RARE LEAD COPPER OXIDE: MURDOCHITE

During my time in Tucson at the 2015 shows I took a break from looking for treasures and headed off on a road trip north toward the community of Hayden, about 70 miles.  That particular area is well known for a variety of metal-producing mines and spectacular collector minerals.  Today, Hayden is the home of smelting operations for ASARCO (American Smelting and Refining Company), a major mining, smelting and refining company and a subsidiary of Grupo Mexico.  ASARCO is a major copper producer in the U.S. and has three open pit mines near Tucson:  Mission Mine, Ray Mine, and the Silver Belle Mine.  It also has ~20 Superfund sites scattered across the country.  I have collected copper nuggets and chrysocolla from the Ray mines---on a sponsored field trip, and visited the Mission Mine from a vantage point.  I could only observe the Silver Bell mine from a distance—no rockhounds allowed :)

The ASARCO web site noted the Hayden Operations consists of a 27,400 ton/day concentrator and a 720,000 ton/year copper smelter, and processes ore from the nearby Ray Mine. Anodes produced at the smelter are then shipped to the Amarillo, Texas, Copper Refinery. The sulfuric acid produced at the acid plant is used in the leaching operations or sold into the market.  I believe that the Hayden smelter is one of only three? operating copper smelters left in the U.S.

One of the more famous mines in the Hayden area is the 79 Mine in the Dripping Springs Mountains.  A former underground Pb-Zn-Cu-Ag-Au-Mo-Sb-V-Fe mine, the operation produced ore (starting ~1879) until about the middle of the 20^th century.  Since then the mine has produced collector specimens---on a sporadic basis.  At almost any rock show in Arizona, and in virtually every rock shop, specimens from the79 Mine may be found “for sale.”  Evidently thousands of mineral specimens have survived from the mine.  MinDat lists 74 valid minerals collected from the 79 Mine including classic butterscotch wulfenite and blue hemimorphite.  I had hoped that perhaps interested rockhounds could get into the area and examine the dumps, but alas, locked gates.

 

Structures at 79 Mine.  Photo courtesy of azgeology.azgs.az.gov.

The 79 Mine includes numerous surface works, the main incline, and in excess of 3000 m of tunnels and stopes.  The oldest rock exposed in the Hayden area is the Proterozoic Precambrian Mescal Formation of the Apache Group.  Above this unit are several thousand feet of Paleozoic rocks (Cambrian to Pennsylvanian).  The major ore body is over 300 meters long and a dozen meters wide and occurs as replacements in the Naco Limestone (Pennsylvanian) and a dike of rhyolite porphyry. The mineralization is most likely Laramide (late Cretaceous and early Tertiary) in age (Keith, 1972). 

In examining trays of minerals at one of the Tucson show venues I came across a single specimen containing, what appeared to the naked eye, as a clutch of metallic looking tiny fish eggs!  It was labeled murdochite (unknown to me at the time) from the 79 Mine so I scooped it up for $3.

The original discovery of murdochite, in the 1950s, was in rocks of the Mammoth-St. Anthony Mine not many miles south of the 79 Mine where “tiny black octahedra of murdochite are found on the surface of and embedded within plates of wulfenite and on the surface of crystals of fluorite” (Fahey, 1955). The mineral seems interesting to me since it contains both chlorine and bromine.  MinDat lists the chemical formula as PbCu₆O_8-x(Cl,Br)_2x where x<=0.5).

 

Mass of murdochite appearing as a druze on a limonite matrix.  However, it is not a druze but hundreds of tiny octahedral crystals.  Width of specimens ~1.3 cm.

Murdochite is usually black in color, a metallic black, with a metallic to submetallic luster; however, the crystal faces reflect light quite nicely and appear adamantine. The mass of “fish eggs” is actually a mass of tiny octahedral or cubic octahedral crystals.  Hardness is rated at ~4 (Mohs) and when rubbed on an unglazed porcelain plate, murdochite gives off a black streak.  Like other metallic luster minerals, murdochite is opaque.

 

Photomicrograph mass of minute, black crystals of murdochite (M), and a crystal of hemimorphite (H). The crystal faces of  these tiny crystals reflect light from the camera. Width of photomicrograph ~5 mm.

Murdochite is a secondary mineral found in the oxidized zones of copper-lead deposits. According to MinDat the primary hypozone lead mineral at the 79 Mine seems to be galena and the oxidized zone includes secondary lead minerals such as cerussite and anglesite (Eastlick, 1968).  The copper primary minerals include the sulfide chalcopyrite and perhaps it provided copper for the several secondary minerals.  Since secondary murdochite includes both copper and lead I suppose the metals must/might have oxidized from solutions passing through this sulfide.  But again, ore mineralogy is certainly not my forte!

And one final note, synthetic metallic oxides whose compositions are inspired by the structure of murdochite “exhibit interesting resistive properties which sound for the possible onset of superconductivity near room temperature. (Djurek and others, 1990).

                                         REFERENCES CITED

Djurek, D., V. Manojlovic, Z. Medunic, N. Martinic, Paljevic, 1990,    Cu-Pb-Ag-O system as a possible superconductor at T>200 K: Journal of the Less Common Metals, v.164-165, pt. 2.

Fahey, J.J., 1955, Murdochite, a new copper lead oxide mineral: American Mineralogist: v. 40.

Keith, S.B., 1972, Mineralogy and paragenesis of the 79 Mine lead-zinc-copper deposit: Mineralogical Record v. 3.