Mimetite

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Mimetite
Mimetite - Congreso-Leon mines, San Pedro Corralitos, Chihuahua, Mexico.jpg
Golden-yellow botryoidal mimetite from Congreso-León mines, Mexico
General
Category Arsenate minerals
Apatite group
Formula Pb5(AsO4)3Cl
IMA symbol Mim [1]
Strunz classification 8.BN.05
Crystal system Hexagonal
Crystal class Dipyramidal (6/m)
(same H-M symbol)
Space group P63/m
Unit cell a = 10.250(2) Å,
c = 7.454(1) Å; Z = 2
Identification
ColorPale to bright yellow, yellowish brown, yellow-orange, white, may be colorless
Crystal habit Prismatic to acicular crystals; reniform, botryoidal, globular,
Twinning Rare on {1122}
Cleavage [1011] Imperfect
Fracture Brittle, conchoidal
Tenacity Brittle
Mohs scale hardness3.5–4
Luster Resinous, subadamantine
Streak White
Diaphaneity Transparent to translucent
Specific gravity 7.1–7.24
Optical propertiesUniaxial (−), anomalously biaxial
Refractive index nω = 2.147 nε = 2.128
Birefringence 0.019
Pleochroism Weak
References [2] [3] [4]

Mimetite is a lead arsenate chloride mineral ( Pb 5(As O 4)3 Cl ) which forms as a secondary mineral in lead deposits, usually by the oxidation of galena and arsenopyrite. [2] The name derives from the Greek Μιμητήςmimetes, meaning "imitator" and refers 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.

Contents

Properties

Mimetite from Namibia Mimetite 21207.jpg
Mimetite from Namibia

Mimetite typically forms short hexagonal crystals that are yellow to brown to orange in color, very brittle, moderately hard (Mohs hardness 3.5–4), and dense (specific gravity 7.24). It is distinctive for its lack of transparency, its resinous to adamantine luster, and its solubility in nitric acid. [5]

Mimetite forms a complete solid solution series with pyromorphite, with phosphate (PO3−
4) substituting for arsenate (AsO3−
4). The two minerals are almost identical in their properties and can be difficult to distinguish except by laboratory tests. Pyromorphite is the more common mineral at most locales. [5]

Campylite is a name applied to mimetite or pyromorphite that crystallizes as distinctive barrel-shaped crystals forming curved hemispherical aggregates. [5] Bellite is a name formerly applied to a chromium-bearing mimetite, or possibly a mixture of crocoite, mimetite, and quartz, which forms attractive orange red crystals, but has been discredited as a distinct mineral species. [6]

Occurrences

Mimetite is found in association with lead and arsenic minerals, including pyromorphite, cerussite, hemimorphite, smithsonite, vanadinite, anglesite, pyrite, mottramite, willemite, and wulfenite. [5] [2] Good specimens have been reported from Gila County, Arizona, US; Ojocaliente, Zacatecas, Mexico; Cumberland, England; Johanngeorgenstadt, Saxony, Germany; Namibia; and Broken Hill, Australia. [5]

References

  1. ↑ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi: 10.1180/mgm.2021.43 . S2CID   235729616.
  2. 1 2 3 Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C. (eds.). Handbook of Mineralogy (PDF). Chantilly, VA: Mineralogical Society of America . Retrieved 2025-05-01.
  3. ↑ "Mimetite". Mindat.org . Retrieved 12 February 2021.
  4. ↑ Barthelmy, David (2014). "Mimetite Mineral Data". Mineralogy Database. Webmineral.com. Retrieved 12 February 2022.
  5. 1 2 3 4 5 Sinkankas, John (1964). Mineralogy for amateurs . Princeton, N.J.: Van Nostrand Reinhold. pp. 419–422. ISBN   0442276249.{{cite book}}: ISBN / Date incompatibility (help)
  6. ↑ Burke, Ernst A.J. (2006-12-01). "A mass discreditation of GQN minerals" . The Canadian Mineralogist. 44 (6): 1557–1560. Bibcode:2006CaMin..44.1557B. doi:10.2113/gscanmin.44.6.1557.
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