MIMETITE: Tsumeb, Namibia (2.81). Photo © Joel E. Arem, PhD, FGA. Used with permission.
Faceted mimetite is one of the rarest of all gems since only one pocket of transparent crystals has ever been found (at Tsumeb), and few of these crystals have been cut. Orange and yellow cabochons are richly colored but are too soft for wear.
| Data | Value |
|---|---|
| Name | Mimetite |
| Colors | Sulfur yellow, yellowish brown, orange-yellow, orange, white, colorless. |
| Crystallography | Monoclinic (pseudo-hexagonal). Crystals acicular; globular; botryoidal. |
| Refractive Index | 2.128-2.147 |
| Luster | Subadamantine to resinous. |
| Hardness | 3.5-4 |
| Fracture | Subconchoidal to uneven |
| Specific Gravity | 7.24 (Lower if Ca replaces Pb). |
| Birefringence | 0.019. |
| Cleavage | None |
| Stone Sizes | Cabochons up to an inch or two can be cut from globular orange and yellow masses from Mexico, and these make unusual and interesting stones. Tsumeb crystals are extremely rare (one pocket found), and most of the crystals are being preserved as specimens and will not be cut. Small broken crystals were cut, yielding some stones up to a few carats in weight, with a maximum of 5-7 carats. |
| Luminescence | Orange-red in LW (Tsumeb, Namibia). |
| Luminescence Present | Yes |
| Luminescence Type | Fluorescent, UV-Long |
| Transparency | Transparent (very rare) to opaque. |
| Absorption Spectrum | Not diagnostic. |
| Formula |
Pb5(AsO4)3Cl. |
| Pleochroism |
Weak in yellow shades. |
| Optics |
o = 2.147; e = 2.128. Biaxial (-); may sometimes be uniaxial. |
| Optic Sign | Uniaxial -, Biaxial - |
Apatite Group, Pyromorphite series.
Optics: o = 2.147; e= 2.128.
Biaxial (—); may sometimes be uniaxial.
Occurrence: A secondary mineral in the oxidized zone of lead deposits.
Pennsylvania.
Scotland; Sweden; France; Germany; USSR; Czechoslovakia; Australia.
Southwestern United States: many localities.
Chihuahua, Mexico: fine globular orange and yellow masses.
Mapimi, Durango, Mexico: yellowish globular masses.
Cornwall and Cumberland, England: a variety called campylite.
Tsumeb, Namibia: fine yellow transparent crystals, up to 1 inch long.
Comments: Faceted mimetite is one of the rarest of all gems since only one pocket of transparent crystals has ever been found (at Tsumeb), and few of these crystals have been cut. Orange and yellow cabochons are richly colored but are too soft for wear.
Name: From a Greek word meaning imitator, because of the resemblance to pyromorphite.
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