Yellow Wavellite from Arkansas
Contributed by: Michael Crawford
Date: Jun 16th, 2025
Locality: Mena, Polk County, Arkansas, USA (See on Mindat)
Size: 6 x 9 cm
Description:
This is a specimen of a vug containing yellow botryoidal wavellite (Al3(PO4)2(OH)3 · 5H2O) originating from Polk County, Arkansas. Low-temperature fluids dissolve various elements from surrounding rocks, leading to the precipitation of secondary minerals such as wavellite in fractures and vugs.
Typically, wavellite from Arkansas has a green color and lacks fluorescence. However, the yellow variant of Arkansas wavellite often exhibits green fluorescence. This particular yellow specimen fluoresces green across all UV wavelengths of illumination and is most intense under shortwave UV illumination. The green fluorescence is activated by trace amounts of the uranyl ion (UO2^2+).
The shortwave emission spectra plot compares uranyl-activated wavellite to another secondary phosphate mineral, autunite, which incorporates a uranyl unit into its crystal chemistry (Ca(UO2)2 (PO4)2 · 10-12H2O). Both minerals\' spectra display vibronic peaks, though these peaks are shifted relative to one another. The variation in peak positions results from distinct coordination environments between the uranyl unit and the attached elements. The peaks for the wavellite specimen are at 497 nm, 518 nm, 541 nm, 566 nm, and 594 nm.
Radioactivity measurements for the wavellite specimen indicated approximately 200 CPM on my Geiger counter, whereas the autunite sample from Washington state showed around 29,000 CPM. This significant difference highlights that only trace amounts of the uranyl ion are necessary to activate green fluorescence.
Summary of luminescence responses:
Wavellite
- Fluorescence under Longwave (365nm LED) UV light: Green
- Fluorescence under Midwave (305nm LED) UV light: Green
- Fluorescence under Shortwave (255nm LED) UV light: Green