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.
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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.
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Skutterudite [(Co,Fe,Ni)As2-3] 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.
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Photomicrograph of skutterudite crystals each ~ 2
mm.
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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.