Baltimorite

Pictured above is a specimen of serpentine collected circa 1940 at the chromite bearing serpentine barrens of Bare Hills in Baltimore County, Maryland. It is one of several such specimens The Natural History Society of Maryland owns. Very likely Charles Ostrander and/or Walter Price, the Society's  prominent curators at the time, collected them. The NHSM labels identified the specimens as  "baltimorite". Minerals of Maryland, the NSHM publication that these two curators co-authored in 1940, may have been the last regional publication to suggest that "baltimorite" was  a species or variety of a species.

 Mindat describes  baltimorite as a "synonym for antigorite." along with 11 other names including "gymnite" and "porcellophite." Interestingly, Minerals of Maryland refers to "gymnite" and "porcellophite" as separate minerals that also occur,  along with "baltimorite," at Bare Hills.  The three names  have since all but disappeared from mineralogical parlance, although  Bernard and Hyrsl's Minerals and Their Localities refers to gymnite as a synonym for deweylite,  which is typically identified as a mixture of several varying different species that can occur  together at some of central Maryland's serpentine outcrops.

In addition to naming twelve antigorite synonyms, Mindat also names eight varieties of antigorite. Two of them, williamsite and chrome antigorite,  are known to those who collect in the serpentinized areas of Central Maryland and Southern Pennsylvania. Highly valued  in lapidary circles, williamsite is  best described as a  solid and translucent apple-green antigorite included with specks of black chromite. Chrome antigorite displays varying amounts of reddish purple coloring attributed to the presence of chromium. It is usually columnar or fibrous and known to occur primarily at the now off-limits Woods Chrome Pits in Lancaster County, Pennsylvania  just a few miles north of the Mayland state line.   

Antigorite, lizardite, and chrystotile are the best known of 13 species in the serpentine subgroup. In addition to these 13 species, the serpentine subgroup as an entity boasts eight varieties, none of them approved as species in their own right.

 One such serpentine subgroup varietiy is picrolite. Mindat defines picrolite primarily by habit  as "a columnar or coarsely fibrous (non - asbestiform) variety of serpentine commonly referred to as a variety of antigorite, but may be other species." The most common of these "other"  species are the serpentine subgroup mineral species lizardite and chrystotile. Picrolite is ubiquitous at nearly all the serpentine exposures in central Maryland and southern Pennsylvania. Note the label on the NHSM specimen at right : "Serpentine, var. baltimorite (Picrolite). It makes sense. Here it can be assumed that the term picrolite was used to to refer to the columnar habit of the sample.  The original material that was given the name baltimorite in 1843  and  wherever else the name was used thereafter matched the Mindat description of picrolite. Typically the color of picrolite is green as shown in the image at left. Interestingly the color of  the several "baltimorite" specimens at NHSM---which are the only specimens anywhere so labeled that we are aware of--- is  brown.  Would it be unreasonable to colloquially refer to picrolite of brown color as "baltimorite?" An interesting aside is that the approved species known as "chrome antigorite," like picrolite, is almost always columnar.

It was Thomas Thomson, a British mineralogist, who came up with the name baltimorite in 1843 (Phil.Mag.22, 1911). He had recently received a specimen from the well-known American scholar and collector Francis Alger. Collected at Bare Hills in Baltimore County, the specimen was opaque with a silky luster and consisted of longitudinal fibers that adhered to each other. Alger had referred to the material as "asbestos and chrome."Thomson studied the specimen. After ruling out the presence of asbestos as well as chromium, he named it baltimorite for the locality.

Two  years later, citing Peggendorff's Annals, Vol. lxii, p. 137, The Edinburgh New Philospophical Journal, Volume 39 published in an article that the German mineralogist Karl Friedrich August Rammelsberg had found that Thomson's "baltimorite" was identical  to other material that had  become known as chrysotile by Kobel.  Franz Kobel  was another German mineralogist, who had originally called the material Schiller asbestos. The conclusion: "As the latter (baltimorite) was described subsequently to the former, the name of baltimorite must be given up."

However, the baltimorite handle endured, only to receive an additional blow in 1855 by a supplement to Dana's The System of Mineralogy. Therein it was noted that two years earlier,  the German mineralogist C. von Hauer had published a new analysis of purported baltimorite in which the chemical composition varied from the original composition given by Thomson. The Dana supplement concluded: 

We have a new analysis of a stone which somebody has labeled baltimorite. It is very wide from the original baltimorite of Thomson (from Bare Hills, Maryland) and is no better entitled to the name than many other fibrous stones that could be gathered from our serpentine regions.

 Eighty one years later in 1936, the American Mineralogical Association placed the final nail in "baltimorite's" coffin. The American Mineralogist in an abstract of a paper by George C. Selfridge, Jr.  of Columbia University entitled "An X-ray and Optical Investigation of the Serpentine Minerals" proclaimed on page 463:

Based on the results of the x-ray and optical studies and the chemical discussion---to drop the names picrosmine, picrolite, williamsite, bowenite, porcellophite, and baltimorite  for the term antigorite. The term serpentinite is suggested for rocks composed of serpentine or antigorite or a mixture of both. 

The recommendation came at about the same time as the mineral aficionados at NHSM were collecting and labeling "baltimorite" specimens from either Bare Hills or the geologically similar Soldiers Delight serpentine outcrops, also in Baltimore County. Interestingly, the NHSM labeled as "picrolite" numerous specimens from serpentine outcrops in Maryland's  Harford County that appeared to differ from those labeled "baltimorite" only by their green color.

Then in 1956, in Vol. 41 of  The American Mineralogist,  a 21 page paper by George T. Faust and Bartholomew Nagy outlined studies showing "that minerals classified as serpentines are either chrysotile,  antigorite, or natural mixtures of these two minerals--- It is suggested that serpentines should be redefined in terms of the relative proportions of antigorite and chrysotile"

Notwithstanding, the endless questions, answers, and names involving  various species and varieties in the Serpentine Subgroup have continued---and undoubtedly will continue--- to evolve without reference to baltimorite  For now, here is the best definition for baltimorite we've been able to come up with:  an obsolete and antiquated synonym for picrolite, a serpentine group variety, which depending upon composition may actually be antigorite, lizardite, chrystotile, or a combination of them.

We are grateful to John S. White, Past Curator in Charge of the Mineral and Gem collection at the Smithsonian's National Museum of Natural History for suggesting this story, turning us on to some of the referenced sources, and proofreading it. 

  

John White and a Rock Pile

Actually we  were trespassing as we peered through a pile of rocks dumped just a few feet off a lonely country road in York County, Pennsylvania near Delta. John White and I  had  tried unsuccessfully to locate the landowner. In the process, we learned that the a local power company had dumped the rocks there in conjunction with a project that never happened. Hopefully, the landowner wouldn't mind.

John had expressed interest in these rocks over the past year and had encouraged me to check them out. He had described the material as "just serpentine rock."  For reasons that hindsight now renders lame,  I had not yet done so. What was wrong with me? When a past Curator-in-Charge of the national mineral collection who also founded Mineralogical Record gets excited about a site less than an hour's drive away, it's worth a look.

Then last week, John emailed me an invite to see some of the rocks he'd purchased at the recent Denver show. Attached was the image shown at left. It pictures cabs from material John had found when he first discovered the aforementioned rock pile last year.  His email suggested that after looking at his Denver acquisitions, we drive there.

We did and ended up crawling over the pile of serpentinite rocks. There were numerous pieces worthy of John's cabs. Antigorite was abundant, especially  the columnar picrolite variety. Colors ranged from  very light green to very dark green. Gemmy coatings of medium to dark green  antigorite occasionally comprised the rocks' surfaces. This antigorite was slightly translucent, but less so than antigorite (var.) williamsite.  It also lacked those chromite spots so definitive of williamsite. The  rock beneath the antigorite was grainy, dull greenish gray and black, possibly chrome-bearing, and iron-bearing almost for sure. Though we didn't have magnets, John had learned  when the cabs from his earlier find were made  that much of the black material here was magnetic.

One specimen that particularly interested me is the quartz (var.) prase shown at right. Is presence along with the serpentinite strongly suggests that the rocks on this pile were from the neighboring Cedar Hill or Penn-Maryland Quarries just a few miles east in Lancaster Couty at Fulton Township. Both quarries exploit the same kind of serpentinite rock that's part of the Baltimore Mafic Complex.

 Another piece that impressed me features an intersection of massive magnetite with pale green antigorite (var.) picrolite. Were it chromite rather than magnetite meeting that picrolite, I suspect some williamsite would be gracing the picture as well. Assuming that the rocks originated at either the Cedar Hill or Penn- Maryland Quarries, such a find would seem reasonable. Chromite as well as magnetite is known to occur at both localities

All told, we spent less than an hour at the rock pile. We departed because we were hungry and still marginally clean enough to be served a hearty lunch at a family restaurant a few miles away.

 Thereafter, we continued about ten miles further on  to check out ---with permission---the edges of another rock pile that John knew about. It consisted mostly of  "Wissahickon schist."  We quickly uncovered some graphite, pyrite, and siderite.  This was a much bigger pile of rocks. Unfortunately,  later commitments limited the amount of time we had to see as much of it as we would have liked. Stay tuned.