This metamorphic rock is extremely useful to find in the field as it catches a chemical reaction in progress. The pale blue mineral is kyanite and the pink mineral is andalusite, both polymorphs of aluminosilicate minerals. These minerals have the exact same chemical formula and will turn from one to the other depending on the pressure and temperature conditions. Kyanite generally is found at higher pressures than andalusite, while andalusite is found at lower pressures and generally higher temperatures (the third polymorph, sillimanite, forms at the highest temperatures).

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Mud-cracks: desiccation and decoration

You’ve probably noticed how cracks develop in dried-up puddles in the summer. Usually these cracks are not simply random lines across the surface, but instead are typically in the form of interconnected polygons. The photo shows mud-cracks on the arid desiccated floor of DeadVlei (dead lake, in Afrikaans), in Namibia. This clay pan was once an oasis, verdant with vegetation and acacia trees. It was the subject of an earlier post here on TES (, where you can read that climate change a millennium ago caused sand dunes to encroach and cut off the river that once flowed into this “lake”.

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