Tuesday, March 8, 2011
Chalcotrichite at Bare Hills in 2011?
I confess to having in the past mistaken micro-organic materials for minerals and only hope that this post wont give some readers occasion to question my credibility. Regardless, no other field collecting find of my life ever excited me as much as the red fibers shown in the photomicrograph above. Could they be anything other than chalcotrichite? Unfortunately, what you see, which measured about a millimeter across, disappeared almost completely as I tried using my forefinger to attach a gummed white arrow to the matrix to note its presence. After recently collecting the specimen near Mt. Washington in Baltimore City just across the line from Baltimore County and discovering the apparent chalcotrichite in it two days later under the scope, to have accidentally trashed it horrified me. I'd like to think this wasn't really chalcotrichite. After all, the rock had bounced around unwrapped in a dirty knapsack for three days before it ever got near the scope, so how likely is it that my clumsy finger, while gently attempting to affix an arrow, would be enough to cause such crystals to vanish?
My initial inclination was to retain the original title I'd planned for this post: Field Collecting Malachite and Chalcopyrite in Baltimore City during March, 2011. Finding the respective specimens shown at left along the banks of a stream about ten yards east of Smith Avenue at the Baltimore City/County line was plenty worthy of a post. Less than a quarter mile to the northwest of where these pieces were collected, the little stream had passed under an entrance to the Bonnie Ridge Apartments. Until about 50 years ago, that entrance was the site of the only remaining dumps from the long covered over Bare Hills Copper Mine, which closed for the final time late in the 19th century.
The malachite probably formed from solutions between water in the stream and ores of copper over which the water flowed. The chalcopyrite, which was the primary ore that had been mined nearby, had obviously washed down to where I found it. With nowhere to park on Smith Avenue, I'd hiked in from a nearby parking lot. The area was overgrown enough to be treacherous, even with minimal early March vegetation. Where the ground wasn't covered with brambles, branches, or soil, most prevalent amongst the few visible rocks were the meta-gabbro and hornblende schist that years before had borne numerous ore veins. The malachite, however, was on a rock mostly comprised of feldspar. I discovered the chalcopyrite upon breaking into three pieces a brown rock that looked unlike the others present around the stream area. The apparent chalcotrichite later showed up under the scope near the edge of a cracked open face of one of those three chunks.
According to the 1940 Natural History of Maryland publication, Minerals of Maryland, by Ostrander and Price, species known to occur at the Bare Hills Copper mine were as follows: amphibole-anthophyllite, cummingtonite, actinolite, tremolite, octahedral magnetite in steatite, magnetite in large masses, malachite, azurite, radiated epidote, honblende crystals, calcite, milky quartz, talc, stilbite, laumontite, albite, chlorite, chalcopyrite, bornite, feldspar, garnet, chalcanthite, blue quartz, chalcocite, chrysocolla---and in the "Lee Collection," radiated actinolite and covellite. No mention was made of cuprite, much less of chalcotrichite, which is a variety of cuprite. To the best of my knowledge, chalcotrichite has never been reported in Maryland.
My first ten minutes of examining the chalcopyrite rocks under the scope revealed some amphibole rock, a bit of massive magnetite, and some plates that looked a bit like covellite but probably weren't. What had most interested me to this point was the crystal (shown at 40x) at right. Chalcocite or hornblende, perhaps?
Then I spied that blotch of red, zeroed in on it at 40x, photographed it, and showed the specimen to my wife. By now it was time for dinner through which I all but pinched myself to be certain this chalcotrichite thing wasn't a dream. After dinner, I excitedly returned to the mineral room and began my attempt to affix the gummed arrow near the little red spot that was barely discernable without a loupe.
After two tries at getting the arrow to stick, the little red spot was no longer visible. I spent the next hour repeatedly looking at the rock under the scope and also scanning the tabletop with my loupe. The only remaining trace of anything similar is pictured at left. The colour and thickness of that one little thread resemble to the hundreds that were there earlier. However, I question whether the apparent curvature could might be something other than a crystal. That would be less distressing to me than what I think happened. Unfortunately I'll probably never know.
My next move will be to further crack up the three chalcopyrite bearing pieces and examine them under the scope hoping to find more crystals. I'm neither optimistic about what that will reveal, nor enthusiastic about the prospect of trampling through those brambles again.