Multi-color Calcite from Terlingua, Texas
Contributed by: Michael Crawford
Date: Jul 20th, 2025
Locality: Little 38 Mine, Brewster County, Texas, USA (See on Mindat)
Size: 7 x 10 cm
Description:
This is a multi-color calcite specimen from the Little 38 Mine, Terlingua, Brewster County, Texas. The specimen contains the classic Terlingua calcite that is pink under longwave illumination, and blue, white under midwave light and blue under shortwave light. It has a strong midwave and shortwave afterglow. There is a weak blue afterglow with longwave exposure. There are also bands of orange and yellow, fluorescent calcite. The fluorescent color is the same in all wavelengths of UV illumination. The yellow band has a honey color in the white light image that may be hydrocarbon inclusions. The honey calcite has strong greenish afterglow with exposure to longwave UV illumination. The orange and yellow, fluorescent bands are likely activated by organics. The most likely explanation of the pink and blue fluorescence of Terlingua calcite is the presence of luminescence centers created by radiation damage.
Spectra of the three calcite types are shown in the last image. The orange and yellow calcite fluorescent spectra are broad peaks. The Terlingua calcite spectra have a peak in the blue region, around 420 nm in all three wavelengths. The longwave spectrum also has a broad peak with a maximum at 595 nm. The mix of blue fluorescence and reddish fluorescence creates the pink color we see under longwave illumination.
Summary of luminescence responses:
Calcite (Mindat) (RRUFF)
- Fluorescence under Longwave (365nm LED) UV light: Pink
- Fluorescence under Longwave (365nm LED) UV light: Yellow
- Fluorescence under Midwave (305nm LED) UV light: Violet
- Fluorescence under Shortwave (255nm LED) UV light: Blue
- Afterglow after exposure to Longwave (365nm LED) UV light: Green
- Afterglow after exposure to Midwave (305nm LED) UV light: Blue
- Afterglow after exposure to Shortwave (255nm LED) UV light: Blue