ADAMITE

By Ronald L. Parker

Senior Geologist, Senior Geologist, Borehole Image Specialists, 6564 S. Heritage Place E. Centennial, CO | ron@bhigeo.com

Adamite, hydrous zinc arsenate - Zn2AsO4(OH), is a secondary mineral found in the oxidized weathering zone of arsenic-bearing zinc deposits. Adamite, once considered rare, is now more plentiful. Adamite occurs in many colors (clear, yellow, green, magenta, purple, brown) that reflect transition metal impurities. Adamite exhibits a complete solid solution series with olivenite, Cu2AsO4(OH). Intermediate compositions between the Zn and Cu end-members, formerly called a variety of names, are now identified as zincolivenite. A prominent physical property of adamite is bright, yellow-green UV fluorescence. Fluorescence is quenched by increases in Cu content. Although adamite is known from worldwide occurrence, the Ojuela mine (Mapimi, Durango, Mexico) is the most significant source of well-crystallized, fluorescent adamite.

This year I was pleased to be able to attend the Denver Gem and Mineral show, in spite of the COVID-19 pandemic. I was heartened to observe that every attendee was required to wear a face covering, that numbers were limited and that social distancing was effective. I went on the second Saturday of the event so I was hoping to find some deals. And, I did. One of my favorite purchases that day was from AJ Lafortune (LaFortunate Minerals, Durango, CO). It was a sparkling thumbnail of translucent, yellow-green adamite that looked like a lemon drop tucked into a mini baseball mitt of orange and brown iron oxides. It was from the famous Ojuela mine in Mapimi, Durango, Mexico. Perhaps because the event was winding down, the piece was half-off the marked price of $10. What really attracted my interest was the adamite’s fluorescence. (Several years ago, I was at the same Gem and Mineral show, and I encountered someone who was brandishing a small handheld UV flashlight to make certain minerals “pop” with fluorescence. I liked that, a lot, so I ordered one - a Convoy S2+, 265 nm LW UV light - online). Poised in front of the adamite at this year’s show, my UV light caused the adamite to flareup with a bright, neon, yellow-green fluorescence. I gave AJ my 5 bucks.

Adamite was named after Gilbert-Joseph Adam, a 19th French Mineralogist who first described it in 1866 from a discovery in the Dolores mine Chanarcillo, Copiapo Province, Atacama Region, Chile (Farndon and Parker, 2011; Jones, 2011). In 1956, an isostructural manganese analogue of adamite was discovered, Mn2 2+AsO4(OH). This new mineral was named eveite, completing the Garden of Eden theme (Cook, 2001). Eveite is rare, but exceptional, because half of the Mn2+ cations are in 5-fold coordination, a previously undocumented, and vanishingly rare, configuration in the mineral kingdom (Moore and Smyth, 1968). Adamite was first discovered in the United States in 1916 from the Tintic District of Utah, then at Cedar Mountain, Mineral County, Nevada and later at Gold Hill, Tooele Count, Utah (Murdoch, 1936). Adamite is an uncommon mineral. Or is it? As Jones (2011) proffers, adamite WAS uncommon – even rare – until the Ojuela mine in Mapimi, Durango, Mexico began extracting huge quantities.

Adamite occurs in a variety of colors including yellow, orange-yellow, honey-yellow, yellow-green, pale yellow, brownish-yellow, brown, violet, magenta, pale green, vibrant lime-green, white or even colorless (Cook, 2001; Southwood et al., 2020). Adamite, of any color or color blend, often occurs as transparent to translucent crystals (Johnsen, 2002). The colorless variety of adamite is compositionally pure. Colored adamites contain transition metal impurities that alter their color: iron turns the mineral yellow, cobalt imparts a pinkish hue, copper turns adamite green and purple adamites are manganese-enriched (Haynes, 2008; Farndon and Parker, 2011).

Adamite is orthorhombic, crystallizing in the 2/ m2/m2/m (dipyramidal) crystal class. Crystal habits are developed by elongation on [010], [001 and rarely on [100]. Adamite commonly occurs as blunt-ended or chisel-tipped, wedge-like prisms in druses, crusts or granular aggregates (Dana and Ford, 1922; Cook, 2001; Farndon and Parker, 2011). Sometimes crystals appear as fan-shaped rosettes or radiating aggregates composed of prismatic crystals that are elongated parallel to the b-crystallographic axis (Johnsen, 2002). The radiating habit can often yield an outer surface that has botryoidal appearance, even creating golf ball-like masses (Jones, 2011). In 1956, a triclinic dimorph of adamite was discovered, subsequently named paradamite (Switzer, 1956). The structural difference between adamite and paradamite lies in the coordination of the Zn (Hawthorne, 1976). Adamite is a soft mineral, having a hardness of 3.5, which permits it to leave a white to pale green streak. Specific gravity is 4.4. There is one perfect cleavage on {101}, poor cleavage on {010} and a brittle fracture. Adamite exhibits a range of lusters, described variously as resinous, vitreous or adamantine (Johnsen, 2002).

Adamite forms a complete solid solution series with olivenite (Cu2AsO4(OH). Although some references indicated that adamite is isostructural with olivenite (Hill, 1976), this is now disproven - olivenite is monoclinic. The change in symmetry from orthorhombic to monoclinic occurs at ~80 mole% copper (Southwood et. al., 2020). The range of compositions intermediate between adamite (Zn) and olivenite (Cu) end-members spawned a variety of names in the mineral community (cuproadamite, cuprian adamite, zinc olivenite and zincian olivenite), These names were based on qualitative properties, not compositional analyses. In 2006, the International Mineralogical Association (IMA) addressed nomenclatural irregularity by approving a new mineral species – zincolivenite, ZnCuAsO4(OH), which, like adamite, is orthorhombic. Although ideal zincolivenite has 50 mole% Zn and 50 mole% Cu, the compositional range of zincolivenite is defined as 25 mole% to 75 mole% Cu. Southwood et al. (2020) show that the relative percentage of copper in zincolivenites from the Tsumeb mine can be estimated by color. They determined that low Cu zincolivenite is pale (spearmint) green, midrange Cu zincolivenite is emerald green and high Cu zincolivenite is bottle green. These color differences are distinguished from the end-members adamite (colorless, yellow, pink and brown) and olivenite (blackish-green). Although most common former name for the intermediate, cuproadamite, is now discredited, collectors and dealers are noted to be reluctant to change specimen labeling (Southwood et. al., 2020).

The fluorescence exhibited by adamite is variable and appears to depend upon trace element composition. Mrose et. al. (1948) describe the fluorescence of Ojuela Mine adamite as “a bright lemon-yellow with a pale lemon-yellow phosphorescence in short-wave ultraviolet radiation; a much weaker reaction is given in long-wave ultraviolet (p. 451). Jones (2011) notes that copper-bearing adamites (zincolivenites) often do not respond to UV illumination, the degree of fluorescence quenching increasing with increasing copper content.

Adamite is associated with other secondary minerals such as hemimorphite, goethite, smithsonite, scorodite, olivenite, mimetite, wulfenite, azurite, malachite, calcite, quartz, and iron and manganese oxides and oxyhydroxides (Cook, 2001; Mineral Data Publishing, 2005). In one adamite mine (Ojuela), the primary mineralization was composed of large emplacements of arsenic-bearing sulfides (arsenopyrite, pyrite, sphalerite and argentiferous galena) into a carbonate country rock (Mrose et. al., 1948; Cook, 2001).

Notable global adamite occurrences include: ancient mines at Laurium, Greece; Cap Garonne, France; Cumbria, England; Tyrol, Austria; Reichenbach, Baden, Germany; Mount Valerio, Tuscany, Italy; Isle of Thasos, Turkey; Ain Achour, near Guelma, Constantine, Algeria; Tsumeb, near Otavi, South Africa; Broken Hill, New South Wales, Australia and, the type locality at Chanarcillo, Atacama, Chile (Dana and Ford, 1922; Cook, 2001).

By far, the most important locality for adamite has to be the Ojuela Mine in Mapimi, in the state of Durango, Mexico. This mine is thought to have provided 95% of the world’s best adamite specimens (Cook, 2001). When Mrose et. al., (1948) wrote about the Ojuela mine in 1946, it was one of the largest mines in Mexico, having been the site of rich silver ore extraction since 1600. Farndon and Parker (2011) describe the Mapimi district. “Mapimi is a limestone replacement deposit rich in rare arsenic minerals. Here, adamite is found not with limonite but in yellow straws and sprays with hemimorphite, austinite and rare minerals such as legrandite and paradimite. All the best specimens of fluorescent adamite come from this location” (p 206). At this locality, the upper oxidized zone is over 1,000’ thick. As Jones (2011) recounts about adamite from Ojuela, “When the mine was working, the number of specimens coming forth was virtually countless and box after box of this fine mineral was saved in quantity.” (p 17). Cook (2001) suggests that adamite would be a boring secondary mineral if not for the Ojuela mine, a “location that has been the source of a seemingly endless supply of exceptionally beautiful specimens for at least four decades.” (p. 40). Jones even suggests that the flow of mineral richness from the Ojuela mine undid the previous consideration that adamite is a rare mineral. The sample that I bought for $5 at the Denver Gem and Mineral Show is from the Ojuela mine.

In the United States, adamite is known from: the Sterling Hill mine, New Jersey; the Funeral Mountains, the Amargosa Range, Inyo County, California; Cedar Mountain, Mineral County, Nevada; Coconino County, Arizona; the Tuckerville District, Hinsdale County, Colorado; and from the Gold Hill mine in Tooele County, Utah. (Cook, 2001; Farndon and Parker, 2011).

Adamite, a treasured, glowing addition to my collection.

https://www.minerals.net/mineral/adamite.aspx

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