PEACOCK ORE; BORNITE OR CHALCOPYRITE?

Labeled Peacock Ore: Missouri. This specimen is chalcopyrite that has been subjected to an acid bath.  Width ~2.5 cm.

Labeled Peacock Ore: Mexico.  Width: ~2.0 cm.  Chalcopyrite and quartz.

I have always found it interesting that in virtually every rock/mineral show, in most rock shops, and in many curio/souvenir shops, especially in the western U.S, visitors will easily locate specimens of “Peacock Ore” for sale.  In most, but not all, instances vendors will simply market the specimens, at a cheap price, to buyers.  Upon request most sellers will explain that peacock ore is a copper mineral while others might state the ore is a variety of bornite or perhaps chalcopyrite.  Only the most honest proprietors will explain to prospective buyers the history behind peacock ore.  It is very inexpensive and the iridescence is quite attractive so just buy it and take it home for the kids.

Chalcopyrite, Sweetwater Mine, Reynolds County, Missouri.  Width ~6.5 cm.

Chalcopyrite [CuFeS₂] is a copper iron sulfide that is often confused with the iron sulfide, pyrite [FeS₂].  Both are sort of a brassy yellow color and tarnish upon exposure to oxygen.  Chalcopyrite is much softer (3.5-4.0 Mohs) than pyrite (6.0-6.5).  Pyrite (Isometric Crystal System) often occurs as cubes but sometimes as octahedrons and pyritohedrons.  Chalcopyrite (Tetragonal Crystal System) crystals are often shaped like two wedges back-to-back (dispenoid).  Both pyrite and chalcopyrite can have striated crystal faces and also occur as botryoidal masses.  They also are found together but there is something about the color, tarnish and luster than distinguishes the two.

Chalcopyrite is a very common mineral and the major ore of copper occurring as massive mineral deposits to veins to disseminated crystals in the porphyry copper deposits so important in the U.S. 

Massive Bornite and chalcopyrite, Flambeau Mine, Rusk County, Wisconsin.  Width ~ 4.4 cm.

Labeled Bornite (massive) and Galena: Montana.  Width: ~4.2 cm.

The sulfide bornite [Cu₅FeS₄] is also an important copper ore (close to 65% by mass), especially in the copper porphyry deposits.  It is Orthorhombic but commonly is massive and/or granular in habit.  It does not display the brassy color of pyrite or chalcopyrite until tarnished but is a mottled brown/black/purple to copper-red but often is mixed with chalcopyrite. It easily tarnishes upon exposure to air and results in purple/blue iridescence on some areas.  In fact, it is often called “peacock ore!”

So, what are the specimens sold as iridescence peacock ore seen in the markets?  Well, mostly it is chalcopyrite that has been in an acid bath! Sometimes chunks of very iridescent bornite may find their place in sales but essentially peacock ore is altered chalcopyrite: 

CuFeS₂ + 2HCl + 5/4O₂-->CuC_l2 + FeOOH + 1/2H₂O +2S. 

So, can an “amateur” turn chalcopyrite into peacock ore?  I found it interesting that an entire forum on “turning copper pyrite into peacock ore” may be found on www.mindat.com. Statements included:  the trick consists of not rinsing the specimens after treatment but to let them dry; the best results I have had is using plain white vinegar, acetic acid, with a little scrap copper, wire or copper pipe, thrown in. I usually let it set outside covered in the sun for a week or until the color has changed. Pull them out and let dry without rinsing. You can rinse after drying if needed; household bleach (sodium hypochlorite) will also do it (in place of acid).

One of my future projects is to locate some chalcopyrite, dump it in some acid, and see if I can produce some non-realistic looking peacock ore.  Well, at least “fake” peacock ore.  Or is it “real” peacock ore?  If possible to produce I will stick it on my shelf besides the non-realistic looking (dyed) agate.  Who will know?

Some people think that the truth can be hidden with a little cover-up and decoration.  But as time goes by, what is true is revealed and what is fake fades away.
           Ismail Haniyeh

CARMINITE: A NICE RED ARSENATE

Specimen of carminite on druzy quartz.  Width of specimen ~3.7 cm.  I am certain that other minerals are present (see photomicrographs below); however, my knowledge certainly does not allow much identification.

 Among the specimens tossed to me (see posting on Duftite) by Mr. Rockhounding the Rockies (he of the large crystals) was one that had a druzy quartz matrix with “blobs” of this gorgeous tiny red mineral.  Who could turn down such a beautiful-colored sample?  Since this suite of specimens, including the duftite, came from the oxidation zone of some lead deposits, I begin to check the references listing possible associated minerals that were red in color, and up popped carminite [PbFe₂(AsO₄)₂(OH)₂, a lead iron arsenate and a fairly uncommon alteration product of arsenopyrite, an iron arsenic sulfide.  In fact, arsenopyrite [FeAsS] is probably the most common arsenic mineral and in many instances represents a major cause of pollution by acid mine drainage where the arsenic (As) and sulfur (S) ions are released into the water.  I certainly wish that I had remembered more of my chemistry class to better understand what happens in the oxidation zone of sulfide ores so that the silver colored and shiny crystals of arsenopyrite are transformed into these carmine red crystals and encrustations. This identification may be a little suspect but to me the color seems very diagnostic.  I believe, but would not bet the farm on it, that the specimen came from the Ojuela Mine, Mapimi, Durango, Mexico. 

Photomicrograph showing tiny crystalline carminite. FOV ~.8 cm.

Photomicrograph showing tiny crystalline carminite. FOV ~1.0 cm.

 

Nature composes some of her loveliest poems for the microscope…

          Theodore Roszak 

QUARTZ: RED AND AUTHIGENIC

Length of crystal ~1.7 cm

At a small venue in Tucson I stumbled upon an interesting specimen labeled “red quartz.”  I immediately shelled out a couple of bucks since it was a doubly terminated, nice floater crystal, and it was red.  It seemed obvious the crystal was authigenic quartz since it had sharp hexagonal crystal faces with simple terminations.  It “looked” essentially identical to the authigenic quartz known as Pecos Diamonds (see Blog posting July 11, 2013) except for the red color.  The red specimen came from a well-known collecting locality at Domeño, Chelva, Valencia, Valencian Community, Spain (www.mindat.org).  Here the Triassic Keuper Formation produces floater red quartz crystals from gypsum bearing marls. 

The Triassic Period (~252-201 Ma) was named for rocks exposed in Germany where there is a tripartite division:  the Bunter is mostly early Triassic with an arbitrary boundary with the Late Permian (mostly terrestrial), the Muschelkalk is Middle Triassic (inland marine with some restricted circulation basins) while the Keuper is Late Triassic (lots of deposition in evaporitic basins with minor marine incursions).  Much of the Keuper deposition is related to basins created by the initial breakup of supercontinent Pangaea---the future North America rifting apart from the future Europe (Laurasia).  Rocks of the Keuper (readers will notice that Keuper is used as a formation/group name as well as a generic name for the Late Triassic) are exposed over much of Europe including the British Isles. The rocks also were extensively deformed by numerous Mesozoic and Tertiary orogenies.  The terrestrial sediments of the Keuper are well-known for producing dinosaurs (and numerous other fossils).

The red colored Spanish quartz is known as ferruginous quartz, Eisenkiesel, or locally Jacintos de Compostela.  The latter name is often used by agencies selling specimens.  The red color is due to inclusions of hematite scattered evenly throughout the crystals and is different from other red-colored crystals created by surficial hematite.  There are very few localities around the world that have produced Eisenkiesel and the Spanish locality is certainly the best known.

So, if readers are interested in collecting interesting and different specimens of quartz look for Jacintos de Compostelas at the next rock show.