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Fluorescence

Trancas geode (a.k.a. Chihuahua geode)

This is one of my favorite rock specimens. This geode is from Estación Trancas, Chihuahua, Mexico, formed in Eocene age rhyolite. It’s age is approximately 37 million years old and formed as gas cavities in volcanic rock that later filled with silica rich fluids, producing quartz and chalcedony.

Trancas geodes put on a spectacular show when exposed to short wave UV-light. Characterized by their dazzling white drusy crystalline centers, these geodes contain a very small amount of uranium that often fluoresces a brilliant bright-green. The highest reading I could get for this specimen on my Geiger counter is 0.17 microsieverts (μSv) (normal background radiation is around 0.1 mrem/hour or 1 μSv/hour).

This video shows the geode in various stages and lighting, beginning with how it looked before I cut it open, after cutting, and how it fluoresces under UV light.

The pink glow on the outside of a Trancas geode under UV light is caused by fluorescent Calcite. This mineral, often found in the outer layers of Mexican geodes, contains impurities such as manganese that activate under UV light, emitting a vibrant pink or red color, frequently accompanying chalcedony. Enjoy:

Our green granite countertop fluoresces pink under UV light:

Willemite and Franklinite in Calcite from the Sterling Hill Mine, New Jersey: