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,

Getting Started in Mineralogy by Gemma Burns

Publisher's note:

Mineral Bliss always been receptive to posts written by guest writers who have something to say that's pertinent.  Gemma Burns, a freelance writer, submitted the first such article that we have received and chosen to publish.  We were impressed as to how she managed to compact the essentials to getting started in our hobby into an article of appropriate for a Mineral Bliss post. 

Mineralogy is a fascinating and rewarding area of interest. Not only is it very good for you in general [1] to have a hobby, but the study of mineralogy can vastly improve your general knowledge and view of the world [2]. However, if you’re just getting started up, it’s easy to make pitfalls, to head in the wrong direction, and to make ill-judged decisions through want of better knowledge. Here, therefore, is a short guide to getting started in mineralogy. There is of course no single surefire route to this, and lots of people get into this in lots of different ways, but these tips might help if you’re a bit uncertain!

A lot of mineralogy can be (and is) learned ‘on the job’ – plenty of people got into mineralogy by finding an interesting rock and experimenting with it. However, if you’re really serious about this science, you’d do well to familiarize yourself with the theoretical basics. There are a wealth of books, magazines, journals [3] and websites out there which can help you to increase your store of rock-based knowledge (including our own website!). If you’re just getting started, it would probably be wise to grab yourself a nuts-and-bolts handbook [4] rather than leaping in at the deep end with the more involved academic stuff.

Once you’ve familiarized yourself with some basic mineralogical theory, it’s time to get down to the real business of learning about rocks on a practical level. A great way of doing this is to get a starter kit which contains samples of rocks all the way up the Mohs Scale of Hardness (well, perhaps not quite all the way up – you’d be hard-pressed to find a kit-maker who’ll put a diamond in a practice kit!). Play around with the samples, learning about what they can and cannot tolerate. Not only will this allow you to get close to the object of your hobby – the rocks themselves – it will also aid your understanding of them on a practical level, and give you some valuable experience which will help you to know how to handle various minerals in the field.

Mineralogy isn’t dangerous if done correctly, but it isn’t always the safest of hobbies either. This is part of what makes it so much fun! Your life as a mineralogist may see you heading for some precipitous rockfaces, or putting yourself in the path of falling rocks. If you’re careful and you know what you’re doing, then you’re unlikely to get into any trouble. But it’s worth brushing up on rock safety procedures and ensuring that you and your kit are ready for all eventualities [5] just in case. Get yourself some steel toe-capped boots, some goggles to protect your eyes when chipping out rocks, a helmet, some tough gloves, a first aid kit, and some hi-viz in order to let workers see you if you’re collecting in a quarry. If you’re really serious about safety, you might also want to grab a Geiger counter in order to monitor the minerals you’re working on. Some rocks can emit surprisingly high levels of radiation [6]!

Apart from safety concerns, you’d also be advised to prepare well for any collecting expedition. Get a good geologists hammer, a chisel, a magnifier, and a bag to put your kit and specimens in. Make sure that you’ve got everything you need before setting out – there’s nothing more irritating than finding a really promising specimen, and being unable to get it out for want of a chisel. It’s also really important to get the permission of any relevant landowner whose land you may wish to work on. Not only will they be able to tell you if you’re likely to be disrupting anything of import with your activities, mineralogists who trespass [7] bring the whole field into disrepute. Finally, you’re advised to obtain things like polish and microscopes to aid you in cleaning up and examining your specimens. Other than that, there’s little else you can do apart from get down to the rockface and learn!

[1] Cynthia Ramnarace, “Why You Need To Have A Hobby”, Business Insider, Mar 2014
[2] R Detrosier, "The benefits of general knowledge; more especially, the sciences of mineralogy, geology, botany, and entomology, being an address delivered at the opening of the Banksian Society, Manchester. On Monday, January 5th, 1829", Hathi Trust
[3] Mineralogical Society Of America, "Select Publications"
[4] Sarah Garlick, “National Geographic Pocket Guide to Rocks and Minerals of North America”, National Geographic
[5] Compare "Looking after your collections"
[6] Thomas Harding, “Museum’s rock collection was highly radioactive”, The Telegraph, Feb 2001

[7] Cornell University Law School, “Trespass”