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An Interesting Fluorite Occurrence in the Limestone Capital of the World

Last Updated: 6th Apr 2018

By Christopher Clemens

After recently having moved to northwestern Indiana, I’d heard stories about a road cut in Lawrence County, in the southern part of the state, that was rumored to produce some little-known fluorescent fluorite and calcite specimens. As a collector of fluorescent minerals, a visit to this location had been on my field trip list for a while, but the drive was a bit far for a single day outing, so I kept putting it off. In June of 2016 I was able to coordinate a visit to the road cut in combination with a trip to the 51st Annual Lawrence County Rock Club Show in Bedford.

The road cut is located in south central Indiana, along State Route 37, just north of the town of Bedford. Bedford is known as the “Limestone Capital of the World,” due to its numerous limestone quarries and as a producer of dimension stone used in the construction of national monuments in Washington, D.C., as well as many notable U.S buildings such as the Pentagon and Empire State Building. At its south end, the road cut begins at the stoplight at the junction with Indiana Route 58, near the Bedford Super 8 Motel (Figure 1).

Figure 1. Welcome to the Route 37 road cut in Bedford, Indiana. View looking southeast.

The road cut extends to the north on both sides of Route 37 for approximately 0.3 miles, and excavates into the fossiliferous layers of the Mississippian age St. Louis limestone (approx. 335 mya)[1]. In places the cut is 20 feet high or more (Figure 2).

Figure 2. Panoramic view of the Route 37 road cut, looking northwest, showing the stratification of the Mississippian age St. Louis limestone.

With directions from Bob Harman, a well-known local collector of geodes and Mindat contributor, it was fairly easy to find the occurrence of fluorite. Walking north from the stoplight, on the west (southbound) side of the highway, the limited zone of fluorite begins approximately 1/3 to 1/2 of the way along the length of the road cut. The fluorite is found as small, well-formed, egg yolk-yellow colored cubes up to 8 mm on an edge, and occurs in small vugs and pockets in the limestone, roughly between knee and head height (Figures 3 and 4).

Figure 3. The fluorite zone, containing numerous vugs and pockets in the limestone host rock. Many of the vugs visible in this photograph contain fluorite crystals. The sheet-like habit of exfoliation of the St. Louis limestone, clearly visible in this photograph, made specimen extraction relatively easy.


Figure 4. Close-up view of one of the many pockets visible in Figure 3 above, in situ, lined with yellow fluorite cubes. The largest cube is approximately 0.4 cm on an edge.

Specimens were reasonably easy to collect off of the face of the road cut, as the St. Louis limestone tends to exfoliate in large sheets with a little help from a pry bar and hammer. With care, in most cases it was possible to remove a sheet of limestone containing the entire crystal-lined pocket. The relatively thin sheet could then be field trimmed to remove excess matrix. The fluorite occurrence was confined to a relatively limited area, and continued for approximately 40 yards. I also had success finding specimens on the opposite, east (northbound) side of the highway, along an area that roughly paralleled the occurrence on the west side.

Farther toward the north end of the road cut, primarily on the west side, I also found several deposits of calcite, including one pocket that contained scalenohedral crystals to about 25mm in length. The calcite was far more difficult to collect, as it was located in host rock that did not exfoliate well, and was therefore hard to extract (Figure 5).

Figure 5. Calcite-filled pocket, photographed in situ, near the north end of the road cut, west side. Note the more conglomeratic nature of the host rock. FOV is approximately 10 inches.

However, I did manage to obtain about a dozen calcite specimens to evaluate for fluorescence.

Back in my motel room later that night, I lamped my finds from earlier in the day. The fluorite was confirmed to be fluorescent under UV, showing a bright white response under short wave (254nm) and mid wave (302nm), and a brighter, warmer, cream colored response under long wave (365nm). The fluorescence was somewhat similar to that seen in the root beer brown fluorite cubes from the notable Clay Center, Ohio, location. This may perhaps be a clue to the presence of an organic activator of fluorescence in the bedford material. The limestone host rock was dimly fluorescent as well, showing a patchy, red/brown response. Perhaps the most interesting aspect of this fluorite occurrence, however, relates to its pattern of phosphorescence. Although all of the specimens that I collected showed a nearly identical fluorescent response under all UV wavelengths, they could be subdivided into two distinct groups based on phosphorescence. The first group (we’ll call it Type I) shows blue phosphorescence following exposure to short wave UV, but is not phosphorescent under mid wave or long wave UV (Figures 6 and 7).

Figure 6. Visible light photograph of one of the many specimens of yellow fluorite cubes that I was able to successfully extract. The largest crystals in this specimen measured approximately 2.5 mm on an edge. This fluorite showed a Type I blue phosphorescent response (see Figure 7, below).


Figure 7. Photographs of the same specimen shown in Figure 6, above, showing short wave UV (254nm) fluorescence (top image) and blue phosphorescence (bottom image) typical of a Type I response. This specimen showed a similar fluorescent response under mid wave (302nm) and long wave (365nm) UV, but was not phosphorescent under either wavelength.

The second group (Type II) displays off-white phosphorescence after short wave UV exposure and green phosphorescence following mid wave and long wave UV (Figures 8 and 9).

Figure 8. Visible light photograph of a fluorite specimen which shows a Type II phosphorescent response. The largest crystal (right of center) is approximately 6.5 x 5.0 mm. See Figure 9, following, for photographs of fluorescence/phosphorescence.


Figure 9. Photographs of the same specimen shown in Figure 8, showing fluorescent and phosphorescent response to short wave (254nm) and long wave (365nm) UV. Note the Type II white and green phosphorescence following exposure to SW and LW UV, respectively.

In all cases, the color of short wave phosphorescence was predictive of the mid and long wave phosphorescent response, or lack thereof. Although these yellow fluorite cubes would make poor visible light display specimens, they show a dramatic fluorescent response under all UV wavelengths, and provide very interesting and worthy additions to any collection of fluorescent minerals.

The majority of the calcite that I collected from the roadcut showed only a dim red/pink response under short wave UV. However, several small specimens of calcite druse showed bright white fluorescence with lasting phosphorescence under all UV wavelengths. Larger specimens of this type, if found, would provide spectacular fluorescent material (Figures 10 and 11).

Figure 10. Specimen of calcite druse, seen under visible light. The size of this piece is 25 x 15 x 8 mm. This material is brightly fluorescent and phosphorescent under UV (see Figure 11, below)


Figure 11. Same specimen of calcite druse shown in Figure 10 above, showing fluorescence under UV (left images) and phosphorescence after UV (right images).

Most interestingly however, several of the clusters of scalenohedral crystals, as well as a few cleavage rhombs, showed a nearly Terlingua-type fluorescent response, with blue/pink triangular zones of fluorescence under short wave UV with lasting blue phosphorescence (Figure 12).

Figure 12. Calcite collected from the north end of the road cut, west side, showing a nearly Terlingua-type fluorescent response, with blue/pink triangular zones of fluorescence under short wave UV (middle image) with lasting blue phosphorescence (bottom image). Under long wave UV (365nm), this piece shows only a dim pink response (not shown). This specimen measures approximately 37 x 34 x 25 mm.

Only a dim pink response was seen under long wave UV in these specimens.

Collecting at this location presents some potential hazards that must be taken into consideration. Traffic is certainly a concern, although the shoulder is very wide, and mostly paved to the base of the road cut. Crossing the highway is very dangerous, and I would recommend wearing a bright-colored safety vest to maximize your visibility to passing drivers. Equally dangerous, if not more so, is the risk of falling rock. Some sections of the road cut were overhung with protruding rock, and there was evidence of frequent rockfalls everywhere. Therefore, a hard hat is highly recommended. The tendency for the St. Louis limestone to exfoliate in large sheets is also a concern, whether the release is by the intent of the collector, or by natural means. These sheets tend to have rather sharp edges, might weigh up to a hundred pounds or more, and could act like an axe blade if they fall on an unsuspecting foot or head. For that reason, steel-toed boots are a smart accessory item. Lastly, due to the somewhat dangerous nature of this site, it would not be a good location to bring children.

In summary, this is an easily accessible, but little known location for collecting interesting fluorescent mineral specimens in the Midwest. If you live near the town of Bedford, Indiana, or will be passing through the area, the Route 37 road cut site is definitely worthy of a visit. Plan to park on the paved shoulder, well off the highway, and not under any overhanging rock. Wear the appropriate personal safety equipment, and with a little patience, the yellow fluorite cubes are easy to find and extract during the day, and make worthy additions to any fluorescent mineral collection. The very limited nature of the extent of this fluorite occurrence makes it even more appealing in terms of the rarity of these specimens, particularly for anyone who specializes in uncommon Midwestern species. There may be some very interesting fluorescent calcite present at this location also, but night collecting with a field lamp would be the recommended method to locate the best specimens. Due to the high frequency with which the limestone sloughs off of the face of the road cut, new specimens are exposed seasonally, so this location is not likely to be collected-out any time soon.

An earlier version of this article was originally published in UV Waves, Volume 46, No. 4 (July-Aug 2016) the bi-monthly publication of the Fluorescent Mineral Society.

References

1. Camp MJ and Richardson, GT. Roadside Geology of Indiana. Mountain Press Publishing Company, Missoula, MT, 1999




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Comments

Nice find Chris - well done.
Thank you for the report.

Keith Compton
5th Apr 2018 12:02pm
I agree with Keith, very well done! I especially enjoyed the large photos and the in situ views of the geology.

Larry Maltby
5th Apr 2018 1:48pm
I do know that in Minerals of Indiana (Erd and Greenberg, 1960, p. 29) they mention that fluorite had been found in the St. Louis Limestone in Harrison County and Monroe County, so I would assume that there may be other localities that have some fluorite cubes as well where St. Louis Formation is exposed.

A very good and detailed article.


Jamison K. Brizendine
6th Apr 2018 8:59pm
A pleasure to read, thank you for article!

A. Mathauser
7th Apr 2018 7:19pm

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