A blue or blue-gray silicate mineral containing sodium, magnesium, and iron, commonly found in metamorphic rocks that formed under high pressure.
From Greek glaukos ('blue-green') + phanos ('appearing'). Named in the 1840s because of its distinctive blue color visible in thin rock sections under a microscope.
Glaucophane is like a geologist's pressure gauge—it only forms under extreme squeeze deep in Earth, so finding it tells you that ancient rocks have been through a cosmic traffic jam.
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