TUCSON 2022 MINERAL CITY: CINNABAR AND GIBBSITE

In my last Post on the Tucson Mineral City Shows I focused on Shannon Family Minerals.  Today I want to point out some of the other selling venues in Mineral City.  In fact, there are several shops that are officially part of Mineral City; however, there are others that are located in the Mineral City area but do not seem affiliated with Mineral City! For example, Barlows Gems have their own permanent location on Lester Street immediately west of “official” Mineral City.  Named the The Rock Yard, this year Barlows had an amazing selection of chrysocolla (hydrated copper silicate) collected from the Bagdad Mine, Eureka Mining District, Yavapai County, Arizona. A beautiful sky-blue color, large pieces from the mineare no longer available (according to a salesperson).

 

The Rock Yard in front of Barlows retail store.

Polished blue chrysocolla in specimen trays.

A very large (?24 inches) polished chrysocolla specimen.

Dennis Beals, a dealer in specimens from Mexico, moved from the downtown hotel district to Mineral City.  Dennis is from the Denver area and a regular in the Colorado show circuit.  As usual, he regaled me with exciting yarns about collecting trips south of the border.  I was impressed with a specimen of calcite with cinnabar collected from the Cocineros Mine in the Santa Eulalia District, Chihuahua, Mexico, so plunked down my monthly allowance. I can’t find out much information about the mine except that it has produced mercury—how much?  When? 

Cinnabar and calcite, Cocineros Mine.  Width FOV ~1.6 cm.
 

   
Don't drink the tap water in rural Mexico so the next best choice is mescal or tequila!

Vendor banners are the big rage.

I spent about three days wandering around the shops in Mineral City and viewed a large number of interesting and beautiful specimens.  Wendel Minerals had a nice case of varied minerals and a nifty specimen of gypsum collected from the Cavnic Mine in Romania.

Wendy’s Minerals had a very nice selection of blue gibbsite collected from the Hongsheke bauxite deposit, Yanshan County, Wenshan, Yunnan, China. I could not afford the $250-$300 specimens but was able to purchase a nice thumbnail. Gibbsite is an aluminum hydroxide [Al(OH)₃] and is one of the three major minerals that make up the rock bauxite, the others being the aluminum hydroxide minerals diaspore and boehmite, plus goethite and hematite (both iron oxides), kaolinite (an aluminum clay), and perhaps a small amount of the titanium minerals anatase and ilmenite.  Actually, bauxite is not a very attractive rock and is usually tan or reddish brown in color with a very dull luster. The formation of bauxite is a quite complex process and is usually classified into two types: 1) carbonate bauxites formed by lateritic weathering of limestone and dolomites leaving behind aluminum rich clays; and 2) the more common lateritic bauxite where lateritic weathering attacks aluminum rich igneous rocks and clays.  In this process there is dissolution of the aluminum-rich clay mineral [Al₂(OH)₄Si₂O₅] kaolinite, where silica is removed, and gibbsite is formed. The problem I have is trying to identify gibbsite in specimens of bauxite (usually 40% to 60%) is that most of the bauxite I have collected in Arkansas, or have seen from other locations, is that nondescript tan to reddish brown color with the component minerals seemingly “mixed up”.  

Pisolitic bauxite specimen from Pulaski County, Arkansas collected late 1960’s. Width ~6.5 cm.

Isolated specimens of gibbsite are found in a variety of colors from white, gray, green, reddish white, blue, and non-colored (clear). The latter variety has a vitreous to sub-vitreous luster, is transparent to translucent, and often forms prismatic Monoclinic crystals.  The colored and massive material (from impurities) have lusters ranging from pearly to earthy/dull. There are some specimens, like mine, that appear nodular but under magnification one can observe vitreous to subvitreous lightly colored (blue in this case), translucent, crystals and nodules. 

w

Gibbsite nodules and crystals.  Width FOV ~1.6 cm.

Photomicrograph of above.

The atomic structure of gibbsite is composed of octahedra (an aluminum ion bonded to six hydroxide groups) that are loosely stacked and weakly bonded; therefore, gibbsite is quite soft (~2.5 Mohs). Interesting, the soft gibbsite atomic structure is identical to the structure of very hard [9 Mohs] corundum.  The difference is that oxygen replaces hydroxide in the chemical make composition [Al₂O₃] and therefore the stacking arrangement of octahedra is well bonded.

One of the nagging questions that I have about gibbsite is---what causes the blue color in some specimens?  I have been unable to locate an answer.  Could the color be similar to blue corundum where impurities of iron and titanium produce several shades of blue sapphire? Or perhaps impurities of iron in the mineral results in the reduction of ferrous iron (3+) to ferric iron (2+) and perhaps the latter form is blue (I am way above my pay grade here, but this guess was based upon a blue color known a ferric blue or even Prussian Blue.  Far out of my league). One of life’s persistent questions.

As for the location of blue gibbsite, China has a number of bauxite deposits, and hence large mines, in Yunnan Province—all seem to have a similar origin: 1) weathering and accumulation of aluminum bearing minerals from “basement rising” rocks; 2) desilication and laterization of the aluminum bearing minerals; and 3) aluminum clay enrichment (Shi and others, 2007).  Since gibbsite is an important constituent of bauxite it evidently is common, or at least collectible, in these mines.

 

REFERENCES CITED

Shi, Zhu-huan, Dong, Jia-long,Yang, Song, 2007: https://en.cnki.com.cn/Article_en/CJFDTOTAL-KCYD200703012.htm