornblende is often described .

literature

  • Walter Ehrenreich Tröger: Optical determination of the rock-forming minerals . 2nd Edition. tape 2 . Schweitzerbart, Stuttgart 1969, p. 465-468 .
  • Hans Pichler, Cornelia Schmitt-Riegraf: Rock -forming minerals in thin sections . 2nd Edition. Enke, Stuttgart 1993, ISBN 3-8274-1260-9 , pp. 102-103, 114-115 .
  • Hans-Ulrich Schmincke: Vulcanism . 4th edition. Primusverlag, 2013, ISBN 978-3-86312-367-3 .
  • David Shelley: Igneous and Metamorphic Rocks under the Microscope . Chapman & Hall, London 1993, ISBN 0-412-44200-0 , pp. 192-194 .

Individual evidence

  1. ↑ SI Tomkeieff: Dictionary of Petrology. Wiley, Chichester, 1983, p. 398
  2. ↑ Ferdinand Zirkel: The microscopic nature of minerals and rocks . Wilhelm Engelmann, Leipzig 1873, p. 171 .
  3. ^ Karl Petzold: Petrographic studies on basalt rocks of the Rhön . Gebauer-Schwetschke'sche Buchdruckerei, Halle a. P. 1883, p. 25-30 .
  4. ↑ Hessisches Landesamt für Bodenforschung (Hrsg.): Explanations for the Geological Map 1: 25000 of Hessen, sheet 5621 Wenings . Wiesbaden 1988, p. 65-67 .
  5. ↑ W. Maresch, H.-P. Schertl, O. Medenbach: Rocks. Systematics, determination, origin. 2nd Edition. Schweizerbart, Stuttgart 2014, ISBN 978-3-510-65285-3 , p. 132-134 .