BLUE AZURITE: TUCSON 2016

Who could leave a Tucson Shades of Blue show without taking home a specimen of Arizona azurite.  One of my purchased specimens is a beautiful group of electric blue azurite balls spread on some sort of a gossan matrix.  It is fairly easy to observe, especially with a loupe, that the aggregates are composed of numerous individual crystals of azurite.

Azurite from Morenci mines, Arizona.  Width ~4.6 cm.

Azurite is one of the more recognizable blue minerals since most people associate an azure blue color with the mineral---and they are correct, although the mineral also shades to lighter hues of blue.  Azurite is a secondary copper carbonate [Cu₃(CO₃)₂(OH)₂] found in the oxidized zone above copper deposits where carbonated water reacts with copper minerals, or with solutions of copper sulfate or copper chloride reacting with limestone (www.MinDat.org).  The crystals are Monoclinic, commonly quite complex, and are displayed in several different forms with over 100 recorded examples (Handbook of Mineralogy, Mineral Data Publishing, www.rruff.info).  My specimen is composed of individual tabular crystals formed into spherical aggregates while other crystal examples commonly are prismatic.  Crystals are transparent to translucent, vitreous in individuals but less so (subvitreous) in aggregates.  The streak is a very pale blue and the crystals have a hardness of around 3.5-4.0 (Mohs).

Photomicrograph of above specimen.  Azurite aggregates composed of individual tabular crystals.  The center aggregate is ~ 5m mm in width.

The specimen that I purchased came from the Morenci Mine, Copper Mountain District, Greenlee County near Clifton, Arizona.  Morenci is one the largest copper producers in the U.S. and the ore, like many southwestern copper mines, is a porphyry copper ore body (large volumes of ore allows mining with copper content of less than 1%). 

At Morenci the reserves contain: millable ore grades at 0.48% copper, crushed ore runs 0.51% copper, while the large deposits of leachable ore averages 0.16% copper. These low figures contrast with the early mining of oxides where the copper content averaged about 20%. 

In a typical copper porphyry ore deposit, such a Morenci, pyrite [FeS₂], chalcopyrite [CuFeS₂], bornite [Cu₅FeS₄] (not important at Morenci) and chalcocite [Cu₂S] are the major copper sulfide minerals in the Primary Zone.  In some copper ore deposits, such as Morenci, other Primary Zone non-copper minerals are also present and sometimes mineable: molybdenite (MoS₂; in production), galena (PbS), and sphalerite (ZnS).

These Primary Zone minerals, in ore deposits, form from superheated aqueous solutions (hydrothermal) that originate at great depths and then migrate upwards where deposition begins, usually in lime rocks..  In some cases, where the sulfide minerals are near the surface, low temperature water (meteoric) oxidizes and alters the Primary Zone minerals and dissolved elements are carried downward and may precipitate out and form new minerals.  These Secondary Zone minerals form both above and below the water table.  The minerals that form above the water table are stable in an oxidizing environment and include minerals such as malachite [Cu₂CO₃OH], smithsonite [ZnCO₃], cuprite [Cu₂O], azurite [Cu₃(CO₃)OH₂], pyromorphite [Pb₅(PO₄0₃Cl], atacamite [Cu₂Cl(OH)₃], chrysocolla [(CuAl)₂H₂Si₂O₅(OH)4-nH₂O], native copper [Cu], and tenorite [CuO]. Minerals precipitating out below the water table, in a reducing environment, are mostly sulfides like covellite [CuS], and chalcocite [Cu₂S]. Above in part from Rakovan (2003).

The Morenci deposits were mined underground until the 1930s and then converted to open pits (www.mining-technology.com/projects/morenci).  In 2015 copper production at Morenci was 902 million pounds and the company employed over 3,200 people (www.freeportinArizona.com). 

Morenci became a critical part of Phelps Dodge and Company in 1881 but is now part of Freeport-McMoRan Inc. with Sumitomo Metal Mining Company a substantial minority partner.  

Aerial view of the open pit mines at Morenci.  Photo courtesy of www.freeportinarizona.com.

Morenci is also famous for once producing, via the “miner’s lunch box” method, a sky-blue turquoise in a pyrite matrix.  If I understand correctly, vintage Native American turquoise jewelry is coveted by collectors and is quite expensive.  I also believe that Morenci produced (produces?) minor amounts of silver and gold as accessory minerals; however, I have been unable to locate production figures.

REFERENCES CITED

Rakovan, J., 2003, Hypogene and supergene: Rocks and Minerals, v.78.