Chagrin (microscopy)

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As Chagrin (English shagreen ) is referred to in the geology an optical effect in microscopy of rocks in the thin section . The chagrin enables a first, rough estimate of the refractive index .

The name "Chagrin" comes from French and is used there for grained leather , which is similar in texture to the Chagrin discussed here.

description

Chagrin is the wrinkled, grained and rough-looking structure inside a mineral. Minerals, whose refractive index differs more strongly from the embedding medium, show a stronger shagreen. As with the relief , a distinction can be made between positive and negative shagreen. Negative shagreen appears in minerals with a refractive index below that of the embedding medium, it seems to bulge spatially away from the observer. Conversely, positive chagrin is formed in minerals with a refractive index higher than that of the embedding medium; it appears to bulge spatially towards the observer.

In particular, the chagrin as an optical property of optically anisotropic minerals is also direction-dependent (see example with calcite below).

The chagrin also depends on the settings of the microscope: for example, closing the field diaphragm increases the chagrin, whereas folding the condenser front lens reduces the chagrin. Therefore, when comparing the shagreen of different minerals, the lighting settings should remain unchanged.

photos

  • Thin section showing a quartz (Qrz), a garnet (Grt) and a pyroxene (Pyx). The quartz has the same light refraction as the embedding medium (1.54) and therefore shows only a slight shagreen. The garnet has a very high refraction (around 1.8), which results in a high shagreen. Minerals of the pyroxene group have a lower refraction than garnet and therefore also have a weaker shagreen. Image height approx. 1 millimeter.

  • Thin section with calcite. The shagreen is clearly visible. Image width 330 microns.

  • The same thin section as above, but rotated 45 degrees clockwise, resulting in a change in the shagrin. This is an example of the directionality of the shagrin. Image width 330 microns.

Emergence

The chagrin is created by the unevenness on the surface of the cut. If this is placed in the embedding medium, this lines the unevenness. The different refractive indices of the embedding medium then lead to optical effects ( light diffraction , light refraction , total reflection ), which are stronger the greater the difference in the refractive indices. This creates a perceptible difference in brightness on these structures in the microscopic image.

Individual evidence

  1. ↑ Volker Schweizer: Dictionary of Geology . German – English, English – German. Springer-Verlag, Berlin / Heidelberg 2012, ISBN 978-3-8274-1825-8 , pp. 51 , doi : 10.1007 / 978-3-8274-2262-0 .
  2. ↑ Martin Okrusch , Siegfried Matthes : Mineralogie . An introduction to special mineralogy, petrology and geology. 9th edition. Springer-Verlag, Berlin Heidelberg 2014, ISBN 978-3-642-34659-0 , p.  21 , doi : 10.1007 / 978-3-642-34660-6 .
  3. ↑ Hans Pichler, Cornelia Schmitt-Riegraf: Rock -forming minerals in thin sections . Enke, Stuttgart 1987, ISBN 3-432-95521-9 , pp. 8-9 .
  4. ^ Gregor Markl: Minerals and Rocks . Mineralogy - Petrology - Geochemistry. 3. Edition. Springer-Verlag, Berlin Heidelberg 2015, ISBN 978-3-662-44627-0 , p. 180-183 , doi : 10.1007 / 978-3-662-44628-7 .
  5. ↑ G. Müller, M. Raith: Methods of Thin Section Microscopy . In: Clausthaler Tektonische Hefte . 2nd Edition. No. 14 . Ellen Pilger, Clausthal-Zellerfeld 1976, p. 51-53 .