BOURNONITE: TUCSON GEM SHOW REPORT: TUCSON CITY CENTER

One of the larger show venues is the Arizona Mineral and Fossil Show centered at the Hotel Tucson City Center north of downtown.  There is a mixture of paleontological/fossil dealers (many in the Fossil Hall) but some scattered in the hotel rooms, high-end mineral dealers, numerous purveyors of nice jewelry, and a variety of smaller dealers selling all things rockhounds might be interested in purchasing.  There is a beautiful hotel courtyard pool with lunch tables scattered in the grass and paleo specimens (dinosaurs) peering at visitors.

Ammolite, a biogenic gemstone.  The ammonite is Placenticeras sp. from the Bearpaw Formation (Cretaceous).      

I enjoy the Paleo Hall as Glen Rockers from my old hometown of Ha in Alberta, Canada.ys, Kansas, always displays a number of fine Cretaceous specimens from the western Kansas chalk beds.  One of my past blogs discussed a fantastic specimen of amber, known locally as Jelenite, from the Dakota Formation (Cretaceous) in central Kansas (9/232012) .  Another specimen is a full-body mount of nice bill fish called Protosphyraena.  He told me that the specimen is about 80% complete, including the skull.  During my time playing in the chalk collectors often found the rostrum (the bill) that had broken from an unknown skull.  In viewing the photo below remember that the rostrum actually supported a ?cartilaginous covering.  The famed fossil collector Charles H. Sternberg postulated in 1917 that the front teeth projected in a forward direction and the bill and teeth were used as a ramming device on other fish or mosasaurs.  I think paleontologists remain uncertain about the use of the bill and teeth; however, Sternberg seemed correct about the projecting teeth.

Protosphyraena, a bill fish from the Cretaceous rocks of western Kansas.  Collected by Glen Rockers.

I was able to locate my mineral of the day for $4, bournonite [PbCuSbS₃], a lead copper, antimony sulfide (sometimes containing arsenic).  The crystals have a metallic luster and usually a steel gray (metallic) color.  Bournonite is a rather soft mineral, ~3 (Mohs), opaque, and sometimes fractures in subchonidal surfaces. The crystals of bournonite are usually short prismatic in nature and are deeply striated. They often form twins that repeat and then form “cogwheels” that resemble deeply striated hexagonal or cubic crystals.  In other instances the crystals are tabular, or even massive.  The twinned crystals seemed to be the best way to identify bournonite from similar looking enargite [Cu₃AsS₄].  Bournonite is the result of hydrothermal solutions and often associated with ore minerals such as galena (lead), tetrahedrite (copper/silver) and sphalerite (zinc). 

Older references, especially from England, often refer to bournonite as "wheel ore" due to its repeated twinning producing "cog wheels."  My specimen was collected at the Quiruvilca Mine (La Libertad Mine; ASARCO Mine), Quiruvilca District, Peru. According to MinDat.org the mine is "a copper-lead-zinc-silver-gold mine owned by ASARCO (80%)."  MinDat lists 37 valid minerals collected from the mine.
  

Bournonite.  Notice the two larger twinned crystals with deep reenterant grooves.  Length of upper large crystal ~6 mm.

Repeating crystals of bournonite forms a "cogwheel."  Figure courtesy of www.smorf.nl/ via www.minerals.net.

Dark silver-gray colored (metallic) bournonite.  Width of photo ~4 cm.

So, it was another successful day of admiring minerals, fossils and hunting for “cheap” mineral specimens.