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Adiscovery of exceptional calcite crystals,
concentrically zoned with marcasite and pyrite inclu-
sions, was recently made in the Chicago metropoli-
tan area at the Conco mine, North Aurora. Crystals are found in
massive solution cavities that were exposed by underground min-
ing for limestone and dolomite. In 2010, Lafarge North America,
owner of the Conco mine, accepted a proposal from Saga Minerals,
Inc. to mine the solution cavities for mineral specimens. Crystals
to 25 cm and specimens weighing many hundreds of pounds have
been recovered. These specimens were first shown to the mineral
collecting community at the 2011 Arizona Mineral and Fossil Show
at the Hotel Tucson City Center in Tucson, Arizona.
Mineralization at the Conco mine is helping researchers map
deep hydrothermal fluid migration across the midcontinent and
better understand regional diagenetic processes. Mississippi Val-
ley–type mineralization in northeastern Illinois (the southwest
Michigan Basin) and structural dip suggest that the Michigan
Basin was a source of mineralizing fluids. Without further study,
THE
CONCO
MINE
North Aurora
Kane County
Illinois
JARED T. FREIBURG
Illinois State Geological Survey
615 E. Peabody Drive
Champaign, Illinois 61820
freiburg@illinois.edu
JOHN RAKOVAN
Department of Geology and Environmental Earth Science
Miami University
Oxford, Ohio 45056
rakovajf@muohio.edu
116 ROCKS & MINERALS
Jared T. Freiburg, a geologist at the Illinois State Geological Survey, is
the founder of Saga Minerals, Inc. (www.sagaminerals.com).
Dr. John Rakovan, an executive editor of Rocks & Minerals, is a
professor of mineralogy and geochemistry at Miami University.
Figure 2. Calcite with inclusions of marcasite and pyrite; the
specimen is 13.5 × 9 cm. Matt Zukowski specimen, Jeff Scovil
photo.
Figure 1. Marcasite stalactite cluster (“Saguaro Cactus”);
the specimen is 6.5 × 3.7 cm. Anthony Mendoza specimen,
Jeff Scovil photo.
Downloaded by [John Rakovan] at 01:32 21 March 2012
however, the Illinois Basin cannot yet be ruled out as a po-
tential source.
Regional Mississippi Valley–Type Deposits
The recent discovery of important calcite specimens from
the Conco mine has put the Chicago metropolitan area on
the mineralogical map of the United States. Located on the
boundary of two sedimentary basins, the Illinois and Michi-
gan basins (fig. 3a), Mississippi Valley–type (MVT) min-
eralization is of no surprise. MVT deposits are epigenetic,
stratabound, carbonate-hosted sulfide bodies composed
predominately of sphalerite, galena, pyrite, marcasite, calcite,
and dolomite (Paridas, Hannigan, and Dewing 2007). MVT
deposits are typically found at shallow depths on the flanks
of sedimentary basins (Leach and Sangster 1993). Although
Volume 87, March/April 2012 117
Figure 3a (above). Major basins in the north-central
United States in relation to the Conco mine and the
Upper Mississippi Valley zinc-lead mining district
(modified from Freiburg, Fouke, and Lasemi 2012).
Figure 4. Satellite
image (2010) of the
Aurora quarry and
Conco mine. The
Black arrow indicates
the decline into
the mine. Image ©
DigitalGlobe.
55
88
90
80
94
90
290
355
290
90
294
Highland Park
Arlington Heights
Aurora
Cicero
Elgin
Evanston
Oak Lawn
Oak Park
Skokie
Hammond
Napierville
North
Aurora Conco Mine
Illinois
Indiana
Elmhurst
Orland Park
Kane
County
Lake
Michigan
Chicago
0 20
0 20
miles
km
Figure 3b (right). Map of the Chicago area and the location of the Conco mine at the intersection of Interstate
88 and the Fox River in North Aurora, Illinois.
Downloaded by [John Rakovan] at 01:32 21 March 2012
118 ROCKS & MINERALS
minor MVT mineralization in northeastern Illinois is found
in the occasional road cut, outcrop, or quarry, it has not yet
been found in significant deposits. Better-known mineral
localities within the region include the historic Upper Mis-
sissippi Valley zinc-lead mining district (UMV) in north-
western Illinois (fig. 3a). Significant specimens of galena,
sphalerite, calcite, pyrite, and marcasite have been recovered
from this now-closed mining district. UMV is the type-lo-
cality for MVT deposits and was an economically significant
deposit of zinc and lead, among other minor metals (Heyl et
al. 1959). Comparative fluid-inclusion and isotopic studies
of UMV mineralization and outlying occurrences of MVT
mineralization suggest minerals were deposited from a ba-
sin-sourced low-temperature hydrothermal saline fluid (e.g.,
Kutz and Spry 1989). It is further suggested that outlying
minor and noneconomic occurrences of MVT mineraliza-
tion are the result of depositional processes similar to UMV
(e.g., Haefner et al. 1988; Carlson 1994; Freiburg 2010). At
these minor occurrences earlier paragenetic ore-stage min-
erals such as galena, sphalerite, and fluorite are rare, whereas
later paragenetic gangue mineralization such as calcite, py-
rite, and marcasite are much more abundant. Other outly-
ing regional occurrences similar to that at Conco include the
Rensselaer quarry, Jasper County, Indiana; Racine Vulcan
Materials quarry, Racine County, Wisconsin; and the Lin-
wood mine, Scott County, Iowa, to name a few.
History and Mining
In the Chicago area, urban sprawl has increased the de-
mand for aggregate materials such as limestone and dolo-
mite. With limited land availability for new quarry opera-
tions, aggregate producers have answered this demand with
underground mining. Such is the case for the Conco mine,
located in North Aurora, Illinois (fig. 3b). In 1991 Conco
Western Stone Company began conversion of the Aurora
quarry into the Galena Platteville mine. In 2006 Lafarge
North America purchased the operation and renamed it the
Conco mine.
The Aurora quarry was extracting Silurian dolomite be-
fore underground mining operations began. To convert the
quarry to the current Conco mine, a decline was drilled and
blasted in the northwest quarry floor (fig. 4), and down ap-
proximately 175 feet through the base of the Silurian dolo-
mite, through the Ordovician Maquoketa Shale, and into the
Ordovician Wise Lake and Dunleith formations.
The mine is divided into two sections: the North mine
(north of Interstate 88) and the South mine (south of Inter-
state 88). The mines currently have two levels with a simple
room-and-pillar layout. The first-level floor is approximately
280 feet below the surface, and the second-level floor is ap-
proximately 360 feet below the surface. The stone is mined
by diamond drilling, ammonium nitrate blasting, and ex-
tracting a 50-foot-wide by 25-foot-deep breast and later
mining a bench another 25 feet deep to leave a 50-foot-wide
by 50-foot-long room. Pillars measuring approximately 50
feet wide by 50 feet long are left for support. It is in these pil-
lars that solution cavities are exposed and collectible. During
the fall of 2010, Saga Minerals. mined its first mineralized
solution cavity (fig. 5). The cavity is located in a pillar on
the level 1 bench in the southeast corner of the North mine
(18 South 21 West). It measured approximately 11 feet wide,
17 feet long (to the mouth of a second cavity), and 2–3 feet
high. The cavity certainly extended further into the pillar,
estimated to be another 10 feet, but no more than 50 feet.
Another cavity, connected to the first by a large opening, is
estimated (not measured) to be 3 feet wide, 2 feet high, and
22 feet long.
To gain access to the cavity, Saga Minerals was allowed
to drive and park a truck next to the pillar. This made haul-
ing tools in and large samples (some up to several hundred
pounds) out much easier. In 2010 fourteen days were spent
mining the first cavity. Before working the cavity for speci-
mens, the chief mine engineer examined it for structural
stability. To increase safety the pillar face above the cavity
was slightly scaled back, removing any loose overhanging
rock. The rock above the cavity entrance was then supported
with two timbers for additional stability. The cavity floor was
lined with thick foam mats to protect crystals from damage
and to cushion crystals extracted from the roof and walls.
Specimens were extracted using the feather and wedge tech-
nique, a Hilti Te 70 hammer drill, hammers and various chis-
els, and a Husqvarna K3000 wet saw when necessary.
Geology
The Conco mine is approximately 40 miles due west of
downtown Chicago along Interstate 88 in North Aurora. It
lies on the east bank of the Fox River and crosses under the
interstate. Geologically, the mine lies east of the intersection
of the Kankakee and Wisconsin arches, on the far southwest
flank of the Michigan Basin, and on the northern end of a
small localized structural low called the Aurora Syncline, the
result of minor faulting in the area. Visible faulting, how-
ever, is rare within the mine. Vertical offsets to 25 cm are
occasionally encountered. It is thought that fractures in
which cavities occur may have a sense of strike-slip displace-
ment (Freiburg, Fouke, and Lasemi 2012), likely parallel to
the major Sandwich Fault, located approximately 25 miles
due south of the mine. Fractures and joint sets trending
northwest-southeast and northeast-southwest are prolific
within the Ordovician bedrock at the mine. Mineralized so-
lution cavities are found dominantly along northwest-south-
east-trending fractures.
The Conco mine is currently operating within the Wise
Lake and Dunleith formations (part of the Ordovician-age
Galena Group) that are approximately 450 million years old.
Level 1 of the mine is in the Wise Lake Formation, and level
2 is in the Dunleith Formation. Most of the solution cavities
are found within the Wise Lake Formation, although they
are not uncommon in the Dunleith. The Wise Lake Forma-
tion is a pale brown to gray, mostly pure dolomite with some
remnant limestone. Typically, it is massively bedded, slightly
vuggy, and contains trace amounts of chert. It is primarily
mudstone and wackestone with rare thin beds of grainstone
and packstone. The facies represents a warm, shallow ma-
rine environment, characterized by low depositional en-
ergy and the occasional storm deposit (Willman and Kolata
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Volume 87, March/April 2012 119
Figure 5 (right). The solution cavity mined
in 2010 from which all specimens displayed
at the 2011 Tucson Show were collected. It is
referred to as the Burrow Cavity for the
dolomitized Thalessinodes trace-fossil
burrows on which many calcite specimens
occurred. Arrow is pointing to a vertical
fracture along the cavity and a sharp bedding
plane along a horizontal bentonite bed. Jared
Freiburg photo.
Figure 6 (below). An inside view of the Burrow
Cavity shown in figure 5. Calcite crystals com-
pletely lined the cavity before blasting. The
doubly terminated calcite crystal in the lower
center of the photograph is approximately 24
cm long. Jared Freiburg photo.
Figure 7 (bottom). An inside view of another,
smaller cavity found on 3 June 2011. The
larger crystals in the photo center are roughly
12 cm tall. Note the selective, partial coating of
marcasite on the calcite. John Rakovan photo.
1978). The Dunleith Formation is
a brown to gray dolomitic lime-
stone that is slightly argillaceous,
slightly vuggy, and contains more
chert than the Wise Lake Forma-
tion. Both formations are highly
bioturbated and contain many
trace fossil burrows; most com-
mon are the horizontal, branching
Thalassinoides burrows. Fauna in
the Wise Lake and Dunleith in-
clude bryozoans, mollusks, bra-
chiopods, and Receptaculites (cal-
careous alga).
Both the Wise Lake and Dun-
leith formations contain multiple
millimeter-thick beds of bentonite
clay (K-bentonite). During the de-
position of these Ordovician car-
bonates a convergent plate bound-
ary formed along the eastern coast
of proto–North America (Kolata,
Frost, and Huff 1986). Associ-
ated volcanoes deposited multiple
widespread ash beds throughout
the midcontinent, some during
deposition of the Galena Group
sediments. Over time, diagenetic
alteration of the ash beds formed
K-bentonite beds (Kolata, Huff,
and Bergstrom 1996) or illitic
clay-rich beds. Mineralized solu-
tion cavities are most commonly
found along fractures just below
these bentonite beds (fig. 5). The
fractures acted as conduits for
mineralizing fluids transported
from deep in the basin, and the
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120 ROCKS & MINERALS
bentonite acted as a seal or aquatard, resulting in fluid
buildup, limestone dissolution, and mineral precipitation
within cavities.
Solution Cavities
Mineralization at Conco occurs as open-space filling in
joints, faults, vugs, and solution-collapse breccias and cavi-
ties. Solution cavities contain the best crystal specimens. So-
lution cavities have been found just large enough for a child
to crawl into and up to cavities that are approximately 15 feet
wide, 5 feet high, and more than 50 feet deep. Cavities are
often linear pipes focused along a fracture and sometimes
elliptically shaped below a pronounced bedding plane typi-
cally consisting of a bentonite. Most cavities are more later-
ally extensive than vertically and have been found extending
from one side of a 50-foot pillar to the other. Certainly these
cavities were more extensive pre-blasting and excavation.
Two cavities have been encountered that are dimensionally
more vertical than horizontal. Before filling one of these cav-
ities with excavated rock, for safety purposes, employees esti-
mated the depth to be approximately 100 feet. In the second
case, visual inspection suggests the cavity extends upward at
least 30 feet.
Host rock surrounding cavities is often dolomitized and
well cemented with calcite. Some areas with cavities are less
cemented, and the dolomitic rock is nearly unconsolidated
and sucrosic in nature. Miners have referred to this dolomite
as “sugar.” Cavities that are barren of calcite and sulfide min-
eralization often contain this sucrosic dolomite. The host
rock commonly contains vast networks of trace-fossil bur-
rows that are typically dolomitized and sucrosic. The cavity
mined in 2010, referred to here as the “Burrow Cavity,” is the
only one so far recognized where these burrows have been
filled by mineralization and are protruding from the cavity
walls. They are cemented with calcite, marcasite, pyrite, and
dolomite, preserving the form and branching nature of the
Thalassinoides trace fossils.
Most cavities contain a thin coating of marcasite and py-
rite and a thick coating of calcite over the dolomitized host
rock. Calcite on cavity walls has been noted to just over a me-
ter thick. The calcite habit within a given cavity is dominated
by scalenohedra (figs. 6 and 7). Rarely do cavities contain
both scalenohedra- and rhombohedra-dominant morphol-
ogies. Sulfide-dominant cavities are much less common. In
one, large modified calcite rhombohedra and scalenohedra
are found with large masses of marcasite including stalac-
tites. In another, massive marcasite to approximately 30 cm
lines the cavity and is coated by a red to copper-colored
druse of millimeter-sized crystals of pyrite with a few modi-
fied calcite scalenohedra.
Paragenesis
The paragenetic sequence of mineralization was studied
in great detail to aid in the interpretation of the timing, com-
position, and source(s) of the mineralizing or diagenetic flu-
ids that migrated through the host rock (Freiburg, Fouke, and
Lasemi 2010, 2012). The mineralogy of cavities at the Conco
mine is quite simple; however, the sequence of mineralizing
events was not (fig. 10). Based on petrographic observations
from a single cavity, referred to here as the “Thesis Pocket,”
Figure 8 (below). Calcite zoned with marcasite and pyrite; the
specimen is 18 × 15 cm. Jared Freiburg specimen, Jeff Scovil photo.
Figure 9 (right). Marcasite stalactite cluster; the specimen is 15 ×
10 cm. Dave Bird specimen, Jeff Scovil photo.
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Volume 87, March/April 2012 121
five events of calcite mineralization occur and are separated
and included by at least five events of marcasite and pyrite
mineralization. Freiburg, Fouke, and Lasemi (2012) further
characterized each calcite, marcasite, and pyrite event with
radiogenic and stable isotopes and trace elements delimitat-
ing mineralization events. Sphalerite and bitumen were not
found in this pocket but have been found in other pockets
and in drill core.
Mineralization at Conco appears to be a localized phe-
nomena resulting from migrating basinal fluids concentrat-
ing in a localized area of fracturing, faulting, and a small
structure, the Aurora Syncline. However, similar occurrences
of mineralization and paragenetic relationships in north-
eastern Illinois, southeastern Wisconsin, and northwestern
Indiana suggest mineralizing fluids may have been more
widespread. In 1985, the Elmhurst quarry, located in Dupage
County approximately 25 miles east of Conco and 15 miles
west of Chicago, started mining the Wise Lake Formation
underground. During mining, an approximately 20-foot by
30-foot cavity was encountered that was completely lined
with calcite crystals with included marcasite (J. Eburno, pers.
comm., 15 March 2010). Calcites were mostly scalenohedra
with some modified by rhombohedra. One specimen recov-
ered was a calcite “scepter” or early scalenohedron capped by
a later rhombohedron overgrowth, reminiscent of the calcite
scepters from UMV, Shullsburg, Wisconsin. Sphalerite was
also found in this cavity at Elmhurst, Illinois. Calcite, mar-
casite, and sphalerite have also been found at the Joliet mine
in Will County, Illinois, and at the Thorton quarry in Cook
County, Illinois.
Mineralogy
Calcite, CaCO3, with multiple phantoms of marcasite and
pyrite (concentrically zoned generations of inclusions) com-
prises the most common mineral specimens found at the
Conco mine. The dominant habit of crystals varies among
pockets. Every calcite released on the market at Tucson 2011
was from the same pocket (Burrow Cavity) in pillar 18 South
21 West of the North mine level 1 bench. Miller indices were
determined for several small crystals by reflected-light go-
niometry by Pete Richards and are reported here relative to
the morphologic unit cell. Based on comparison of larger
crystals to those from which these measurements were made,
it appears that the majority of calcites from this pocket are
dominated by the {211} scalenohedra. Most are modified by
{101}, {315}, and {725} (e.g., figs. 16 and 17), and most ex-
hibit multiple phantoms of marcasite and pyrite. The largest
crystals are doubly terminated and up to 25 cm. Calcites on
the floor of the cavity were the largest and tend to exhibit
dissolution or etching features and oxidation of exposed
marcasite and pyrite. Some calcites
on the walls of the cavity and all
calcites on the roof were completely
protected from this dissolution event
and have a very high luster. Almost
all calcite specimens have at least one
Figure 10. Simplified paragenetic sequence at the Conco mine
(modified from Freiburg 2010).
Figure 11 (left). Twinned calcite with marcasite
and pyrite; the specimen is 6.1 cm high. Mike
and Sally Bergmann specimen, Jeff Scovil photo.
Figure 12 (center). Calcite with marcasite and
pyrite after Thalassinoides burrow; the speci-
men is 10 × 2 cm. Jared Freiburg specimen, Jeff
Scovil photo.
Figure 13 (right). Calcite with marcasite and
pyrite on dolomite; the specimen is 9 × 4 cm.
Rivany DaSilva specimen, Jeff Scovil photo.
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122 ROCKS & MINERALS
face that was protected during dissolution, leaving a clear
window into the internal zones of beautiful sulfide crystals.
Calcites found on the floor and walls are primarily growing
on dolomitized Thalassinoides trace-fossil burrows. Rare cal-
cite twins were also found in the pocket. Although the mor-
phology is variable, all twins studied so far exhibit a (001)
twin plane.
Other pockets with distinct calcite morphologies include
a large vertical pocket that was back-filled for safety. In it,
crystals are dominated by the {101} rhombohedra with mi-
nor modifications by other forms. The {101} rhombohedra
exhibit very shallow stepped sur-
faces that may be vicinal surfaces or
high index scalenahedral faces (e.g.,
figs. 18 and 19). In another, much
smaller pocket, calcite crystals with
almost no inclusions were found
that exhibit very complex morphol-
ogies with multiple scalenahedral,
rhombohedral, and prism forms as
well as macroscopically rounded and
concave surfaces (figs. 21 and 22).
Marcasite, FeS2, comprises the
second most abundant species at
Conco. The abundance of marca-
site varies from pocket to pocket,
and globular masses to 70 cm have
been recorded. Stalactitic marcasite
is the most common habit found.
Marcasite stalactites are commonly
found in blast piles and have been
seen in-situ in one pocket. Stalac-
tites are commonly iridescent with
blue, purple, green, and gold colors.
Stalactites have a radiating structure
from the center of an inner core.
One vein of teal-green iridescent
marcasite on a dolomite matrix was
discovered approximately ten years
ago. In it, individual crystals are tabular and up to 6 mm.
About fifteen specimens were retrieved that are up to 18 × 12
cm. These marcasite specimens are nicknamed “Brian’s Em-
erald” after the miner who retrieved them. Other marcasites
with such brilliant color have not found.
Figure 14 (left). Calcite zoned
with marcasite and pyrite; the
specimen is 9 × 5.5 cm. Jared
Freiburg specimen, Terry Huiz-
ing photo.
Figure 15 (above). Crystal draw-
ing of the habit exhibited in
figure 14. Miller indices based on
the morphologic unit cell.
Figure 16 (right). Crystal
drawing of the habit ex-
hibited in figure 17. Miller
indices are based on the
morphologic unit cell and
measured on a smaller crys-
tal from the same pocket by
Pete Richards.
Figure 17 (far right). Calcite
zoned with marcasite and
pyrite; the specimen is 10 ×
7 cm. Jared Freiburg speci-
men, Terry Huizing photo.
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Volume 87, March/April 2012 123
Pyrite, FeS2, specimens are uncommon at Conco. Typi-
cally, these are globular in habit with partial terminations
of slightly distorted cubes protruding from the globular
mass. When found as inclusions in calcite crystals, pyrite is
micron-sized growing on marcasite, sometimes epitaxically
(fig. 20a). Similar pyrite/marcasite intergrowths are found
Figure 18 (far left). Calcite
zoned with marcasite and
pyrite; the specimen is 12 × 5
cm. Jared Freiburg specimen,
Terry Huizing photo.
Figure 19 (left). Crystal draw-
ing of the habit exhibited in
figure 18. Miller indices based
on the morphologic unit cell.
Figure 20a (top). SEM photomicrograph showing a tabular,
twinned marcasite crystal with epitaxic pyrite crystal.
Figure 20b (above). SEM photomicrograph showing filament-
like crystals composed of stacked pyrite octahedra.
Figure 21 (above left). Calcite crystals with minor sulfide inclusions;
the specimen is 7 × 4 cm and was collected from the Gem Pocket on 3
June 2011. Jared Freiburg specimen, Terry Huizing photo.
Figure 22 (left). Calcite crystal with minor sulfide inclusions; the speci-
men is 4 × 2.5 cm and was collected from the Gem Pocket on 3 June
2011. Jared Freiburg specimen, Terry Huizing photo.
Downloaded by [John Rakovan] at 01:32 21 March 2012
124 ROCKS & MINERALS
Figure 23 (above left). Marcasite on dolomite (“Brian’s Emerald”); the
specimen is 14 × 7.5 cm. Jared Freiburg specimen, Jeff Scovil photo.
Figure 24 (above right). Cubic pyrite cluster; the specimen is 10 × 6.5 cm.
Jared Freiburg specimen, Jeff Scovil photo.
Figure 25 (right). Calcite with pyrite and marcasite; the crystal is 4 × 3
cm and was collected from the Thesis Cavity on 22 September 2011. Jared
Freiburg specimen and photo.
Figure 26 (below left). Calcite zoned with marcasite and pyrite; the speci-
men is 9 × 6 cm. Brian Washkowiak specimen, Terry Huizing photo.
Figure 27 (below right). Calcite twin with pyrite and marcasite inclusions
and calcite scalenohedra with pyrite and marcasite after Thalassinoides
burrows. Notice cross-section of a Thalassinoides burrow at the base and
center of the specimen. Burrow is filled with buff-colored dolomite with
marcasite, pyrite, and calcite surrounding dolomite. The specimen is 7 × 5
cm and was collected from the Burrow Cavity on 23 September 2011. Jared
Freiburg specimen and photo.
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Volume 87, March/April 2012 125
in the Chicago area (Rakovan et al. 1995). Crystal forms
include octahedra, pyritohedra, cuboctahedra, and, rarely,
simple cubes. Similar to marcasite, pyrite habits vary from
growth zone to growth zone.
Sphalerite, ZnS, is extremely rare at Conco. Miners recov-
ered two specimens before completely blasting out the cavi-
ties in which they were found. One of the specimens mea-
sures more than 30 × 30 cm and was donated to the Illinois
State Geological Survey. It is coated with a sphalerite druse
with etched pyrite- and marcasite-included calcite scaleno-
hedra to 4 cm growing over the sphalerite. Sphalerite crystals
to 7 mm occur and are complex and distorted.
With plans for continued mining and the specimen re-
covery agreement between Lafarge North America and Saga
Minerals, the potential for new specimens to reach the mar-
ket looks bright for the foreseeable future. Furthermore, the
demand for aggregate around the Chicago area is forcing
quarry operations to consider underground mining. With
limited land availability resulting from urban sprawl, estab-
lished quarry operations have limited options. New mineral
specimen discoveries in the region are imminent.
ACKNOWLEDGMENTS
We thank Lafarge North America and the employees at the
Conco mine. Specifically, we are grateful to Brian Washkowiak for
his support and interest in the geology and mineralogy at Conco.
We are grateful to Pete Richards for his goniometric measurements
of several calcite crystals reported herein. We also thank Dave
Morse and Curt Abert at the Illinois State Geological Survey for
reviewing the manuscript and assisting with the cartography, Terry
Huizing and Jeff Scovil for their photography, William Besse for
the locality map, and Terry Huizing for reviewing the manuscript.
Publication was authorized by the director of the Illinois State Geo-
logical Survey.
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