Wednesday, July 22, 2015

A Truly Great Overlooked Maryland Locality

Deserving  serious recognition in Maryland's mineralogical circles for the specimens it once yielded is the defunct Farmers Cooperative Limestone Quarry near the crossroads of New London in Frederick County. The quarry, which produced a dolostone primarily of Wakefield Marble laced with phyllite, ceased operations in 1973. 

The locality found its way to Mindat  thanks to Dr. William Cordua,  now retired  as Professor of Geology at the University of Wisconsin. Over the past 30 years, Dr. Cordua has written more than 50 professional publications, most relating to Wisconsin's geology and/or mineralogy. Among them is  a compendium of Wisconsin species that appeared in  Rocks and Minerals. More thorough is the database of Wisconsin minerals he maintains for the Wisconsin Geological and Natural History Survey. Now a member of Mindat's Management Group, he has enhanced that site's listings for species and/or localities in nine other states, including  Maryland's  Farmers Cooperative Limestone Quarry. Dr. Cordua collected there between 1967 and 1969 while  a geology student living near Washington DC.

He recently decided to  "de-acquisition" some of his collection.  Included were the specimens he collected at Farmers Cooperative,  His wish was for them to “wind up with people who appreciate Maryland minerals.”

The suite appears worthy of consideration as one the most interesting and significant Maryland mineralogical finds in last 50 years. We are not aware of a more aesthetically pleasing Maryland-collected aurichalcite specimen  than the one pictured in our title image. Somewhat visually drowned  out  by virtue of colorless transparency are numerous well defined accompanying hemimorphite crystals. The image at right better captures similar crystals from  another rock in the suite. As with the aurichalcite, we are unaware of a better example of what little Maryland hemimorphite is known to exist.

None of the specimens  in the suite amazed us more than the
transparent green sphalerite crystals on calcite pictured at left. Of obvious gem quality, these crystals form an aggregate extending about  8 centimeters. 

Also significant and pictured at right is green/gray botryoidal crystalline smithsonite inside a 6.5 millimeter vug at a contact point with quartz. It is in a rock richly graced with sparkling blebs of galena. Interestingly, galena is not one of the more abundant species among the primary ore minerals from Farmers Cooperative. Far more common are sphalerite, bornite, and chalcopyrite.

 All  three aforementioned sulfides are ubiquitous in thin ore filled veins occasionally running through the Wakefield Marble for which the quarry was mined. Less common and found separately, were crystals of chalcopyrite, rare in Maryland, especially in association with calcite. The several examples that Dr. Cordua collected  rested on marble topped by drusy calcite, as pictured at left..

Of particular interest is  the specimen shown in the photomicrograph at right. The main crystal is goethite pseudomorph after chalcopyrite with epitaxial prisms of malachite. As most of the malachite from  the quarry was of  was of a darker green hue, Dr. Cordua had originally suspected that these beautiful crystals could be dioptase.

Farmers Cooperative Limestone Quarry is not mentioned in The Natural History Society of Maryland's 1940  Minerals of Maryland, by Ostrander and Price. We suspect the quarry may not  have existed then. However, it is one of but two Frederick County localities mentioned in Maryland Geological Survey's 1981 Minerals of the Washington, DC Area by
Lawrence Bernstein. Among minerals observed in Dr. Cordua's suite  that we have not covered but are are listed in that publication are baryte, hematite, manganese oxides (pyrolusite) as dendrites, and chlorite. Bernstein also noted calcite crystals, but may not have been aware that some of the smaller ones werer as beautiful as the twins  in the image at left, which  Dr. Cordua shot.

Bernstein did not mention hydrozincite, It occurs in white crusts that are less than remarkable to observe until placed under shortwave ultraviolet light as shown  at right.

Minerals of the Washington DC Area  does  mentions gold, cuprite, linarite, and rosasite, all on the basis of "oral communication." The late Herb Corbett provided the oral communication regarding rosasite, which he described as "crusts of tiny green crystals."  However, an x-ray analysis that Dr. Cordua submitted.suggests that this material was aurichalcite, which the book did not mention.

Another publication with particularly interesting information regarding the geology of Farmers Cooperative is  Heyl and Pearre's 72 page Copper, Zinc, Lead, Iron, Cobalt, and Barite Deposits in the Piedmont Upland of Maryland, published in 1965 by the  Maryland Geological Survey. Minerals it names from Farmers Cooperative include are limited to  sphalerite, galena, chalcopyrite "and their oxidation products."  These "sulfides, Heyl and Pearre noted, "occur as replacements of the (Wakefield) marble at the contact of the adjacent Ijamsville Phyllite, and in thin replacement veins, bunches and stringers within the marble band."  The veins, they added, are similar to those at the New London (copper) Mine.

It is intriguing how many species species included in Dr. Cordua's suite have been reported  at three nearby long defunct and grown or built over localities: the aforementioned New London Copper Mine; the Unionville Zinc Mine; and the the Mountain View Lead Mine.. All of these localities, along with Farmers Cooperative Limestone Quarry, are  located within a few miles of each other. The ore and collectible  minerals in Dr. Cordua's  suite comprise a significant partial composite of such species,

In a recent email, Dr. Cordua contributed some updated thoughts:.
The similarity in mineralogy at Mountain View and so forth - distinctive stuff like the green sphalerite - I think prove a commonality in origin to the deposits.  Not sure if the formation names haven't changed as a consequence of subsequent remapping. Certainly their interpretation has. I recall plate tectonics just coming in when I was an undergraduate. The Piedmont is certainly a mess of interlocking terranes, likely hacked up now by major faults but, hey, that made for the diverse mineralogy of the state.
None of the literature mentioned thus far provides as much detail as Dr. Cordua's  field notes taken when he collected these specimens.  In addition to species thus far described, they cite manganoan calcite, chalcocite, tenorite (var.) melaconite) covellite, siderite, and celestine.  The notes suggest varying levels of uncertainty regarding  a presence of native sulfur as well as secondary lead minerals  anglesite, cerussite, all known to have been found in microscopic quantities at the nearby Mountain View Lead Mine.

Today, what is left of the Farmers Cooperative Limestone Quarry is filled with water and posted with a no trespassing sign. The remains of an old wooden building still stand, and evidence of trenches and a small foundation are discernible. We saw no evidence of any remaining dumps, especially through the thick midsummer vegetation,

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