Tuesday, June 1, 2010

Deweylite Confusion

How I wish the Maryland Academy of Sciences still maintained its display of Maryland minerals on the third (or fourth) floor of Baltimore's Enoch Pratt Free Library. That was several decades before the Academy's move in 1980 to its Inner Harbor domicile, which to the best of my knowledge has yet to house any minerals from Maryland or anywhere else.

Right next to an enormous Montgomery County gold nugget embedded in a quartz boulder was an humongous chunk of Bare Hills deweylite that oddly enough fascinated me equally as much. Three closeup images of lesser Baltimore County deweylite pieces from my personal collection compose the title picture for this post. From left to right, their localities are the Bare Hills serpentine barrens along Falls Road less than a mile north of the city line, the former Dyer Quarry at Soldiers Delight, and the still active Blue Mount Trap Quarry north of Monkton. Wherever it is found, deweylite is never a species, but a combination of species that seem to vary not only from locality to locality, but according to whom you ask. All that appears to be certain is that serpentine is part of the equation.

The image at left graced the Maryland Minerals web site slide show until about a month ago. It disappeared pursuant to a lesson learned from John S. White, one that seemed to me a bit arcane for mention in our previous post. Even John's advice struck me as a bit convoluted at first, though ultimately it proved to be more concise than anything my subsequent research could uncover. "The best approach," he suggested, " is probably to label it Serpentine "Deweylite."'

Bernard and Hyrsl's Minerals and Their Localities lists Deweylite in italics with the following description: "Deweylite, synonym gymnite----A mixture of serpentine, stevensite or talc minerals, fine grained, yellow, green , red. As resinous crusts in serpentinites at-----(several localities). " Mindat refers to deweylite as "a mixture of various poorly ordered trioctahedral 1:1 and 2:1 layer silicates, mainly lizardite and stevensite. Mindat describes gymnite as both a synonym for antigorite and also an "obsolete name for an apparently amorphous antigorite," and it describes lizardite as a species "closely related to" antigorite and also chrysotile." Antigorite, lizardite, and chrysotile are all prominent members of the serpentine group.

A third definition of "gymnite" from Webster's Dictionary on the Internet proved to be the most interesting part of my research, referring to it as a "hydrous silicate of magnesium coming from Bare Hills, Maryland." Since Minerals and their Localities defines gymnite as a synonym for deweylite itself, and Mindat refers to it as a synonym for antigorite, a reasonable conclusion would be that antigorite is a major component of deweylite from Bare Hills. While our photographs of deweylite from Bare Hills (as well as two other Baltimore County localities) depict antigorite with a brownish colour that might appear far from typical, the species occurs in numerous colors, can range from fibrous to nodular, and has been referred to by a variety of names including , picrolite, and baltimorite. As for other species in the deweylite mix, a magnesite presence is obvious in each of the three Baltimore County specimens.

On the other hand, I've never encountered mention of lizardite from any of deweylite's three Baltimore County localities. At other localities reporting deweylite where lizardite is known to occur, I'd assume that the combination of species could be different. At least John S. White managed to keep things simple. Our Bare Hills "deweylite" image will soon reappear at Maryland Minerals web site with the label that he suggested: Serpentine "Deweylite."

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