Reduction spots

Reduction spots; the long axis is parallel to the grain of the slate
Green area in a purple slate due to localised reduction of iron

These are areas of green in otherwise dark generally purple slates. They are due to the chemical reaction between iron and organic matter present in the original sediments.  They may be present as discrete bands which formed from organic-rich strata present in the original muds. They may also be present as ovoids, which appear as ovals or circles on the cleavage surface. Assuming that the ovoids were initially spheres which became flattened and stretched during metamorphism,  the ratio of the long axis to short axis gives a measure of the amount of deformation which has taken place; the higher the ratio the more developed the grain of the slate. For example the typical value  for a Scottish Ballachulish slate is 6:1 which has a pronounced grain. In contrast reduction spots in Cumbrian slate are circular, which is indicative of flattening with uniform stretching in all directions,. These slates  have no grain.

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