Saturday, 3 May 2014

Micrograph of the Month: Dissolution of Spherulites

This post follows on from the Manganese Micrograph Mystery I posted a while ago. Blog followers will remember I posted some images of layered ruminant dung which had a distinctive black 'speckled' appearance, a bit similar but not quite the same as Mn staining.
Thanks to my readers the mystery was solved through the kind contributions of Hans Huisman. Huisman is a geoarchaeologist with expertise in degradation and preservation of archaeological materials. He suggests that manganese would produce more dendritic like patterns, and that instead this appears to be iron sulphide staining. Iron sulphides include minerals such as pyrite, which can oxidise to rust. This type of staining can be recognised in OIL as having a 'metallic' lustre, or smaller particles, such as we have here, can be examined using reflectance microscopy at a high magnification. So I'll be doing that asap. For now I wanted to share this image showing the possible impact this staining has had on the dung spherulites. In the top image (PPL) we can see the fragments of dung (1) and the speckled appearance of the metal salt staining (2). In the lower image (XPL) we can see the typical birefringance of the animal dung and the calcareous spherulite particles that are produced in the animal gut. However some of the spherulites (i.e. those that have the staining) have started to dissolve. The staining appears to 'infill' the spherulites, masking the distinctive cross pattern that is used to identify them in XPL. Interestingly similar features can be seen e.g. on diatoms in lake sediment.

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