Tuesday, November 25, 2014

An Historic and Obscure Zeolite Occurrence at Monkton, Maryland

Pictured above is an historic specimen displaying numerous microscopic chabazite and heulandite crystals at a contact point on a slab of Baltimore gneiss. Pursuant to the labels, it was collected at Little Falls near Monkton in Baltimore County, Maryland. The orignial label is from from the collection--- or dealership--- of Germany's late Dr. August Krantz (1809-1872) and identifies the featured species to be haydenite and beaumontite, as chabazite and heulandite  respectively were referred to at the time. The specimen was later in the collection of Jeff Weissman, a renowned mineral photographer and rare species expert.
I acqurired it from John Betts with labels that go well beyond the call of duty to document the provenance. Though  not noted on the label, John also expressed a belief that the specimen could very likely have been collected from excavations near Monkton for the Northern Central Railroad line during the 1830's.

After ceasing operations in 1972, the former NCR rail line was converted into a popular rail/trail in 1984. It  is known today as the Torrey C. Brown Trail and crosses Little Falls about 1.7 miles north of Monkton just before it flows into the Gunpowder River  The Baltimore gneiss in which the chabazite and heulandite are present is indigenous to this specific area. It is more prevalent, however, farther south in Baltimore City and was quarried there extensively in the 19th century for building stone. Interestingly, the chabazite and heulandite appear on the gneiss in a manner that is visually similar to known historic specimens collected  at the Jones Falls Quarries. Some of these can be viewed on  Mindat.

Such historic specimens from Maryland localities of which no remnants still exist can be fascinating to those with interest in the regional mineralogy. For certain, we have much to learn from them. On the other hand, the localities attributed to them can become misleading when the names by which their localies were once known change and become forgotten or confused.

With this specimen, however, the documentation is sufficient to suggest that it is everything the labels claim. To the best of our knowledge, it could well be the only currently known occurrence of these two zeolite species to be reported from the Baltimore Gneiss in Baltimore County (as opposed to the City of Baltimore) or for that matter the only chabazite and/or heulandite specimens to be reported from anywhere in Baltimore County.