Carrollite and the Cobalt Sulphides of Carroll County, Maryland

Pictured from left to right: Carrollite from the Patapsco Mines near Finksburg, siegenite-carrollite from the Mineral Hill Mine in Louisville, and linnaeite from the dumps of the Springfield Mine near Sykesville. So state the labels in my personal collection. While the information to back up these identifications have sustenance, only the Mineral Hill siegenite-carrollite identification bears total certainty. For that matter, identification uncertainties plague even the origninal carrollite that derived its name from Carroll County, Maryland, where the Patapsco Mines receive credit as the type locality.

Carrollite, siegenite, and linnaeite belong to the linnaeite mineral group---or series. They occur along with the the more prevalent copper and iron minerals in veins throughout the shists, gneiss, and ultramafic rocks that geologists refer to as the Sykesville Formation. Described in Bulletin #28 of the Maryland Geological Survey, the Sykesville Formation extends "from the Baltimore-Carroll County Line about 2 1/2 miles south of Finksburg southwestward through Sykesville and thence across Howard County."

Carrollite is the most renowned mineral in the linnaeite series. Collectors everywhere treasure the magnificent octahedral crystals from Katanga in the Congo. Maryland Carrollite, though nowhere near so visibly spectacular, is difficult to come by on the market and likely to command top dollar wherever and whenever it appears. Any remote chances of field-collecting carrrollite at its type locality were removed by the folks who fenced off, posted, and then buried the last remaining dump from the Patapsco Mines beneath tons of heavy trash.

Questions have arisen over many years regarding whether the mineral discovered at the Patapsco Mines that became known as carrollite was not in fact linnaeite with copper impurities contributed by grains of associated chalcopyrite, chalcocite, and bornite unwittingly included in the grainy mix being tested. Johnny Johnsson's feature about the history of the Patapsco Mines and the discovery of carrollite, published during the summer of 1998 in the late Jay Lininger's Matrix Journal, states that "significant doubts linger as to whether carrollite actually exists in the Patapsco Mines where it was initially 'discovered." More than a century’s worth of testing Maryland's carrollite, siegenite, and linnaeite have demonstated extensive gradations in chemical composition amongst all three minerals, particularly with respect to copper content.


Let's return now to the specimens pictured above and the cases for their identification.

• Carrollite from the Patapsco Mines: Once part of the Neil Wintringham collection, I purchased this specimen for $77 at auction on eBay. The label naming Sykesville as the locality was probably a generalization. The presence in the rock of malachite (possibly brochantite) and chalcocite was typical at and probably unique to the Patapsco Mines (Finksburg, Wildesen, or Orchard). The grayish black chalcocite, in fact, would seem to have replaced much of what may once have been bright shiny silvery colored carrollite.
• Carrollite-siegenite from Mineral Hill Mine: The Mineral Hill Mine is the only locality where the composition of "carrollite" was ever found to include nickel along with cobalt and sulphur. I obtained the specimen from Fred Parker, a stickler about accurate identification, which he personally verifies based on x-ray diffraction and microanalysis. Interestingly, Joseph Vadjke once showed Parker an end-member Mineral Hill Mine carrollite specimen that had been analyzed in Czechoslovakia.
• Linnaeite from the Springfield Mine: I found this when breaking up a rock from a small rock pile adjacent to the foundation sitting uphill from the main Springfield dump. My confidence was boosted upon reading in Lawrence R. Bernstein's Minerals of the Washington D.C. Area of "cuprian" linnaeite in"silvery to pinkish-gray metallic masses" as the only cobalt bearing mineral named from the Springfield Mine. Upon showing the piece to Fred Parker a couple weeks later and being told: "Yes, that’s linnaeite," I became convinced.

Finally, we have not mentioned a fourth cobalt bearing mineral known to have occurred at these localities, namely cobaltiferous gahnite. Mineral Bliss has that story on the backburner.