Mimetite
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(′mim·ə′tīt)
(mineralogy) Pb5(AsO4)3Cl A yellow to yellowish-brown mineral of the apatite group, commonly containing calcium or phosphate; a minor ore of lead. Also known as mimetene; mimetesite.
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Pb
Hexagonal -- Hexagonal bipyramidal
Environment
Secondary (weathered) zone of lead ore deposits.
Crystal descriptionSlender to thick needles, sometimes in yellowish mammillary crusts. Orange-yellow rounded crystals (melon-shaped) called campylite.
Physical propertiesWhite, yellow, yellow-orange to brown. Luster resinous; hardness 3Ɖ; specific gravity 7.0-7.3; fracture uneven; cleavage pyramidal. Brittle; transparent to translucent.
CompositionLead chloroarsenate (74.6% PbO, 23.2% As 2 O 2.4% Cl).
TestsFuses readily on charcoal; suddenly boils, giving off arsenic fumes (garlic odor), and reduces to a lead bead.
Distinguishing characteristicsAppearance, associations, and occurrence show it to be a member of this group. Distinguished from pyromorphite and vanadinite by arsenic smell.
OccurrenceLike pyromorphite and vanadinite, mimetite is a secondary mineral. It is rather rare, being far less common than the other two, and only occasionally is it of importance as an ore of lead. Best distributed in collections are the old campylite specimens. Campylite occurs in small (to 蕀 in.; 1 cm) crystals at several British localities. Mimetite is rare in the U.S. Most frequent occurrence is across the border in Durango, Mexico, where it formed orange-yellow botryoidal coatings of great charm at San Pedro Corralitos. Brilliant orange blobs illuminate thin yellow wulfenites at Cerro Prieto, Sonora, Mexico, and at the Rowley and 79 Mines in Theba and Hayden, Arizona. It has been found at Eureka, Utah. The best and largest isolated crystals of former times were found in the old mine visited by Goethe at Johanngeorgenstadt, Saxony. Remarkable, gemmy, pale yellow, beautifully terminated 2-in. (4-5 cm) crystals have lately been found at Tsumeb, Namibia, and provide what is probably the all-time great for this mineral.
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Mimetite
| Mimetite | |
|---|---|
 Mimetite, Bilbao mine, Zacatecas, Mexico. Size: 7.9 x 5.8 x 3.4 cm |
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| General | |
| Category | Arsenate minerals |
| Chemical formula | Pb5(AsO4)3Cl |
| Strunz classification | 08.BN.05 |
| Crystal symmetry | Hexagonal dipyramidal H-M symbol: (6/m) Space group: P63/m |
| Unit cell | a = 10.250(2) Å, c = 7.454(1) Å; Z=2 |
| Identification | |
| Color | Pale to bright yellow, yellowish brown, yellow-orange, white, may be colorless |
| Crystal habit | Prismatic to acicular crystals; reniform, botryoidal, globular, |
| Crystal system | Hexagonal |
| Twinning | Rare on {1122} |
| Cleavage | [1011] Imperfect |
| Fracture | Brittle, conchoidal |
| Tenacity | Brittle |
| Mohs scale hardness | 3.5 - 4 |
| Luster | Resinous, subadamantine |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 7.1 - 7.24 |
| Optical properties | Uniaxial (-), anomalously biaxial |
| Refractive index | nω = 2.147 nε = 2.128 |
| Birefringence | δ = 0.019 |
| Pleochroism | Weak |
| References | [1][2][3] |
Mimetite, whose name derives from the Greek Μιμητής mimethes, meaning "imitator", is a lead arsenate chloride mineral (Pb5(AsO4)3Cl) which forms as a secondary mineral in lead deposits, usually by the oxidation of galena and arsenopyrite. The name is a reference to mimetite's resemblance to the mineral pyromorphite. This resemblance is not coincidental, as mimetite forms a mineral series with pyromorphite (Pb5(PO4)3Cl) and with vanadinite (Pb5(VO4)3Cl). Notable occurrences are Mapimi, Durango, Mexico and Tsumeb, Namibia.
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Contents
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Uses of mimetite
Industrially, mimetite is a minor ore of lead. The chief use of mimetite is as a collector's specimen, often creating attractive botryoidal crusts on the surface of the specimen. Though mimetite is also found in prismatic crystal forms, it is not used as a gemstone due to its softness. Quality prismatic forms have been found in Johanngeorgenstadt in Saxony and Wheal Unity in Cornwall, England.
Associated minerals
Mimetite is found in association with lead and arsenic minerals, including those minerals with which it forms a series. Some associated minerals include: calcite, galena, pyromorphite, smithsonite, vanadinite, and wulfenite.
Alternative names
Alternative names of mimetite include arsenopyromorphite, mimetesite, and prixite. Campylite is the name for a variety with barrel shaped crystals of a brownish-red or orange-yellow color and containing a considerable proportion of phosphoric acid.
Notes for identification
Useful information pertaining to the field identification of mimetite include its habit, color, and high density. However, this mineral's similarity to pyromorphite can be problematic, especially since these minerals are known to share colors. Pyromorphite is typically green, and mimetite is typically yellow, but specimens of each are known in the other's colors. As a result, some identification may require lab analysis.
Mimetite specimen gallery
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Mimetite, Pingtouling Mine, Guangdong Province, China. Size: 2.2 x 2.1 x 1.8 cm
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Mimetite, Santa Eulalia, Chihuahua, Mexico. Size: 5.4 x 3.9 x 1.5 cm
References
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Wikisource has the text of the 1911 Encyclopædia Britannica article Mimetite. |
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Wikimedia Commons has media related to: Mimetite |
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McGraw-Hill Science & Technology Dictionary
McGraw-Hill Dictionary of Scientific and Technical Terms. Copyright © 2003, 1994, 1989, 1984, 1978, 1976, 1974 by McGraw-Hill Companies, Inc. All rights reserved.
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Peterson Field Guide to Rocks & Minerals
Peterson Field Guide to Rocks and Minerals, by Frederick H. Pough. Copyright © 1998 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.
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