CAMPBELLITE

POLISHED "CAMPBELLITE" FROM CAMPBELL SHAFT

I once purchased a specimen labeled as “chalcotrichite, Campbell Shaft, 1800’-2300’ level, Bisbee, AZ”.  In doing some research on this partially-polished specimen, I found some interesting aspects concerning the mineral.  Chalcotrichite is “a variety of cuprite consisting of fibrous sprays or mats of hair-like crystals” (MinDat, 2011).  However, my specimen seems a composition of blood red cuprite (copper oxide, Cu₂O), white calcite (CaCO₃), and perhaps some native copper (Cu)—no hair like crystals are present. This specimen, collected from the Campbell Shaft or mine near Bisbee, Arizona, is known locally as "campbellite". The mixture is valued as polished specimens or cabochons.  The mine seems a prolific producer of minerals and MinDat has listed 110 different minerals.  Mineralization at the Campbell Mine ore body is largely oxidized copper in Cambrian, Devonian and Mississippian limestones (MinDat, 2011).  At one time the underground mine was a major producer of copper and collectable specimens of azurite, cerussite and malachite. 

 

PROTECTING PYRITE

SEE BLOG POSTING OF 5-12-13 FOR BETTER DESCRIPTION

Causal collectors of minerals often find pyrite, iron disulfide (FeS₂), appealing as a display specimen; most give little thought to keeping it protected over the years. On the other hand, paleontologists, but especially vertebrate paleontologists and museum workers, are quite concerned about “pyrite disease” eating up their specimens.  If fossil “bones” (as an example) contain even small amounts of pyrite and are exposed to conditions of high humidity, then the mineral begins to oxidize and forms iron sulfate (perhaps the mineral melanterite), FeSO₄.  Iron sulfate is of greater volume that iron sulfide (pyrite) and so causes expansion in the specimens and breakage and crumbling soon follow.  In addition, sulfuric acid forms and discolors the specimens.  Once pyrite disease begins the process seems irreversible.  That is one reason museum collections are stored in dry conditions, 45% humidity or less.  And, individual collectors should keep their prized specimens in a sealed container along with a package of silica gel, a desiccant.  Without proper storage they may begin to oxidize within a few months.  Larger specimens of pyrite cubes are slower to oxidize and most collectors will only see some “tarnishing” on their specimens.  This is the same chemical reaction, plus water and perhaps microbes, that produces the very acidic (sulfuric acid) acid mine drainage.

Masss of intergrown pyrite cubes displaying striations on crystal faces.  Photo courtesy of Wikipedia.com.