Color Changing Scheelite from South Korea
Contributed by: Michael Crawford
Date: Nov 25th, 2025
Locality: Tae Hwa Mine, Neungam-ri (Neung Am-ri; Noungam-ri), Angseong-myeon, Chungju City, North Chungcheong Province, South Korea (See on Mindat)
Size: 7 x 9 cm
Description:
This cluster of scheelite (Ca(WO4)) crystals exhibits both intrinsic fluorescence from tungstate in the scheelite structure and extrinsic fluorescence due to impurities of rare earth elements replacing calcium in the scheelite structure. The presence of these two forms of fluorescence causes the fluorescent color to be different under different wavelengths of UV illumination. The intrinsic fluorescence activated by the tungstate ion is light blue under shortwave light. The extrinsic fluorescence caused by dysprosium, samarium, and neodymium is orange red under midwave light and yellow under longwave light.
The specimen comes from the Tae Hwa Mine, North Chungcheong Province, South Korea. This mine produced tin and tungsten from 1902 until 1973. The specimen also contains a black, non-fluorescent mineral that may be the tin mineral cassiterite or ferberite, an iron tungstate mineral.
The shortwave emission spectrum is a broad peak with a maximum at 428 nm and a shoulder that extends into the ultraviolet region. There are also two sharp peaks at 542 nm and 572 nm. These peaks may be caused by the rare earth dysprosium replacing calcium.
The midwave emission spectrum contains numerous sharp peaks. The red midwave fluorescence is mainly caused by samarium (Sm3+) that produces several sharp peaks (598 nm, 607 nm, and 645 nm). There is another set of sharp peaks in the near infrared (807 nm, 879 nm and 889 nm) that are activated by neodymium (Nd3+). The black and white image shows the near infrared fluorescence activated by the neodymium. There are more peaks in the midwave spectrum that are unassigned to a specific rare earth element.
The longwave emission spectra also contain numerous sharp peaks. The peaks are caused by samarium, dysprosium, and other rare activators. The dominate yellow fluorescence is caused by dysprosium and other rare earths that cause the peaks at 522 nm, 529 nm, 542 nm and 550 nm. The area on the specimen where the longwave fluorescence is red has an emission spectrum where the peaks (522 nm, 529 nm, 542 nm and 550 nm) are less intense and the samarium peaks (596 nm, 605 nm, and 644nm) in the red are brighter. The yellow and red fluorescence have the same emission peaks and same activators, but the color difference is caused by different concentrations of the impurities activating the fluorescence.
Summary of luminescence responses:
Scheelite (Mindat) (RRUFF)
- Fluorescence under Longwave (365nm LED) UV light: Yellow
- Fluorescence under Midwave (305nm LED) UV light: Red
- Fluorescence under Shortwave (255nm LED) UV light: Blue