Erythrite

By Cook, Robert B

BOU AZZER, MOROCCO Were it not for secondary minerals of the common base metals, the mineral collecting world would be a drab place indeed. Gone would be azurite, malachite, and the plethora of related supergene copper minerals; lead-dominated cerussite, anglesite, mimetite, and wulfenite would likewise disappear, as would the zinc species smithsonite, adamite, and hemimorphite. There are other metals, however, of far lesser abundance and more restricted distribution for which secondary minerals produced by oxidation of their primary sulfides are of equal beauty but of much greater rarity, particularly in fine specimens. One of these metals is cobalt. Its common sulfides and arsenides are the “parents” of the highly collectible species erythrite. Specimens of this mineral are available in modest to good quality at many mineral shows, mostly from Moroccan localities in the Bou Azzer area. One such specimen has been nominated as this issue’s Connoisseur’s Choice.

Erythrite is best known for its crimson to peach-red color, which may vary to a pale rose or pink. It exhibits a sub-adamantine luster that tends to be pearly on cleavages. It is very soft with a Mohs hardness of 1.5 -2.5 and is sectile and flexible in thin {010} laminae. Its cleavage is perfect on {010} and poor on {100} and {102}. Its measured density is 3.06.

Erythrite is monoclinic (2/m) and is a member of the vivianite group. Crystals are typically flattened on [010] and may be elongated and prismatic on [001]. Large crystals are relatively rare but are known up to 6 cm in length. It is commonly seen in radial or stellate aggregates of small crystals, in fibrous and drusy crusts, or in powdery masses.

Ideally, erythrite is hydrated cobalt arsenate with the formula Co3(AsO4)2*8H2O. It forms a solid-solution series with both annabergite (Ni end member) and hornesite (Mg end member). Early descriptive literature mentions red- and green-zoned crystals, suggesting color changes with varying cobalt and nickel content. Some analyses show iron and manganese in amounts up to several percent by weight.

Erythrite forms in the oxide zone of deposits containing cobalt, commonly those containing primary Co-Ni-As mineral assemblages, often with accessory silver. Associated minerals include cobaltite, skutterudite, arsenopyrite, lollingite, symplesite, roselite, scorodite, pharmacosiderite, adamite, morenosite, retgersite, and manganese oxides. In some climates, stains and thin pink crusts of erythrite are ubiquitous around cobalt occurrences, so much so that they have been called “cobalt bloom,” an excellent prospector’s indicator of the presence of cobalt.

Erythrite is of rare and generally undistinguished occurrence in the eastern and central United States, generally forming pale pink, thin patches and crusts. It is reported from such historically interesting localities as the old cobalt mine at Great Hill, East Hampton, Middlesex County, Con-necticut; the Franklin and Sterling Hill zinc mines, Sussex County, New Jersey; the old Tilly Foster iron mine near Brewster, Putnam County, New York; Pennsylvania occurrences including the Cornwall iron mines in Lebanon County, the French Creek iron mines and Wheatley lead mines in Chester County, and the Dyer, Robeson, and Hay Creek quarries in Berks County. It is reported from the Alberene and Old Dominion soapstone quarries, Albemarle County, Virginia; and the Buick, Sweetwater, and other lead mines of the ViburnumTrend, Iron and Reynolds counties, Missouri. It has been found in the Martin mine of the Upper Mississippi Valley lead-zinc district, Lafayette County, Wisconsin; and the Mohawk mine, Keweenaw County, Michigan.

In the western United States, erythrite is known from a variety of metal deposits, a few of which have produced interesting specimens. It occurs at several Colorado localities (Eckel 1997) including the Clyde Long property in Elk Park, San Juan County, where it forms thin microcrystalline patches on fracture surfaces and fills small vugs. Here, stel-late groups of crystals to 3 mm have been found generally associated with a variety of minerals including cobaltite, skutterudite, and uraninite. Erythrite has long been known associated with a thucolite-bearing vein exposed in the Evans mine at Placerville, San Miguel County, Colorado. Attractive crusts of small erythrite crystals occur coating fractures and in cavities at mines of the Blackbird cobalt district, Lemhi County, Idaho (Ream 2004; Shannon 1926). In the Blackbird mine proper, it forms rose-red fibrous and crystalline coatings in a black tourmaline-quartz rock and has been found in both the pit and upper underground workings. Thin blades and rosettes of rose-red erythrite occur in the Bulkley mine and Brooklyn claim of the same district. Crusts and druses of rose-red erythrite microcrystals occur with weathered cobalt-bearing gersdorffite at a prospect above the mouth of Slate Creek in Shoshone County. Small sprays of tiny erythrite crystals occur with tetrahedrite, azurite, annabergite, brochantite, chalcophyllite, and morenosite at the Lovelock and Nickel mines, Table Mountain district, Churchill County, Nevada. Crusts and small groups of microcrystalline erythrite occur with the silver ores of the Alhambra and nearby mines of the Black Hawk (Bullards Peak) district, Grant County, New Mexico; it has also been reported from the Ponil district, Colfax County, of that state. Erythrite specimens have been found at the Happy Jack and W. N. mines in the White Canyon district, San Juan County, and on Temple Mountain, Emery County, Utah.

Several interesting Arizona erythrite occurrences have been documented. It is reportedly a rare mineral in the famous Orphan mine on the south rim of the Grand Canyon, Coconino County, and occurs in the Mule Shoe Bend area of the Salt River in Gila County. It has also been described from cobalt-bearing prospects in the Black Hills district, Yavapai County, including the Cobalt and Walker prospects and the Shylock mine area.

Small amounts of erythrite have been reported from several California occurrences (Pemberton 1983). It occurs with skutterudite at the Mar John mine near Sheepranch, Calaveras County, and with skutterudite, acanthite, and annabergite at the Bishop silver- cobalt mine on the west side of Chocolate Peak, Inyo County. A similar association of ery-thrite, acanthite, and skutterudite occurs in the Kelsey and O. K. mines near Morris Reservoir in San Gabriel Canyon, Los Angeles County. It has also been found in small amounts as an alteration product of cobalt-bearing arsenopyrite at the historically important Pine Tree and Josephine gold mines, Mariposa County. Other California occurrences include the Goleta mine, Mono County, and the Friday mine, San Diego County, where the mineral is found associated with more-nosite. In Washington, it has been reported as pink crusts after cobalt-bearing arsenides at the Monte Cristo mine, Snohomish County, and recently from the Crown Jewel gold deposit, Okanogan County.

Traill (1983) documents a wide variety of Canadian ery-thrite localities, most of which have not produced collectible specimens. It is known from at least nine mines and prospects in British Columbia, of which the Evening Star claims near Rossland have produced bladed crystals of the mineral. It is particularly common, generally as stains and powdery crusts, in many of the silver mines and prospects of the Temiskaming district, Ontario. Erythrite is widespread as an accessory mineral in shallow portions of many silver deposits of the Cobalt district, Ontario. Specimens attributed to the Princess, Nipissing 404, Nipissing, Nova Scotia, Timiskaming, Harrison-Hibbert, Cobalt Contact, Agaunico, Christopher, Hargrave, Silver Queen, Cross Lake, O’Brien, LaRose, Chambers Ferland, Coniagas, and Hudson Bay mines of this district are retained in various public and private collections. Other relatively well-known Ontario ery-thrite localities include silver occurrences near Gowganda, the Frontier and Keeley mines at the abandoned settlement of Silver Centre in South Lorrain Township, the Langis mine in Casey Township, and prospects at Sauve Lake.

Several Mexican localities have produced fine erythrite specimens (Panczner 1987). Tufts of crystals to 8 mm on goethite have been collected at the La Luz mine, Maguarich-ic, Chihuahua, and similar specimens have come from the Veta Rica mine, Sierra Mohada, Coahuila, where it is associated with native silver, acanthite, skutterudite, and proustite in veins cutting serpentinite. The finest Mexican erythrite, however, is that from the cobalt-nickel mine near San Ber-nardo, some 44 kilometers north of Alamos, Sonora. Here, crystals to 1 cm and aggregates to almost 30 cm across were found associated with heterogenite (“stainierite”), cobalt-bearing mansfieldite, and goethite.

A number of European localities have produced good to excellent erythrite specimens, and a far greater number are known for erythrite micromounts. Interesting erythrite occurred historically in a number of the more famous Cornish mines, and good micromount specimens are collectible at the Tynebottom mine, South Tynedale, Alston Moor district, Cumbria, England. Miniature-sized specimens with erythrite sprays in vuggy matrix are found on the Trial level of the Vigra mine, Bontddu, Gwynedd, Wales. The finest European erythrites are the somewhat prismatic, striated crystals to several centimeters in length once found at Schneeberg, Saxony; among the dozens (see, for instance, mindat.org) of other lesser German localities are Richels-dorf, Hess, Gluksbrunn, and Kemsdorf, Thuringia, and Wit-tichen in the Schwarzwald Mountains. Erythrite has been reported from many localities in Austria, with perhaps the best occurrence represented by beautiful micromounts of bright red, radial aggregates of erythrite on and with dark blue azurite from prospects on Silberberg Mountain, Inn Valley, North Tyrol. Earthy erythrite crusts and small crystals occurred at Jachymov and Abertamy, and more recently good micromount specimens have been collected at mines in the Steinknochen area, Usti region of Bohemia, Czech Republic; similar specimens are found at Dobsina, Slovakia. French erythrite occurrences include Sainte-Marie-aux Mines in the Vosges Mountains, and Les Chalanches near Allemont, Isere Department. Very attractive thumbnail, and miniature-sized erythrite specimens have been collected at the Traversella mine, Canavese district, Torino, Piemonte, Italy. Good micromount erythrite occurs at Cerro Minado, Almeria, and with conichalcite in the La Encontrada mine, Granada, Andalusia, Spain. By far the best contemporary erythrite localities are the cobalt deposits of the Bou Azzer area, Ouarzazate Province, in southern Morocco. Specific localities include the Bou Azzer deposit itself and outlying mines at Aghbar, Agoudal, Ait Ahmane, Ightem, Khder, Mechoui, Oumlil, and Tam-drost (Favreau and Dietrich 2006). Here, individual ery-thrite crystals to 6 cm long and groups coating matrix slabs to 25 cm across have been collected. Associated minerals are roselite and heterogenite. A spectacular find in the district in June and July 2000 produced what are likely the finest ery-thrites known. Many groups were in cavities in white quartz, making an unusual and attractive association. Others were on broken massive skutterudite. Erythrite is also reported from the Imiter mine of the same province, better known for its silver and acanthite specimens.

Erythrite has been reported from a variety of Australian localities, generally as quite colorful crusts of microcrys-tals. It is relatively abundant at Mount Cobalt near Selwyn, Cloncurry district, some 200 kilometers southeast of Mount Isa, Queensland, where it also occurs in good contemporary specimens. Other Queensland localities include the Big-genden mine, approximately 90 kilometers west of Mary-borough, where it occured with a variety of bismuth minerals and arsenopyrite (Dunstan 1913). Attractive erythrite specimens from the Bulldog gold mine, associated with weathered cobaltite, and well-crystallized crusts of erythrite crystals to 4 mm long from the Alice Mary copper mine, both near Ravensthorpe, Western Austalia, are described by Simpson (1951). Erythrite as pseudomorphs after cobalt-bearing arsenopyrite and as crusts of minute crystals is reported from the Dome Rock copper mine, Olary Province, South Australia, by Noble, Just, and Johnson (1983). Segnit (1978) reports erythrite from this same mine as pseudo-morphs after cobaltite, flat crystals to 1 cm in length, and fractured rock faces “several feet across” coated with radiating groups of crystals. Minute tufts of acicular erythrite crystals on glaucodot from Carcoar, small stellate crystal groups on limonite and encrustations on cobaltite from the Consols mine at Broken Hill, and erythrite efflorescence on “smaltite” at the Bismuth mine, Torrington, New South Wales, have been described by Smith (1926).

Other interesting erythrite specimen localities include Talmessi and Meskani in the Anarak district, Esfahan Province, Iran; the Dashkesan cobalt deposit, Azerbaijan; the Kakanda mine, Kambove, Zaire; the Kruisriver cobalt mine, Mpumalanga Province, South Africa; the Naganobori and Dohgatani mines, Honshu Island, Japan; the Khovu-Aksy Co-Ni deposit, Eastern-Siberian Region, Russia; the Mar-camarcani mine at Sorata, La Paz Department, Bolivia; and the Veta Negra mine, Copiapo Province, La Blanco mine, Huasco Province, and Minillas mine, Elqui Province, Chile.

ACKNOWLEDGEMENTS

The manuscript for this article was improved considerably by the thoughtful reviews of John S. White, Stewartstown, Pennsylvania, and Bob Ramik of the Royal Ontario Museum, Toronto, Canada. Locality information supplied by Carl Francis of the Harvard Mineralogical Museum is also gratefully acknowledged.

COLOR SPONSORS for the Connoisseur’s Choice column during 2007 are the Mineral Section of the Houston Gem and Mineral Society and the Cincinnati Mineral Society.

REFERENCES

Dunstan, B. 1913. Queensland mineral index and guide. Queensland Geological Survey publication 241.

Eckel, E. B. 1997. Minerals of Colorado. Updated and revised by R. R. Cobban et al. Golden, CO: Fulcrum Publishing.

Favreau, G., and J. E. Dietrich. 2006. Die Mineralien von Bou Azzer. Lapis 12 (3): 68.

Noble, R. J., J. Just, and J. E. Johnson. 1983. Catalogue of South Australian minerals: 1983. South Australian Department of Mines and Energy handbook 7.

Panczner, W. D. 1987. Minerals of Mexico. New York: Van Nostrand Reinhold.

Pemberton, H. E. 1983. Minerals of California. New York: Van Nos- trand Reinhold.

Ream, L. R. 2004. Idaho minerals. Coeur d’Alene: Museum of North Idaho.

Segnit, E. R. 1978. Further minerals from the Dome Rock mine, South Australia. Australian Mineralogist 15:73-74.

Shannon, E. V. 1926. The minerals of Idaho. U.S. National Museum bulletin 131.

Simpson, E. S. 1951. Minerals of Western Australia. Vol. 2. Perth: Government Printing Office.

Smith, G. 1926. Mineralogy of New South Wales. New South Wales Department of Mines, Mineral Resources bulletin 34.

Traill, R. J. 1983. Catalogue of Canadian minerals. Geological Survey of Canada paper 80-18.

ROBERT B. COOK

Department of Geology and Geography

Auburn University

Auburn, Alabama 36849

cookrob@auburn.edu

Dr. Robert B. Cook, an executive editor of Rocks & Minerals, is a professor ermeritus in the Department of Geology and Geography at Auburn University, Auburn, Alabama. He welcomes suggestions for this column and can be contacted at the addresses above.

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