Datolite and Calcite from Healdsburg, Sonoma County, California

Contributed by: Michael Crawford
Date: Aug 15th, 2025
Locality: Healdsburg, Sonoma County, California, USA (See on Mindat)
Size: 11 x 11 cm

Description:
Datolite (CaB(SiO4)(OH)) and calcite (CaCO3) from Healdsburg, Sonoma County, California. The calcite and datolite occur as veins and veinlets in highly fractured, black fine grained crystalline matrix. The matrix is either igneous or metamorphic rock. A 1.5 cm thick vein of calcite on the left side of the specimen cuts across the fractured matrix.

Datolite from Northern California has been called “esperite of the West” because of its bright yellow fluorescence. This datolite fluoresces bright yellow under all wavelengths of UV light. True esperite from Franklin, New Jersey fluorescestt bright yellow only under shortwave light. The fluorescence of the calcite and datolite is the same under longwave and shortwave UV illumination.

The longwave and shortwave emission spectra of the calcite and datolite are nearly identical. Fluomin.org lists ytterbium (Yb2+) with a peak at 520 nm as a possible activator for yellow fluorescence in datolite. The measured emission peak of datolite is at 518 nm. Calcite has a broad peak with a maximum at 495 nm and a smaller peak at 443 nm. These peaks do not correspond to any trace element known to activate calcite fluorescence. Organics are possible activator for the calcite and possibly the datolite fluorescence instead of ytterbium.