SKUTTERUDITE: COBALT-NICKEL ARSENIDE

I have been interested in the mineral skutterudite since my mineralogy class a number of years ago.  It was a bright shiny mineral with nice crystals, and the name---who could not appreciate a name like skutterudite?  It just rolls off the tongue!  Several years later I drove through the nickel mining area of Sudbury, Ontario, and was amazed at what fumes from a nickel smelter could do to living organisms.  But, at a museum I noticed a beautiful specimen of cobalt-nickel arsenide—skutterudite—and decided that someday I would have some of those shiny crystals!  That day came on Friday when I visited the Spring Colorado Mineral and Fossil Show in Denver.

Specimen of skutterudite from Morocco with nice octahedral crystals on fresh surface. The majority of the surface, bottom and sides of specimen, is massive and granular.  White mineral is calcite.  Specimen is difficult to photograph as bright metallic luster reflects light.  Length of specimen ~4 cm.

Skutterudite [(Co,Fe,Ni)As_2-3] has a very metallic luster, sort of a silver to tin-white color, and a hardness of 6 (Mohs) or a little less.  If crystals are present they are usually cubic or octahedral (Isometric) but often specimens are massive or granular.  The mineral is a major ore of cobalt and nickel.

The specimen I purchased came from the 12 Irhtem Mine in the Bou Azzer Mining District of the Anti-Atlas Mountains of southern Morocco, an area “famous” for producing the world’s best specimens of skutterite (and erythrite, roselite, talmessite, wendwilsonite, and gersdoffite). The Bou Azzer skutterudite has a fairly high cobalt content (12%-18.5%) with nickel coming in at 7%-7.5% and iron at 1.4%-8.85%.  In addition, the skutterudite has an unusually high gold content of 120 grams per ton.  It is interesting to note that the local population (Berbers) knew about the toxicity of the outcropping arsenates long before the onset of commercial mining—they used it for insect control and rat poison.

Photomicrograph of skutterudite crystals each ~ 2 mm.

Serpentinization (formation of serpentine group minerals by low temperature metamorphism) and weathering processes of the nickel-cobalt ores begin sometime in the Precambrian (prior to 800 Ma) but may have reached a peak during an orogenic volcanic episode about 550 Ma.  The process finally reached an end during an intense deformation and faulting about 250 Ma.  The highest grade ores are always associated with the serpentine group minerals, and found in areas of intense deformation.

Nickel oxidizes and corrodes quite slowly and therefore is commonly used as a plating agent, and in alloys with other metals (commonly with steel as in stainless steel).  Cobalt is also used in alloys, commonly to strengthen steel.  However, the most people know about “cobalt blue” where cobalt silicate imparts a beautiful blue color to glass, ceramics, and paint.

Skutterudite (cobalt-rich) also is in solid solution with choanthite (nickel-rich) and smaltite (intermediate).  It is my understanding that identification of hand specimens by physical appearance is quite difficult.

Information about mining at Bou Azzer was gleaned from: www.blnz.com/news/2008/05/14/Famous_mineral_localities_AZZER_MOROCCO_9115.html.

A NICE BLUE BORON MINERAL: DUMORTIERITE

At one of the rock shows in Arizona this spring I saw a beautiful cabochon labeled “Blue Quartz” with attributes of being able to stimulate new and better relationships, and to remove personal fear of others.  Although I am far from a mineralogist, something didn’t look quite right to me--quartz it was not. The gentleman selling the cab did volunteer that that the original pre-cab rock came from an old collection that in turn was garnered from “a clip mine near Yuma”.  He was uncertain what a clip mine was!

 

Top section of dumortierite slab.  The pyrophyllite? seems stained with iron.  The upper portion is “fresh” dumortierite from a “break”.  The slab is ~ 12 cm. in length.

I inquired about “blue quartz’ later on in the month at a dusty little rock shop.  Wow, the proprietor took me over to a beer flat and showed me slabs of blue material and told me it was a local mineral called dumortierite---and came from the Clip Mine down near Yuma in southern Azizona.  Serendipity!  So, I purchased a nice slab of the mineral complete with some wonderful “blue markings”.

 

Reverse of above photo; slab is sawed.  Note that I am uncertain if both shades of blue, light and dark, represent dumortierite, or if they are two different minerals.

Dumortierite is a a rather uncommon aluminum borosilicate, (Al.Fe)₇BO₃(SiO₄)₃O₃.  Although it may form nice slender crystals, most of the time the mineral is composed of  fibrous aggregates that generally are blue to violet in color (although green, pink, brown and green colors sometimes appear).  Dumortierite is about as hard as quartz (7 Mohs), has a vitreous luster, and occurs both in metamorphic and igneous rocks (aluminum and boron rich).  The rock shop proprietor told me that polished specimens/slabs are commonly observed in southern Arizona and adjacent California but seem rare elsewhere.

Photomicrograph of slab—note reflected parallel saw mark.  White mineral is quartz. Width of blue band is ~ 1 cm.  

The specimen that I purchased is a mixture of dumortierite, quartz, a soft clay-like mineral (probably pyrophyllite) and some other unidentified minerals. In places the pyrophyllite? on the slab seems covered by iron staining.

As for the Clip Mine, www.mindat.com noted it is “a former underground Pb-Ag-V-Fe-Mn-Sr-Baryte-Fluorspar-Cu (Cl-Br) mine…located about 28 miles north of Yuma, Arizona… Mineralization is a linear ore body with argentiferous, oxidized lead minerals with silver chloride and bromide, vanadinite, and some malachite in a gangue of ferruginous and manganiferous calcite, quartz, fluorite, barite, pyrolusite, iron oxides, celestine (?), gouge and brecciated wall rock, in a lensing vein in a fault zone cutting Tertiary andesitic to dacitic flows, tuffs and breccias…It was mined mainly in 1883 to 1887 and to a lesser degree in 1925 through 1929… Ag/T. Silver produced was more than $1,000,000 (period values) and an additional 7,000 oz. Ag (1928-1929).”

I am going to leave my specimen of dumortierite right here on my desk since it has a strong ability to aid intellectual activity within the brain.... and will assist [me]to develop enhanced mental abilities (Healing Crystals for You, 2013).  I certainly need all the help that I can get.