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.