Monday, July 28, 2014


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. 

Side view of large pyrrhotite crystals.  Length of top crystal ~2 cm.

Top view of a large hexagonal-shaped pyrrhotite crystal.  Width ~1.1 cm.
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.

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.

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.

Two tiny siderite rosettes situated on white calcite below a cross sectional view of pyrrhotite.  Width of each rosette ~2 mm.
 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!


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.