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,