Shield-cut mimetite, 0.85 cts, 6 x 3.4 mm, Tsumeb, Namibia. © The Gem Trader. Used with permission.
Transparent mimetite crystals are extremely rare, and very few have ever been faceted. Cabochons with rich orange and yellow colors have been cut, but this material is too soft to wear as jewelry.
| Data | Value |
|---|---|
| Name | Mimetite |
| Formula | Pb5(AsO4)3Cl |
| Colors | Sulfur yellow, yellowish brown, orange-yellow, orange, orangish red, white, colorless, green (rare). |
| Fracture | Subconchoidal to uneven |
| Hardness | 3.5-4 |
| Cleavage | None |
| Crystallography | Hexagonal. Crystals acicular; globular; botryoidal; prismatic. |
| Refractive Index | 2.128-2.147 |
| Birefringence | 0.019 |
| Luminescence | Orange-red in LW (Tsumeb, Namibia) |
| Luminescence Present | Yes |
| Luminescence Type | Fluorescent, UV-Long |
| Absorption Spectrum | Not diagnostic |
| Pleochroism | Weak in yellow shades |
| Optics | o = 2.147; e = 2.128. Uniaxial (-); may sometimes be anomalously biaxial. |
| Optic Sign | Uniaxial - |
| Luster | Subadamantine to resinous |
| Specific Gravity | 7.24 (Lower if Ca replaces Pb) |
| Transparency | Transparent (very rare) to opaque |
| Etymology | From a Greek word meaning “imitator,” because of its resemblance to pyromorphite. |
| Occurrence | A secondary mineral in the oxidized zone of lead deposits. |
Botryoidal mimetites, 3.8 x 3.1 x 2.4 cm, Haus Baden Mine, Badenweiler, Schwarzwald (Black Forest), Germany. © Rob Lavinsky, www.iRocks.com. Used with permission.
Mimetite belongs to the apatite mineral group and occurs in many locations around the world. However, to date, only Namibia and China have produced very limited quantities of facetable material. With a hardness range of just 3.5 to 4, any cut mimetites are usually reserved for display in gem collections.
Mimetite also forms mineral series with both pyromorphite and vanadinite as the arsenic (As) end member.
Green to yellow mimetites, 9.0 x 6.0 x 3.0 cm, Elura Mine, Cobar, New South Wales, Australia. © Rob Lavinsky, www.iRocks.com. Used with permission.
With over-the-limit (OTL) refractive indices of 2.128 and 2.147, a very high specific gravity (SG) of 7.24, and piezoelectric properties, mimetite can usually be readily distinguished from more commonly encountered gemstones. However, it shares some of these characteristics as well as color ranges with a few other rarely faceted gemstones: cassiterite, vanadinite, and wulfenite.
Cassiterite and vanadinite aren’t piezoelectric and typically have lower SG than mimetite. However, mimetites may have lower SG values if calcium (Ca) replaces lead (Pb) in its composition. Vanadinites usually have higher refractive indices (RIs) than mimetites, while cassiterites usually have lower RIs.
Wulfenites are well-known for their rich, red colors, but some may have yellow, orange, or brown colors. They also have piezoelectric properties and SG values close to mimetites but typically higher RIs.
All these gems are rare, but you’re more likely to encounter faceted cassiterites and wulfenites than faceted mimetites and less likely to encounter faceted vanadinites. They all have distinctive hardness values, but conducting scratch tests on finished gems, especially ones so rare, isn’t recommended.
Mimetites also have colors and properties that overlap or approximate those of pyromorphites. Distinguishing them may require advanced chemical analysis.
Scientists have synthesized mimetites for a variety of purposes, including research into the removal of arsenic from contaminated sites. However, there’s no known jewelry use for this lab-created material.
No known gemstone treatments or enhancements.
Tsumeb, Namibia has produced fine, yellow, transparent crystals, up to 1 inch long. Most of the limited facetable material available comes from here.
More recently, Guangdong Province, China has also yielded some facetable material.
Freeform step-cut mimetite, 0,57 cts, 4.2 x 3.8 mm, Guangdong Province, China. © The Gem Trader. Used with permission.
Chihuahua and Mapimi, Durango, Mexico produce fine globular masses in orange and yellow.
Botryoidal mimetites, 3.1 x 2.7 x 1.8 cm (largest sphere 1.5 cm), San Pedro Correlitos, Chihuahua, Mexico. © Rob Lavinsky, www.iRocks.com. Used with permission.
An opaque, banded variety of mimetite, possibly from Bolivia, has recently emerged as a lapidary material that resembles schalenblende sphalerite and “wood tin” cassiterite.
Other notable sources include the following locations:
Ochre-colored, barrel-shaped campylite crystals in a quartz vug, 8.9 x 6.1 x 4.4 cm (largest crystal almost 1 cm), Dry Gill Mine, Caldbeck Fells, North and Western Region (Cumberland), Cumbria, England, UK. © Rob Lavinsky, www.iRocks.com. Used with permission.
Mimetite crystals, Hat Yai District, Songkhia Province, Thailand. Photo by Géry Parent. Public Domain.
Gem cutters have made unusual, interesting cabochons up to an inch or two in length from globular Mexican mimetites.
Most transparent crystals, particularly from Tsumeb, are extremely rare and are usually preserved as mineral specimens rather than cut. However, small broken crystals have been cut, yielding some stones up to a few carats in weight, to a maximum of 5-7 carats.
You’re more likely to find mimetites, if at all, in mineral collections than jewelry collections. Their low hardness makes them very susceptible to scratching, so reserve any mimetite jewelry for occasional wear and use protective settings.
Mimetites contain arsenic and lead, both highly toxic. While this shouldn’t pose a problem for wear or display, faceters should take precautions against inhaling or ingesting particles, protect their eyes, wash their hands, and clean their workspace after cutting mimetites. They should wear protective masks and, ideally, use a glovebox during cutting, polishing, and cleaning.
For more safety recommendations, consult our articles on lapidary safety tips and handling toxic gems.
Clean mimetites only with a soft brush, mild detergent, and warm water. For more care recommendations, consult our gemstone jewelry cleaning guide.
Mimetite: Tsumeb, Namibia (2.81). Photo © Joel E. Arem, PhD, FGA. Used with permission.
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