Mimetite

minerals

pyromorphite

cerussite

vanadite

Images
Formula: Pb5(AsO4)3Cl
Arsenate, apatite group, forms a series with pyromorphite and with vanadite

Varieties

Campylite is a variety of mimetite forming barrel-shaped crystals

Specific gravity: 7.1
Hardness: 3½ to 4
Streak: White
Colour: Colourless, white, brown, orange, yellow, green, grey
Solubility: Slightly soluble in hydrochloric acid and sulphuric acid; moderately soluble in nitric acid and KOH
Common impurities: Ca,F,Cr,V
Environments:

Hydrothermal environments

Mimetite is a common secondary mineral found in the oxidised zones of lead mineral deposits, and in other settings where lead and arsenic occur together (Mindat, Dana, HOM).
Associated minerals include cerussite, anglesite, bayldonite, caledonite, descloizite, hemimorphite, leadhillite, limonite, malachite, mottramite, smithsonite, vanadinite, willemite, pyromorphite and wulfenite (HOM, Mindat).

Localities

At the Endeavor Mine (Elura Mine), Cobar, Robinson county, New South Wales, Australia, mimetite is widely distributed along with beudantite in the oxidised zone. goethite and other iron oxides coats much of the mimetite (AJM 11.2.86-88).

At the New Cobar Mine, Cobar, Robinson county, New South Wales, Australia, mimetite occurs with inclusions of duftite and bayldonite, and associated with bayldonite, duftite, segnitite and gartrellite. It is usually one of the last minerals to form. Rare pseudomorphs of bayldonite and duftite after mimetite have been found (AJM 11.2.80).

At the Kintore open cut, Broken Hill, Yancowinna county, New South Wales, Australia, mimetite is widespread in the oxidised zone, often occurring on its own, especially on a coronadite - goethite matrix. Mimetite is part of a solid solution series including end members segnitite, kintoreite, beudantite, philipsbornite and hidalgoite, and these minerals may have partly or completely replaced mimetite in some specimens. Crystals to 1.5 cm have been found with a thin coating of bayldonite. Associated minerals include bayldonite, olivenite, conichalcite, carminite and cryptomelane. In one area mimetite has been found with wulfenite and fornacite - vauquelinite, and in another area mimetite occurs with duftite and mawbyite (AJM 3.1.49).

In the Dundas area, Tasmania, Australia, mimetite occurs at the Adelaide mine, associated with cerussite on goethite and pyrite. At the West Comet mine, mimetite occurs in weathered galena and with crocoite, anglesite and cerussite (AJM 12.2.83).

At Saint-Pierre-de-Broughton Quebec, Canada, mimetite, occurs as a rare alteration product of galena, as small masses and grains associated with cerussite crusts around partially decomposed crystals of galena (R&M 85.6.13).

At the Pingtouling mine, Liannan county, Guangdong province, China, excellent crystals of mimetite occur on a limonitic gossan matrix (Minrec 38.1.25-29).

At the Clara Mine, Oberwolfach, Ortenaukreis, Freiburg, Baden-Württemberg, Germany, mimetite and pyromorphite are the most common members of the lead zone, and they are found here together (R&M 90.1.45).

The type locality is the Treue Freundschaft Mine, Johanngeorgenstadt, Erzgebirgskreis, Saxony, Germany.

In Central Iran, in the oxidized zone of lead and zinc bearing ore deposits, mimetite is one of the most common arsenates, sometimes in association with vanadinite.
At the Chah-Milleh Mine mineralisation occurs along the crushed contact zone of marble and schist, cemented by coarse crystalline calcite. Here mimetite occurs in the oxidised ore associated with wulfenite. Additional minerals found at this locality include aragonite, baryte, bornite, calcite, caledonite, chalcopyrite, chalcocite, copper, galena, hemimorphite, hydrozincite, leadhillite, malachite, plattnerite, sphalerite and willemite. The willemite is noticeable for its pseudomorphs after descloizite, and on rare occasions the descloizite has been replaced by mimetite.
At the Gowd mine the zinc-lead mineralisation formed as nests, veinlets, and impregnations in the fault zones, in the marble or at the contact zone between marble and schist. Mimetite has been found perched on calcite crystals or hydrozincite. Many of the large crystals show a secondary growth of small vanadinite crystals clustered in the centre of their largest crystal faces.
The Chah-Khouni Mine is characterised by polymetallic gold mineralisation. The host rock is a tectonised dolomitic marble. The oxidised zone is represented by dolomitised breccia with remnants of such minerals as chalcopyrite, galena, pyrite, sphalerite, and magnetite. About thirty different minerals have been listed, including anglesite, atacamite, azurite, calcite, cerussite, chrysocolla, cotunnite, covellite, descloizite, diaboleite/pseudoboleite fornacite, gold, hemimorphite, herbertsmithite, hetaerolite, hydrozincite, iranite, magnetite, malachite, murdochite, phosgenite, plattnerite, pyromorphite-mimetite series, smithsonite, rosasite, tenorite, vanadinite, willemite and wulfenite.
At the Nakhlak Mine the lead mineralisation is situated mainly in the lower part of the Late Cretaceous carbonate rocks, where sandstone and conglomerate are more common. The orebodies occur within dolostone and dolomitised sandy limestone as quartz - calcite - baryte veins or vein clusters of massive galena. The primary ore seems to be monomineralic, consisting solely of galena. Cerussite is common in the oxidised zone. A list of identified minerals includes anglesite, baryte, cerussite, chalcopyrite, covellite, galena, magnetite, mimetite, minium, plattnerite, pyrite, smithsonite, and wulfenite. Prismatic and sometimes doubly terminated microcrystals of mimetite have been found richly scattered on the blades of “snowflake" cerussite (R&M 91.5.402-413).

At San Pedro Corralitos, Chihuahua, Mexico, mimetite occurs with wulfenite (Minrec 35.6.15).

At the San Francisco mine, Sonora, Mexico, mimetite occurs with wulfenite (Minrec 35.6.55-57).

At the Imiter Mine, Tinghir Province, Drâa-Tafilalet Region, Morocco, mimetite occurs as crystals to 1 mm occur associated with wulfenite (Minrec 42.2.126) and has been found as pseudomorphs after it (KL p205).

At Touissit, Oujda, Morocco, mimetite pseudomorphs after vanadinite have been found (KL p207).

At the Tsumeb mine, Oshikoto Region, Namibia, some specimens of mimetite and cerussite rank among the finest in the world. Both are secondary minerals formed from pre-existing lead minerals, the cerussite forming first, probably from the weathering of galena. Then the mimetite crystallised on the cerussite, after which the chemistry of the environment changed to dissolve away the cerussite, leaving an epimorph of mimetite; some specimens retain a solid cerussite core, others have only residual cerussite, while still others are devoid of cerussite altogether. Some specimens retain clear evidence of original twinning. Associated minerals, other than goethite and hematite, are relatively rare. One large pocket produced an abundance of distinctive specimens in which the cerussite was incompletely coated with powdery yellow mimetite and was associated with calcium-rich duftite (R&M 96.4.352-357 ). Frequently arsentsumebite forms pseudomorphs after mimetite with unaltered mimetite still visible in the cores of some of the crystals (R&M 93.6.546). Tsumcorite and bayldonite pseudomorphs after mimetite have been found here (KL p198, 199), as well as the mimetite pseudomorphs after cerussite and anglesite (KL p205).

At the Driggith Mine and Sandbed mine, Caldbeck, Allerdale, Cumbria, England, mimetite is commonly associated with bariopharmacosiderite, beudantite and scorodite. It may also occur with adamite and bayldonite in thin fractures in quartz veinstone on the main dumps. Rarely, mimetite forms pseudomorphs after cerussite. It also occurs in the erythrite-bearing assemblage at Sandbed mine. In addition to its association with the supergene arsenate minerals described above, mimetite commonly occurs with malachite and cerussite (JRS 9.25-26).
Structures in which pyrite and arsenopyrite have partly oxidised to produce carminite, mimetite, beudantite and segnitite in quartz have been found at other localities in the Caldbeck Fells including Sandbed Mine and Deer Hills (JRS 11.25-26).

At Dry Gill mine phosphate-rich world-class mimetite variety campylite occurs in a gangue of quartz, baryte and manganese oxides (Minrec 41.1 supplement pages 28-33).

At Burdell Gill, Coombe Height, Caldbeck, Allerdale, Cumbria, England, UK, mimetite has been found as tiny crystals on ferruginous quartz, and also associated with beudantite or with drusy carminite. Hollow epimorphs of iron oxide after mimetite are not infrequent. Hollow drusy epimorphs of beudantite after mimetite also occur occasionally (JRS 8-1.5)

At Ingray Gill, Caldbeck, Allerdale, Cumbria, England, UK, mimetite is occasionally associated with pyromorphite in epimorphs after galena (JRS 12.38).

At Roughton Gill, Caldbeck, Allerdale, Cumbria, England, UK, mimetite is fairly common. The primary mineralogy comprises galena, sphalerite and chalcopyrite in quartz veinstone with minor baryte and carbonates (JRS 11.25-26).

At Silver Gill, Roughton Gill, Caldbeck, Allerdale, Cumbria, England, UK, mimetite has been found associated with crusts of beudantite on a single specimen. Crusts of phosphate-rich mimetite, some forming pseudomorphs after cerussite, have also been identified on joint planes in vein quartz (JRS 8-2.92).

At Arm O'Grain, Coombe Height, Mungrisdale, Eden, Cumbria, England, UK mimetite occurs as barrel-shaped to platy crystals. Mimetite-pyromorphite and mimetite-vanadinite intermediates also occur (JRS 9.48).

At the Mendip Hills, Somerset, England, UK, mimetite is common, most obviously when it forms typical prismatic hexagonal crystals. It also occurs as a massive mineral forming blobs in mendipite and calcite, as patches on calcite and in fissures and cavities in manganese oxide (JRS 13.29).

At the Gallagher Vanadium Property and Manila Mine, Cochise County, Arizona, USA, mimetite occurs as hexagonal barrel-shaped crystals that are generally associated with wulfenite, vanadinite and pyromorphite (R&M 90.4.344).

At the Fred Glaze claim in the Panamint Range, Death valley, California, USA, mimetite epimorphs after cerussite have been found. Fine-grained mimetite forms a thin shell in the shape of earlier-formed V-twinned cerussite crystals, which in most cases have subsequently dissolved away. They also often have small red wulfenite crystals on them. Cerussite is a secondary mineral, so it probably grew as a weathering product of primary galena. The cerussite subsequently became unstable, causing it to dissolve away. Following the dissolution of cerussite, wulfenite crystals grew on many of the casts. Because wulfenite crystals appear on both the inside and outside of the casts, the wulfenite growth must have occurred last. This means the cerussite was already dissolved before the wulfenite grew (R&M 91.3.246-249).

At the Brown Monster mine, Inyo county, California, USA, mimetite is relatively common. It occurs in quartz veins near a contact with marble, or protruding from fracture surfaces in limestone, commonly associated with wulfenite. In one area it is associated with mottramite. At the nearby Reward mine, mimetite is associated with duftite-conichalcite (Minrec 42.2.189).

Alteration

Solubility of mimetite
Pb5(AsO4)3Cl (solid) + 6H+ (aqueous) ⇌ 5Pb2+ (aqueous) + 3H2AsO-4 (aqueous) + Cl- (aqueous)
(Min Mag June 1989 Vol 53 pp363-371)

cerussite and aqueous H2AsO4-, Cl- and H+ to mimetite and aqueous H2CO3
5PbCO3 + 3H2AsO4- + Cl- + 7H+ ⇌ Pb5(AsO4)3Cl + 5H2CO3
or
5PbCO3 + 3HAsO42- + Cl- + 4H+ ⇌ Pb5(AsO4)3Cl + 5H2CO3
cerussite and mimetite can co-exist only under basic conditions at rather high PCO2 (Min Mag June 1989 Vol 53 pps363-371).

The Activity-pH diagram below was calculated at 298.2 K for the main Cu2+ and Pb2+ arsenate minerals. Boundaries are calculated for constant activities (roughly equivalent to concentrations) of Pb2+ and Cl- ions in solution, over a range of values of pH and of Cu2+ activity. Higher chloride activites make the stability fields of bayldonite and duftite vanish, with respect to the encroachment of that of mimetite (LMW p269).
stability Cu Pb.jpg

The arsenate mineral formulae are:
philipsbornite PbAl3(AsO4)(AsO3OH)(OH)6
mimetite Pb5(AsO4)3Cl
duftite PbCu(AsO4)(OH)
bayldonite Cu3PbO(AsO3OH)2(OH)2
olivenite Cu2(AsO4)(OH)
cornwallite Cu5(AsO4)2(OH)4
clinoclase Cu3(AsO4)(OH)3










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