Dennis Coskren and the Rarest of the Rare Earths

The 2 millimeter field of view microphotograph at left is of coskrenite. It was discovered by and is named after the mineralogist pictured above it, Dennis Coskren. At the Baltimore Mineral Society's 2006 Desautels Micromount Symposium, I obtained the piece in a trade with Dr. Coskren in exchange for a bjarebyite micromount that I'd just purchased from a dealer there.

This was my introduction to micromounting. I didn't even own a microscope yet and had shown up at the conference to observe what was going on and make a determination as to whether micromounting was a hobby that would be to my liking. An explanation of how a greener than bjarebyite novice got so quickly into wheeling and dealing with minerals of such complexity will be saved for a later post.

Coskrenite is a rare earth sulfate containing cerium, neodymium, and lanthanum. It is one of the first three naturally occurring rare-earth sulfates or oxalates known to sicence, a distinction it shares with levinsonite and zugshunstite. The type locality for all three of these minerals happens to be one of the best known and most popular hiking destinations in Tennessee's Great Smoky Mountain National Park, namely Alum Cave Bluff. It hardly matters that collecting here---or for that matter anywhere in the park---is forbidden. For one thing, crystals of the pertinent rare earth minerals are much too miniscule to observe with the naked eye, or even through a loupe. Probably less than a cumulative gram's worth of all three species is known to have been collected at Alum Cave Bluff or anywhere else. Even so, the three minerals are products of a geochemical process that actively occurs at a number of other spots in addition to Alum Cave Bluff. This suggests not only a reasonable possibility that they exist elsewhere, but also that they could renew themselves within a relatively short time frame.

Dr. Coskren has spent more than a dozen years studying every aspect of the mineralogy of Alum Cave Bluff. The fruits of his labors are encapsulated in the article "The Minerals of Alum Cave Bluff," which appeared in The Mineralogical Record, Volumne 31, March-April, 2000. This precisely documented treatise makes for a most interesting read about a fascinating mineralogical realm.

Aside from being rare and and interesting, many of the rare earth minerals occuring at Alum Cave Bluff make beautiful micromounts. To say they're difficult to come by is understatement. I'm aware of only two such pieces that have ever sold, both of them on eBay. The first was a micro-speck of levinsonite for which someone in Russia paid more than $500 at auction . The other was a coskrenite micromount that was purchased from an an eBay store by scientist on behalf of a university in Australia.

This all goes to say that however dear that bjarebyite mount, I couldn't be happier about this trade that turned me into a micromount enthusiast three years ago. The bounty it reaped bore in addition to coskrenite the levinsonite crystal pictured at right---all in the same piece. I keep looking and looking and looking through my scope. Then I refer to Mindat and can't keep myself from wondering whether one of those little pink crystals near the coskrenite couldn't possibly be zugshunstite.