Smythite

Hidden for 50 years

Over at Mineral Matters (highly recommended if you like minerals), Steve Sorrell sometimes posts about his “hidden collection” — minerals in specimens that are concealed, unrecognized, overlooked, and/or not on the label. 

Here’s one of mine.

One of the fun things about having a LOT of rocks and minerals is that on a cold winter day I can explore and make “discoveries” in the comfort of my living room (actually my microscope is on the dining room table, but whatever). This chunk of rock from southern Indiana, geode-like but not really a geode, has some cavities that have barite (barium sulfate), partially encrusted with little pyrite crystals. I’ve known that since I collected it in 1969.

But one barite crystal has some tiny, quarter-millimeter platy inclusions that are not pyrite – they are most likely the iron-nickel sulfide smythite, which in 1969 I had certainly never heard of, but its type locality is near where I collected this piece. “Most likely,” because it could be pyrrhotite, iron sulfide, Fe_1-xS, which is visually identical to smythite. Both the black and golden-orange colors in the plates are consistent with either smythite or pyrrhotite, but I suspect (well, hope) this is smythite. Smythite is trigonal and pyrrhotite is monoclinic (the polytype from these rocks is) but often forms pseudohexagonal twins.

I “discovered” this smythite in 2019, fifty years after I collected it.

Collecting geodes from Mississippian (Lower Carboniferous) limestones in southern Indiana, 1971. Scans of cheap Instamatic slides.

Smythite, officially (Fe,Ni)_3+xS₄ (where x ≈ 0-0.3), was described in 1956 (so I could have known about it in 1969, if I had been paying attention) from the type locality in a roadcut on Highway 37 north of Bloomington, Indiana. I don’t know absolutely that I collected my specimen from the defined outcrop(s) of the type locality, but it was certainly very close to it. It was named by Erd and others (1956, J. of the American Chemical Society, 78:9, p. 2017) for Charles Henry Smyth, Jr. (1866-1937), Professor of Economic Geology at Princeton University. Smyth was one of the first to recognize that pyrrhotite occurs in sedimentary rocks.

Erd and others (1957, American Mineralogist 42:5-6, p. 309) used fluid inclusion studies to infer that the smythite crystallized with the surrounding calcite or barite at low pressures and temperatures (25° to 40° C; 77° to 104° F, something like some hot spring waters), probably from ground water percolating through the rocks long after they had lithified during the Mississippian period about 330 million years ago (but still probably millions of years ago).

Erd and others initially defined smythite as Fe₃S₄ but recognized that it had traces of nickel in it. Over time, it was redefined several times, including in 1972 when the formula was given as (Fe,Ni)₉S₁₁ (Taylor and Williams, 1972, American Mineralogist 57:11-12, p 1571). Bon and Rakovan (2011, 38^th Rochester Mineralogical Symposium Program and Abstracts, p. 10) referred to it as (Fe,Ni)₃S₄. I think the fractional value for iron and nickel in the official formula reflects variability in the iron and nickel content.

Smythite is probably included in calcite more often than not, but mine is in barite.