During my recent trip to the AFMS-RMFMS show in
Tulsa, Oklahoma (Blog July 21, 2014) I was on a
quest to secure additional specimens that may contain nickel, especially
pentlandite, an iron-nickel sulfide [(FeNi)9S8. Alas, I could not locate a dealer with specimens. I suppose that a road trip to Sudbury might
be in order since pentlandite was/is the major ore producing mineral at the
major nickel mines (see Blog July 8, 2014 ).
Pentlandite is often closely associated with
pyrrhotite [Fe1-xS] so the best thing was to pick up a specimen of this
iron sulfide---none in Tulsa but several at Ackley’s in Colorado Springs. My two particular specimens were not
collected at Sudbury (where most pyrrhotite seems massive) but from the Santa
Eulalia District, Mun. de Aquilies Serdan, Chiluahua, Mexico (no exact mine listed). Santa Eulalia has been an amazing producer of
a wide array of specimen minerals and the best that I can tell is mine were
collected in the 1970s. The initial
mining for silver started in the early 1700s and lead and zinc were added
later. I am uncertain about the current
status of the mines. What I do know is
that mineral shows and rock shops are full of magnificent specimens of minerals
gleaned from the rocks at Santa Eulalia and, these rocks produce some of the
world’s finest specimens of pyrrhotite.
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Side view of large pyrrhotite crystals. Length of top crystal ~2 cm.
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Top view of a large hexagonal-shaped pyrrhotite
crystal. Width ~1.1 cm.
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Although, the Santa Eulalia mines do not produce
nickel and pentlandite is lacking, my specimens of pyrrhotite are really quite
impressive, and rather strange—at least to a muddler like me. For starters, although pyrrhotite is simply
called iron sulfide, it has a variable amount iron present and is listed as Fe1-XS. Pure iron sulfide (FeS where x in the formula
is 0---50 atomic percent iron) is a mineral called troilite that is quite rare
on Earth but seems common in meteorites. There seems to be a complete solid
solution between troilite and “typical” pyrrhotite where x in the formula is .2---44.9
atomic percent iron (Klein, 2002). That
indicates the pyrrhotite has about 20% vacancy of iron in the crystal
structure. For reasons that seem unclear
to my mineralogical mind, troilite is non-magnetic while pyrrhotite is
magnetic. Perhaps not as much as
magnetite but never-the-less, magnetic and easily attracts a magnet. In the solid solution series, the magnetism
then varies with the iron deficiency (more iron, less magnetism) but if heated
to ~320OC all magnetism disappears.
Another interesting fact is that “good”
mineralogists can tell the difference between: 1) low-temperature pyrrhotite
where monoclinic symmetry is stable from 0 to ~254O C; 2)
high-temperature pyrrhotite with stability from 1190OC (melting
point) to ~400OC and stable hexagonal symmetry; and 3) mixed
monoclinic-hexagonal symmetry between 254OC and 400OC
(Klein, 2002). I am not a “good”
mineralogist!
Nice crystals of pyrrhotite are brassy to a dark
brown in color. They have a metallic
luster and an almost black streak on unglazed porcelain. Hardness seems to range on either side of 4
(Mohs). Crystals on my specimens are tabular in cross section but hexagonal
when viewed down the C axis. However, a
portion of the specimens are composed of a massive sulfide, evidently
pyrrhotite since it also is magnetic.
Upon closer examination, small gemmy quartz crystals, calcite, galena, perhaps
fluorite and a few brownish siderite rosettes may be observed.
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Tiny gemmy and terminated crystals of quartz, a “top”
view of a pyrrhotite crystal and a cross sectional view of the “stacks.” Length of large quartz crystal ~5 mm.
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Mixed in with the pyrrhotite are several cubes
of shiny galena (a common associate) with what appears to be a “clear” cube of
?fluorite. Width of galena cube ~2 mm.
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Two tiny siderite rosettes situated on white calcite
below a cross sectional view of pyrrhotite. Width of each rosette ~2 mm.
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Pyrrhotite is usually associated with igneous rocks,
some contact metamorphic zones and even in a few pegmatites. It is extensively mined, as at Sudbury, but
only for the associated minerals rich in things like nickel, copper and
platinum.
In Colorado, Eckel and others (1997) reported that
“pyrrhotite is widespread…..but nowhere very abundant.” I find the most interesting occurrence is in
the Green River Formation (early Tertiary) in the Piceance Basin of western
Colorado. The Green River is a generally
a lacustrine (lake) deposit but the oil shale beds contain crystals of pyrite,
marcasite and pyrrhotite. I need to
check that out!
REFERENCE CITED
Eckel, E. B., and others, 1997, Minerals of Colorado: Fulcrum Publishing and friends of Mineralogy--Colorado Chapter.
Klein, C., 2002, The 22nd edition of the Manual of Mineral Science (after James D. Dana): John Wiley and Sons, inc.