Cummingtonite, Amphibole-Anthophyllite, or Bronzite?

The above image, photographed by Smithsonian National Museum of Natural History volunteer Ken Larsen, and identified as the amphibole mineral cummingtonite, is from among several dozen selected Maryland-collected mineral specimens that the museum's mineral sciences department has been kind enough to electronically transmit.

The only available locality information simply states "Maryland." To the best of my knowledge, the only Maryland locality where this relatively obscure mineral has ever been reported was the Bare Hills Copper Mine in Baltimore County. The occurrence was noted in by authors Charles W. Ostrander and William E. Price, Jr. in Minerals of Maryland, the definitive 1940 publication by the Natural History Society of Maryland. The Smithsonian image particularly intrigued me because as a child in the 1950's I collected and am still holding similar material from the Bare Hills Copper Mine dumps, which back then were open and assessible along the north side of Smith Avenue at what has since become the main entrance to the Bonnie Ridge Apartments.

Based on texts available during my childhood, I'd concluded my find to have been the better known species amphibole-anthophyllite, which Ostrander and Price had also reported from the Bare Hills Copper Mine. And so it was until about two years ago when I obtained the specimen pictured at right from the late Larry Krause. Its 1939 label proclaimed it to be amphibole anthophyllite and bore the stamp of none other than the late Charles Ostrander. Despite visual similarities, the colours appeared too dark, and the blades too long for this to be the same species as that in the Smithsonian's as well as my own collection.

For clarification, I looked to Mindat, which, even though the two are named as separate species in Minerals of Maryland, identified "cummingtonite" as a "synonym for amphibole anthophyllite." Further confusion followed: Another species also reported by Ostrander and Price from the Bare Hills Copper Mine was bronzite, a variety of the pyroxene mineral enstatite. Interestingly, a photograph of bronzite copyrighted by Dr. Rob Lavinsky, is but one of two images on Mindat of minerals from the Bare Hills Copper Mine. The only other is my own more recent image of chalcotrichite that was featured in our post of March 8, here at Mineral Bliss. As best as I can ascertain, the material in Dr. Lavinsky's photograph matches our title image from the Smithsonian as well as the piece I collected 50 years ago.

I am certainly not one to be questioning identifications from the Smithsonian, Charles W. Ostrander, Dr. Robert Lavinsky, or for that matter Mindat. The dilemma begs for analysis, albeit a chore likely to rank low amidst the priorities of any lab that might be available to undertake it.

Better news is that our next post features a more timely identification quandary that led to the confirmed analysis of a new find of an unrelated species not previously reported from the Baltimore area. Bob Simonoff uncovered it this past March not more than a few miles from the locality for our amphibole-anthophyllite, cummingtonite, bronzite, or whatever it turns out to be was collected. Stay tuned.