The Minerals of WEST-CENTRAL ILLINOIS

Paul L Garvin and Gene Tribbey
Mineralogical Record

Aug 31, 2005 20:00 EDT

LOCATION

The minerals featured in this report were collected from five operating limestone quarries near the towns of Biggsville and Smithshire in Henderson County, Illinois, and near the towns of Dallas City, Hamilton and Plymouth in Hancock County, Illinois (Fig. 1). Both counties are bordered on the west by the Mississippi River. The cluster of localities will be referred to collectively as west-central Illinois (Fig. 2).

HISTORY

The history of limestone use in west-central Illinois extends back to the time of early European settlement, beginning in the 1830's. At that time, limestone was quarried as "dimension stone" for use in constructing foundations for houses and other buildings. Because the stone was readily obtainable in flat slabs and, owing to its softness, was easily dressed, it was widely used for window sills and lintels, for exterior facings and as ornamental building stone. Surviving examples include the Dallas City High School (Hancock County), which was built about 1890 (Fig. 3), and the Mormon Temple at Nauvoo (Hancock County), construction of which was begun in 1841 and completed in 1846 (Fig. 4). The temple was rebuilt in 2002, using the same site and exterior architectural plans; however, because the old quarry had been flooded by the 1912 Keokuk Dam, the limestone for the building had to be obtained elsewhere.

Another early (and continuing) use of limestone was in the manufacture of "quicklime" (CaO). Quicklime was produced by a process that involved heating the limestone (CaCO^sub 3^) to a high temperature in kilns in order to drive off the CO2 and leave calcium oxide. Quicklime was then hydrated to form calcium hydroxide (Ca(OH)^sub 2^). Calcium hydroxide reacts with water and atmospheric CO2 to produce a fine-grained calcium carbonate which crystallizes and hardens. Because of this property of hardening upon addition of water, quicklime was used in the manufacture of mortar and plaster, and was also used as a flux in the production of steel, among other uses.

As the railroads reached western Illinois in the middle and late 19th century, limestone was widely used in building bridge abutments. The masonry for the Burlington Railroad Bridge came from the Burlington Limestone, which was quarried in Hancock County. This single-track bridge, built in 1868, connected the towns of Burlington, Iowa and East Burlington (now Gulfport), Illinois. It was replaced in 1898 by a double-track bridge.

Since the advent of motor vehicles, crushed limestone has been in demand for surfacing gravel roads and as aggregate in concrete for road surfaces, bridges and overpasses. Much of the limestone quarried in western Illinois today is used in road-building. High-strength limestone, however, is used as aggregate in structural concrete, and high-calcium limestone is used in a wide variety of industrial applications, including steel manufacturing, water treatment, agricultural liming and feed supplements, glass making, ore and petroleum refining and paint manufacturing.

GEOLOGY

The rocks hosting the west-central Illinois mineral deposits belong to the Burlington Limestone and Keokuk Limestone formations, of Mississippian (Valmeyeran) age. Both rock units form prominent ledges that can be seen along the Mississippi River (Figs. 5 and 6). The Burlington Limestone is medium-grained to coarse-grained and highly fossiliferous. The upper and middle parts are characterized by the presence of beds and large nodules of chert, and the limestone is locally glauconitic. The basal Burlington "Quincy Lime" is mined for its high-purity limestone (Goodwin and Harvey, 1980).

The Keokuk Limestone includes thick beds of medium-grained to coarse-grained limestone, as well as thinner limestone beds and intercalated layers of argillaceous dolostone and calcareous shale. The shales in the upper part are similar to those in the overlying Warsaw Shale, and they contain scattered quartz-lined geodes similar to those for which the Keokuk Formation is well known (Goodwin and Harvey, 1980). The Warsaw Shale is present in the uppermost exposed rock at Hamilton.

MINERALOGY

The minerals of west-central Illinois occur primarily as linings in small cavities in the Burlington and Keokuk formations. Some Keokuk mineralization occurs in cryptocrystalline quartz-lined spheroidal cavities (geodes). Locally, geodes are found crushed in situ and the associated mineralization occurs as breccia cement. Since the Keokuk Formation, where present, occurs at the top of quarry exposures, the minerals are considerably weathered, and commonly show etched and iron-stained surfaces. Most of the highest-quality mineral specimens have been taken from the Burlington and the Warsaw formations.

Aragonite CaCO^sub 3^

Aragonite is a rare mineral in west-central Illinois, having been observed in a single sample from Biggsville. It consists of a spray of elongated prismatic to acicular crystals up to 1.5 cm long. The pale amber color of the crystals may be due to iron oxide staining.

Barite BaSO^sub 4^

Barite is rare in west-central Illinois, known only as a single crystal in a specimen from Hamilton. The tabular, pale yellow crystal is perched on drusy quartz.

Calcite CaCO^sub 3^

Calcite is present in all five of the west-central Illinois deposits and in general is the most abundant mineral of the region, with individual crystals ranging in size from microscopic to 8 cm. As is typical of calcites that form in carbonate host rocks in the midcontinental United States, west-central Illinois calcite occurs in several crystal habits (types). Seven distinct types have been observed (Figs. 7-10). Simple and modified obtuse rhombohedral forms (type I) are the most abundant. Type II crystals exhibit marked flattening with respect to the c-axis. These appear similar to calcites found in geodes in western Illinois and adjacent Iowa and Missouri. The well-known acute scalenohedral form ("dog-tooth spar") is uncommon in the five deposits. Type II crystals are generally striated on crystal faces of one, but not both, of the rhombohedral forms.

Three types of contact twinning have been observed: (1) simple basal twinning on (0001) involving obtuse rhombohedral individuals; (2) simple rhombohedral twinning involving obtuse rhombohedral individuals; (3) simple basal twinning involving acute scalenohedral individuals (Palache et al., 1951) (Figs. 8 and 10). At Biggsville, microcrystalline calcite pseudomorphically has replaced crinoid ossicles, and microcrystals of calcite occur aligned in rows along individual needles of millerite.

Calcite is typically colorless to milky white and is variably etched. Locally, Type III calcite is coated with an iridescent, highly reflective brown material, which is probably iron oxide or manganese oxide. Under long-wave and shortwave ultraviolet light Types I, II and III calcite fluoresce pink, yellow or white. Type IV is nonfluorescent. The brown coating is also nonfluorescent (Figs. 11-15).

The calcite types, their crystal forms, their colors (under visible and ultraviolet light) and the quarries where they have been found are shown in Table 1.

Chalcopyrite CuFeS^sub 2^

Chalcopyrite is present, though not abundant, in all deposits except at Hamilton. It occurs as inclusions in calcite and as microscopic crystals perched on calcite and sphalerite. In the latter case, the Chalcopyrite crystals occur in parallel alignment on sphalerite crystal surfaces. The crystal form is the tetragonal disphenoid ("pseudotetrahedron") (Figs. 16 and 17).

Dolomite CaMg(CO^sub 3^)^sub 2^

Dolomite has been identified only at Hamilton and Smithshire. In both localities crystals are approximately 5 mm across and exhibit the typical curved rhombohedral ("saddle") habit. Color ranges from pinkish yellow to yellow-brown.

Galena PbS

Galena is rare in west-central Illinois, occurring only at Biggsville and Plymouth. Small cubes (less than 1 cm) occur in association with TVpe II calcite.

Marcasite FeS^sub 2^

Marcasite is present, though not abundant, at Biggsville, Dallas City and Plymouth. It has not been observed at Hamilton and Smithshire. Individual crystals, ranging in size from microscopic up to a few millimeters long, are typically bladed and wedge-shaped. Marcasite crystals are also found as inclusions in calcite. Striations are parallel to the long axis of the crystal blades. Wedges are polysynthetically twinned; twinning Striations observed on this variety have a chevron-like appearance. At Dallas City, blades are locally aggregated in spheroidal clusters and as sheaf-like subparallel intergrowths.

Marcasite exhibits typical iridescence resulting from thin coatings of iron oxide (Figs. 16 and 18).

Millerite NiS^sub 2^

Millerite is present in all of the five west-central Illinois deposits. It occurs as sprays and felted masses of acicular crystals, with individual crystals up to 2 cm long. It can be found on quartz, dolomite, iron sulfide minerals and chalcopyrite. In some small cavities, felted millerite masses lie loose, i.e. they do not appear to be attached to the cavity walls. Millerite is also commonly included in calcite; where needle diameters are very small and needle density high, heavily included calcite appears black.

Millerite is pale brassy yellow, tarnishing to bronze or gray.

Polydymite Ni^sub 3^S^sub 4^

Polydymite, a relatively rare mineral, has been confirmed in specimens from Hamilton by X-ray powder diffraction (White, 1970). The polydymite occurs included in calcite, as curved filaments from which radiate short, densely distributed blue-black needles. Black acicular crystals have also been observed at Plymouth and Dallas City, suggesting the possibility that violante and/or polydymite may also be present at these localities (Fig. 19).

Pyrite FeS^sub 2^

Pyrite has been found in all five of the west-central Illinois deposits. Individual crystals range in size from microscopic up to 5 mm across. Crystal forms are simple cubes, octahedrons and cuboctahedrons. At Plymouth, pyrite cuboctahedrons are stacked in rod-shaped aggregates. In one specimen from Biggsville, pyrite crystals are localized in recesses between quartz crystals; in another, tiny crystals are perched on corners and edges of rhombohedral calcite crystals. Like marcasite, pyrite also occurs as inclusions in calcite and in some cases shows iridescence (Figs. 16 and 20).

Quartz SiO^sub 2^

Quartz is present at all five of the west-central Illinois localities, as both cryptocrystalline and macrocrystalline varieties. Botryoidal cryptocrystalline chalcedony was observed at Dallas City and Plymouth.

Macrocrystals of quartz range in size from very small (as components of cavity-lining druses) to 1 cm long. Larger crystals are typically doubly terminated and are loosely aggregated in irregular masses. At Plymouth the aggregates appear somewhat like slabs that are stacked in subparallel alignment. Individual slabs range up to 8 cm across. These aggregates are frequently found loose in the cavities, exhibiting no obvious point(s) of attachment to cavity walls (Fig. 21).

Smithsonite ZnCO^sub 3^

Smithsonite is rare in west-central Illinois, currently known in a single pale amber-colored specimen from Dallas City. It occurs as very small (ca. 1 mm) crystals displaying a short-prismatic habit. The rounded, etched crystals are perched on sphalerite and calcite. The Smithsonite was confirmed by X-ray powder diffraction (Kile, 2002).

Sphalerite (Zn,Fe)S

Sphalerite has been found at all five of the west-central Illinois quarries. Individual crystals range from microscopic to more than 8 cm across. Sphalerite most often appears as complex intergrowths of malformed tetrahedral crystals and as cleavable masses. Polysynthetic twinning is common, especially in the larger masses. At Biggsville and Smithshire, well-formed crystals exhibiting subequally developed positive and negative tetrahedrons ("pseudooctahedrons") occur. At Biggsville and Dallas City, large tristetrahedrons (up to 3 cm across) have been found. At Biggsville, sphalerite on chert occurs as a druse of nearly black crystals with adamantine luster. Also at Biggsville, tiny crystals of sphalerite are perched along acicular crystals of millerite. All sphalerite is dark brown to black, indicating a relatively high iron content. Crystal surfaces are locally iridescent (Figs. 22-27).

Violarite Ni^sub 2^FeS^sub 4^

Violarite is a relatively rare mineral. In west-central Illinois it occurs in intimate association with millerite. In fact, it is very difficult to visually distinguish the two minerals. Violarite is reported to be black (Palache et al., 1944), and sprays of acicular violarite crystals occurring on calcite at Plymouth do indeed appear black, but under magnification (30X) the needles are seen to be bronze-colored with a black coating. X-ray powder diffraction analysis of a single specimen revealed the presence of both millerite and violarite (Table 2). It is possible that the black coating alone is violarite.

PARAGENESIS

Paragenetic sequences were determined for each of the five mineral deposits. From these a composite sequence for west-central Illinois was derived (Fig. 28). No individual deposit contains all of the minerals found in the general area, but the order of appearance of the minerals at each of the five deposits suggests that the general paragenetic sequence is roughly the same throughout the area.

In general, quartz (including chalcedony) is always the earliest mineral to form and calcite is always late, except at Biggsville, where Type I calcite follows quartz and precedes or overlaps sulfides. It is generally before and during the deposition of calcite that sulfides form. In cases where multiple generations of sulfides occur (e.g. pyrite and sphalerite), the latest generation (always as scattered microcrystals) was deposited after calcite. The positions of the types of calcite relative to each other in the sequence could not be determined with certainty since, with the exception of Types I and II, they do not occur together in the same sample. That each calcite represents a separate depositional event is suggested by differences in fluorescence under ultraviolet light (suggesting differences in trace-element chemistry) and crystal habit (Table 1).

The positions of galena and barite in the paragenetic sequence could not be determined because they are not in contact with other minerals, except calcite.

MINERAL ORIGINS

By studying the compositions, crystal morphologies and parageneses of the minerals in these deposits, and by making comparisons with similar deposits in the upper Mississippi Valley region whose origins are relatively well established, it is possible to reach some general conclusions about the environments of formation of the minerals. More definitive statements await fluid-inclusion and stable-isotope and radiogenic-isotope analyses.

Carbonate-hosted, sulfide-bearing mineral deposits are widespread throughout the mid-continental United States. The ground for them was prepared by the dissolution of carbonate rock along bedding plane fractures and transverse fractures, resulting in openings ranging in size from small vugs to large caves. In west-central Illinois, cavities are relatively small (up to 25 cm) and tend to be elongated parallel to bedding planes in the host rock. Dissolution, which may have been localized by the presence of preexisting fossils or concretions in the rock, was accomplished by aqueous fluids that were undersaturated with respect to calcium carbonate. A controlling variable here was pH. Low-pH fluids may have been derived through leaching of microscopic iron-sulfide minerals contained in carbonate host rocks or in karst-filling muds (Garvin, 1995). Precipitation of all minerals appears to have taken place under phreatic conditions (below the groundwater table). This is suggested by the fact that calcite occurs as euhedral crystals and not as spelaean forms (Jennings, 1985; Ford, 1988), and by the presence of sulfide minerals throughout the paragenetic sequence. Sulfide mineral formation requires an environment with low oxygen activity. It is generally agreed that these conditions occur below the groundwater table.

The occurrence of multiple generations of calcite, a prominent feature in the mineral deposits of west-central Illinois, is also observed elsewhere in the upper Mississippi Valley, e.g. in the Upper Mississippi Valley Zn-Pb District (Heyl et al., 1959), and at the Linwood mine in Scott County, Iowa (Garvin, 1995). Differences in crystal habit and ultraviolet fluorescence among the several calcite types reflect variations in the physicochemical conditions in the growth environment. It has been demonstrated experimentally that the crystal habit of calcite can be modified by varying the magnesium content of the growth fluid (Devery and Ehlmann, 1981). Other variables that are known to influence calcite crystal habit include pH, temperature and growth rate. Ultraviolet fluorescence in calcite is caused by the presence of activator elements in the crystal lattice. The pink color is probably caused by Mn^sup 2+^; the yellow color may be due to the presence of Fe^sup 2+^. Both activators are common substituents in the crystal lattice of calcite.

Examination of specimens from these deposits frequently suggests that an earlier mineral or other material affected the growth of a later one. Epitactic overgrowths of small rhombohedral calcite crystals on single large scalenohedral calcite crystals have been found at Plymouth and Dallas City. Epitactic chalcopyrite on sphalerite occurs at Biggsville and Smithshire. Growth of microcrystals of pyrite on selected faces of calcite has taken place at Biggsville. Millerite needles were sites of nucleation of microscopic pyrite, sphalerite and calcite at Biggsville, the result appearing as strings of beads. Boxwork calcite and pyrite have been observed: the host material here was probably clay that occupied the cavity prior to mineralization; dessication of the clay created fractures along which calcite crystals (at Plymouth) and pyrite crystals (at Biggsville) grew. Following the removal of the clay by subsurface water flow, euhedral calcite crystals grew on the earlier-formed pyrite. The formation of boxwork structures indicates that these cavities were subjected to alternating vadose and phreatic conditions, reflecting climate-driven fluctuations in the groundwater table (Garvin, 1995). The branching and rib-like aggregates of quartz crystals observed at all localities but Smithshire, some showing no obvious points of attachment with cavity walls, might also be a consequence of growth in, and subsequent removal of, some preexisting material.

COMPARISONS

Numerous minor accumulations of sulfides and associated calcite and other minerals fringe the formerly commercial Upper Mississippi Valley Zinc-Lead District in Illinois, Iowa, Wisconsin and Minnesota (see Heyl et al., 1959, Fig. 101). Upper Mississippi Valley Zinc-Lead deposits are carbonate rock-hosted and consist mainly of calcite, dolomite, quartz, marcasite, pyrite, sphalerite and galena (Heyl et al., 1959). Fringing deposits also have carbonate rock hosts, and they generally contain the same minerals as the Upper Mississippi Valley deposits, but individual deposits may be missing galena, dolomite and/or quartz. Table 3 compares minerals and mineral characteristics for west-central Illinois and Upper Mississippi Valley Zinc-Lead deposits. It is clear that these two types of geologic environment are closely similar, as are their paragenetic sequences. In both, quartz and dolomite precede other minerals, sulfides are early and calcite is late. Iron sulfides generally precede sphalerite. These similarities suggest that the two types of deposits are cogenetic, and that both may be products of mineral-forming processes involving hydrothermal fluids (T>50

ACKNOWLEDGMENTS

We express our appreciation to the owners and operators of the Cessford quarries at Biggsville and Dallas City, the O'Neal quarry at Plymouth, the Gray's quarry at Hamilton and the Galbraith quarry at Smithshire for permission to enter their properties for the purposes of studying the geology and collecting mineral specimens. We are grateful to Zak Lasemi and Rod Norby of the Illinois Geological Survey for assistance with field work and for making Survey literature available to us. The specimen photography was supplied by Dr. Wendell Wilson.

Source: Mineralogical Record