There’s a Giant Flaw in Human History
- By sjastro
- Physical & Life Sciences
- 1508 Replies
Thanks for your input, I sourced the Google image under the search 'Granite cross polarization image', but the greater problem there were no images for vitrified granite so I used Obsidian as an example what a vitrified granite should look like in a glassy amorphous state under a polarizing microscope.At the risk of going off topic, the image on the left would not be the sort of evidence I would wish to use for any purpose. It is labelled as a granite, but its appearance counts against this. Granite is primarily made up of feldspars and quartz. These have low-order interference colours under polarised light and these should dominate the image. Moreover there is no evidence of the twinning one would expect of the plagioclase feldspars and crystal shape is atypical. Micas, an important minor constituent of granites, are also seemingly absent. In contrast several of the crystals have interference colour, shapes and character associated with pyroxenes. This is certainly an igneous rock, but it is not a granite.
Note: it is some decades since I've had occassion to examine rocks in thin section, but petrography was one of my favourite aspects of practical geology. Nevertheless, I considered two alternative explanations that might justify the identification as granite. First, birefringence colours are increased in intensity if the thin section thickness is too great. Against this explanation, a) one does not publish or work with thin sections not prepared to exacting measures and the crystal shapes are abnornal for granite. b)If the micrograph was photographed in uv light then unusual colours might be produced, but in that case the image would be properly identified as taken in that uncommon way.
Ironically @stevevw's link to the Nature article provided all the information I needed.
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