Grossular from Stifle Claims, El Dorado County, California
Contributed by: Michael Crawford
Date: Jul 7th, 2025
Locality: Stifle claims (Stifle prospects; Stifle Memorial claim; Traverse Cr. Special Interest Area; Traverse Creek Special Interest Area), Meadow Brook, Georgetown District, East Segment Mother Lode, El Dorado County, California, USA (See on Mindat)
Size: 5 x 6
Description:
This white grossular garnet (Ca3Al2(SiO4)3) comes from the Stifle claims, Georgetown, El Dorado County, California. Local rocks include ultramafic rocks. This specimen came from James Horste and this is his description about its source.
The find was a lens abut about 3 meters long and up to 1 meter thick which has been completely mined out. About 1,200 Grossular specimens were recovered, from thumbnail to cabinet size, but of these only about 10 to 15% were good fluorescent specimens. The best fluorescent specimens were collected along fracture zones that represent a late stage in the formation of the lens. Some specimens have distinct fluorescent and non-fluorescent zones. This find was reported in The Mineralogical Record, volume 50, November- December, 2019. These specimens were analyzed by XRD and it was confirmed they were pure end-member Grossular with only trace amounts of iron.
The specimen fluoresces red under longwave UV illumination. The fluorescence is most prominent on the rims of the crystals. It has weak to no fluorescence under midwave and shortwave UV. The longwave emission spectrum has five sharp peaks. The multiple sharp peaks are characteristic of a chromium activator. The chromium likely came from the local ultramafic rocks.
Summary of luminescence responses:
Grossular (Mindat) (RRUFF)
- Fluorescence under Longwave (365nm LED) UV light: Red