Cordierite Hornfels
“Baked to hardness by magma's heat”
Cordierite hornfels is a dense, fine-grained contact metamorphic rock characterized by the growth of cordierite crystals, often giving the rock a spotted or mottled appearance. It forms when clay-rich sedimentary rocks are baked at high temperatures near an igneous intrusion.
cordierite, biotite, andalusite, quartz, feldspar, spinel
Dartmoor (England), Oslo region (Norway), Bushveld Complex contact zones (South Africa), Skye (Scotland), Sierra Nevada (California, USA)
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Cordierite — also known as the gem iolite — shows different colors when viewed from different angles, and Vikings may have used it as a polarizing filter to find the sun on overcast days for navigation.
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How might Vikings have used cordierite (iolite) for navigation on cloudy days?
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