Gem News International Gems & Gemology, Summer 2013, Vol. 49, No. 2

Banded Mimetite

Figure 1. This mimetite cabochon, likely from Bolivia, weighs 177.75 ct and has an agate-like structure of yellow-brown and dark brown layers. Photo by Jaroslav Hyršl.

Mimetite is a member of the apatite group and a lead chloro-arsenate, known to collectors as one of the rarest faceted stones. Some yellow transparent specimens weighing up to 1 ct are known from old finds in Tsumeb, Namibia. Small orange translucent stones under 0.5 ct have been faceted from Chinese mimetite, and a new type of lapidary mimetite (figure 1), likely from Bolivia but purchased in Tucson without a reported locality, was identified recently.

The cabochon measured 40.5 × 31.7 × 7.2 mm and weighed 177.75 ct, with a very high luster. Its lapidary work was of low quality, but the stone had a very nice agate-like structure, with alternating yellow-brown and dark brown layers. It closely resembled two unusual ornamental stones: the colloidal variety of sphalerite (figure 2) known by the German name schalenblende, and the colloidal cassiterite variety called “wood tin.” Schalenblende is known from many localities worldwide, mainly Poland but also Bolivia, where it is very difficult to visually distinguish from mimetite. Wood tin is known to be from Bolivia and Mexico.

Figure 2. The sphalerite variety schalenblende has characteristics similar to those of banded mimetite. This specimen weighs 106.20 ct. Photo by Jaroslav Hyršl.

The RI of all three minerals is very high, over the refractometer limit. Loose stones can be easily distinguished by their hardness and specific gravity, and the hardness of mimetite and schalenblende is the same, about 3.5 on the Mohs scale. Wood tin is much harder, about 6.5. The SG of the mimetite was 6.31, considerably higher than the 3.8–4.2 of a typical schalenblende. The SG of wood tin can vary from 5.2 to 6.6, likely depending on the presence of thin chalcedony layers. Mimetite and wood tin are also inert in UV light, while schalenblende is usually yellow in long-wave UV. The mimetite was identified by powder X-ray diffraction by John Attard of San Diego, but its exact origin could not be determined. Thin veins of mimetite deposited from hot springs are known to have been mined from a deposit in Lomitas, located in La Paz Department, Bolivia.