Introduction to the Mineral Glauconite
It is widely accepted that the majority of glauconite forms authigenically as it alters numerous materials exposed on the substrate which includes different mineral grains, especially micas and feldspars. In 1981, Odin and Matter coined a number of terms in an effort to better reference the different stages of glauconite. These terms include: “glaucony (for the green grains in general), glauconitic smectite (dominant mineral of the immature, yellowish green grains) and glauconitic mica (dominant mineral of the mature, dark green grains)” (Erzsébet, T., 2010). As glauconite increases in age it trends towards becoming more potassium rich ranging from K-poor smectites to K-rich glauconitic micas (Krinsley and Pye, 2005). The maturity can be seen in the evolution of the grains morphology which are sub-divided into four morphological stages: nascent, slightly evolved, evolved, and highly evolved. Recent studies have estimated that it may take hundreds of thousands of years in order for highly evolved glauconites to form. Within the evolved to highly evolved stages, the granular substrate cracks and is later filled. This process greatly increases a deposits potential for radiometric dating using the 40K/40AR method. Historically, the greensand deposits of central and southern New Jersey were locally quarried by 18th century farmers for use as a fertilizer in order to keep their land fertile for use year after year. In the early to mid 19th century, several marl companies were established along the nutrient rich inner coastal plain. As result of extensive quarrying, the fossilized remains of a wide diversity of cretaceous period species had been unearthed including the first nearly complete dinosaur skeleton, Hadrosaurus foulkii in 1858.
Glauconite or glaucony as preferred by some authors, was named in 1828 as part of the Mica group derived from the Greek word glaukos meaning “blue-green”. It's rich green color is a distinctive characteristic and also a diagnostic property of this mineral. In fact, glauconitic sedimentary deposits are simply referred to as “greensands” representing its most distinguishing feature. There are many factors controlling development including water depth, sedimentation rates, Eh and pH levels, degree of agitation and oxidation-reduction, starting subtrate material (degradation of the original phyllosilicates), as well as a generous length of time. Because glauconite forms in a specific range of environmental conditions it is useful as an indicator for interpreting depositional environments in the rock record. These conditions are also favorable for preserving a wide variety of lifeforms exemplified by the cretaceous deposits of the Atlantic Coastal Plain. Formations such as the Navesink and Hornerstown are abundantly fossiliferous and contain key index fossils such as the oyster exogyra and ammonite baculites.
The mineral glauconite is a dioctahedral layer potassium phyllosilicate made up of complex potassium, iron, aluminum silicates with greater than fifteen percent iron (III) oxide. It is comparatively described as an iron-rich variety of illite with a structure much alike muscovite. The difference is that the most abundant cation contained on the octahedra sites of glauconite are Fe3+, with an accessory of Mg2+ , Fe2+ and Al3+. As a result, the tetrahedra sites of glauconite contain lower amounts of Al3+ than muscovite in order to compensate for the divalent cations in the octahedral sites (Nesse, 2000). In the structure of mica, two silica tetrahedral sheets face one central octahedral sheet. The tips of the tetrahedra point toward the central unit, and the three sheets are combined into a single layer with a suitable replacement of OH and O. As much as 25% of the silicon may be replaced by aluminum, and the resulting charge deficiency is balanced by inter-layered potassium ions. In glauconite, aluminum can substitute for 4.2% to 17.5% of the silicon in tertahedral sheets. Most glauconite is not pure dioctahedral phyllosilicate but rather a mixture of 10A and 14A expandable lattice phyllosilicates (Dooley, 2006).
Chemical and Morphological Genesis
Glauconization begins with the degradation of the original phyllosilicate as it loses octahedral cations. These phyllosilicates can be either authigenic or a detrital and can degrade as a result of both inorganic and organic processes. McConchie (1979) references: “bacterial metabolism (Prather, 1905) and / or the passage of sediments through the guts of organisms” (Pryor, 1975) to the list of organic processes. Regardless of how the phyllosilicates degraded they must have had a high positive charge deficiency lacking a sufficient amount of cations within the octahedral layer. This charge deficiency would have been balanced temporarily by highly mobile and abundant marine cations like magnesium, potassium, calcium and sodium until they could be replaced by structural iron (McConchie, 1979). The iron is introduced as Fe2+ and through the ion exchange with the stabilizing cations it is oxidized to Fe3+. As the Fe2+ is oxidized to Fe3+, electrons are released and then work to aid the surrounding reduction reactions of the environment. It should be noted that not all the Fe2+ will necessarily be oxidized and thus can be used as a measure of what was occurring in the environment. For example these processes could end with changes in Eh or abundance of Fe. The necessity of slightly reducing conditions (Eh < 0) and a pH of 7 -8 are strictly governing factors in the development of glauconite. It is thought that the reducing conditions are most important in the early stages of the proto-glauconite, but slightly oxidizing conditions become dominant overall (McConchie, 1979). The Eh levels are largely as a result of the organic matter so there is also a delicate balance of how much organics is preferred for glauconite development. In addition to the Eh becoming too negative, chelation can limit the amount of Fe available, organic acids could degrade the glauconite at rates highly than that of its formation, and relatively large quantities of sulfur can form iron sulfides instead of glauconite (McConchie, 1979).
Glauconite characteristically occurs as rounded aggregates and pellets of fine grained, scaly particles (Bonewitz, 2008). These pellets form almost exclusively in agitated, oxidized, normal shallow marine waters at depths of 50 – 200 m (Prothero and Schwab, 2004). Because glauconite forms in a specific range of environmental conditions it is useful as an indicator for interpreting depositional environments in the rock record. These conditions typically occur on the upper continental slope being the ideal low energy environment for glauconite development. Another crucial factor of developing concentrations of glauconite is the amount of clastic sediment imput as glauconite is favorably produced in shallow shelf environments with very slow sedimentation rates. It isestimated that in order for glauconites to form they must reside at the sediment-water interface for 1000 to 10,000 years, and development of highly complex glauconites takes much longer (Prothero and Schwab, 2004). This is not a surprise when we think of the lengthy processes involved in development.In the context of a system-tract, glauconite forms within times of transgression where the continental shelf becomes sediment starved as a result of rapid deepening increasing the accommodation space (Obasi, 2011). It is recognized that the majority of glauconite forms authigenically as it alters numerous materials exposed on the substrate which includes different mineral grains, especially micas and feldspars. “The clay (phyllosilicate) fraction in the greensand matrix consists of mainly illite/smectite, illite, micas, ferric illite, kaolinite, and chlorite” (Dooley, 2006). In 1981, Odin and Matter coined a number of terms in an effort to better reference the different stages of glauconite. These terms include: “glaucony (for the green grains in general), glauconitic smectite (dominant mineral of the immature, yellowish green grains) and glauconitic mica (dominant mineral of the mature, dark green grains)” (Erzsébet, T., 2010). As glauconite increases in age it trends towards becoming more potassium rich ranging from K-poor smectites to K-rich glauconitic micas (Krinsley and Pye, 2005).
Glauconites also form casts of forminifera and other microfossils because it is typically produced under reducing conditions where it biogenetically replaces things like decaying fecal pellets due to the extensive burrowing of annelids for example and also the tissues of forminifera. In some cases, interpretation of the depositional environment has been difficult because of glauconites ability to be reworked in an energetic, well-oxygenated setting (Prothero and Schwab, 2004). When autochthonous (forms where found; authogenic), glauconite is well sorted as the pellets are remarkably uniform in size, but when allochthonously reworked the sediments are typically poorly sorted (Dooley, 2006). It should be noted that there is evidence in the modern that on rare and unique occasions glauconite does develop outside of a marine environment where not only the chemistry of nonmarine solutions match, but also a lengthy list of factors creating the ideal conditions that enable development. For example, the energy of a system and the level of agitation caused will be a major factor in development. Because glauconite forms best in a low energy environment we would not expect to see it within current generated structures which occurrences would have had to be due to the pellets being reworked. Of course the least challenging greensands to interpret occur in a shallowing upward marine sequence directly underneath an unconformity. At the Inversand Co. Mine, in Sewell, NJ, sorting was used to determine whether or not a mass burial of vertebrates in what is called the Main Fossiliferous Layer (MFL) of the lower Hornerstown formation were buried in situ or were accumulated as a result of reworking. The glauconite pellets within the MFL were determined to be well sorted with little siliclastics, and also morphologically highly evolved. This coupled with the consistent REE signatures of the fossils and the fact that there are semi to fully articulated vertebrates suggests that the MFL glauconite formed authigenically and the vertebrates were buried in situ.
Glauconite is further utilized for absolute radiometric dating using the 40K/40AR method. However, this method is limited to the condition of the pellets. The textures of glauconite have been extensively studied by use of backscattered electron microscopy (BSE) in order to aid in both determination of depositional environment and also in the reliability of dating. By examining the individual glauconite grains with BSE we can identify fissures as evidence of an open system or we can locate felspar nuclei grains containing 40K and thus may cause inaccurate radiometric dating. Often glauconite pellets fracture and are then later filled in by a newer generation with some compositional differences (Krinsley and Pye, 2005). This process of fracturing and filling creates the opening and closing of the system. Whether referred to as fissures or cracks, in either case the terminology is not only related to its structural appearance but also as a description of physically becoming fissured or cracked by the nature of its growth without any outside forces acting on it. Cracks are defined as inward narrowing micro-crevices that develop due to a differential crystal growth process that is more efficient in the center of the grain (Odin, G., 1988). Therefore, it is predictable that as the grain increases in size, cracks will develop and increase in size proportionately. Grains which have cracks are termed evolved and those in which have been later filled are considered to be highly evolved. The stages of evolution are said to be nascent, slightly evolved, evolved, and highly evolved. In the nascent stage, the shape of the pellets are simple and have a low concentration of K2O. At the slightly-evolved stage streaks of glauconite are formed within the substrate. In the evolved and highly evolved stages the granular substrate cracks enabling the diffusion of K+ to the pellets center. During the highly-evolved stage the potassium reaches its concentration limit. It is at this time that the cracks are filled creating a bulbous shaped pellet (Obasi, 2011).
In the late 1700's to the mid 1800's throughout the farming communities of central and southern New Jersey, greensands or marls as they were called were dug up for use as fertilizer. During this time farmers were thriving off the fertility of their lands, but in order to use the same plot of land repeatedly, year after year, fresh marls were needed to keep the lands fertile. As a result, greensand for use as a fertilizer was in high demand. The plentiful land of the Cretaceous belt or inner coastal plain was nutrient rich and so farmers and marl companies had strong interest in quarrying. Because of the extensive amount of quarrying many fossils had begun to be unearthed. Invertebrate and vertebrate alike, the greensands have preserved a wide range of cretaceous animal life. In 1842, British comparative anatomist Richard Owen (1804-1892) coined the term dinosaur, “fearfully great lizard”, to classify a group of extinct reptiles. This was based on only isolated bones and teeth of three genera of animals, Iguanodon, Megalosaurus, and Hylaeosaurus (Gallagher, 1997). At the time, no nearly complete skeletons had been discovered. All that would change on a warm summer day in 1858, while William Parker Foulke, a member of the academy of Natural Sciences was vacationing in Haddonfield, New Jersey. As the story goes, Foulke was taking in the country air when he had become acquainted with John Hopkins, proprietor of Birdwood Farm. After learning of Foulke's interest in the natural sciences, Hopkins talked about an unusual discovery made some twenty plus years earlier (Foulke White, 1991). While digging in a marl pit, along a tributary on the Cooper River, near Maple Avenue he had unearthed several large bones (Weishample and Young, 1996). Never having any scientific interest in them, Hopkins gave the bones away to visitors. Foulke wondered if any more bones remained, and was granted permission to reopen the old marl pit in hopes of making a discovery. The following fall Foulke assembled a team of diggers and started to dig down through the marl. After about ten feet the diggers began to bring up bones. Eash was sketched in position, measured, and then carefully extracted. They were then wrapped in cloth and carried up to a cart filled with hay and transported to Foulke who was staying a short distance away (Gallagher, 1997). At Foulke's notice, Dr. Joseph Leidy (1823-1891), and an invertebrate specialist Isaac Lea (1792-1886), traveled to Haddonfield to aid the excavation. Together they mapped out the area and identified many of the shells found in the formation (Woodbury Fm.) containing the bones. Although the skull was never found, the world's first nearly complete skeleton was officially found. Dr. Leidy named the dinosaur Hadrosaurus foulkii, “Foulke's bulky lizard” in honor of William P. Foulke. In 1868, the entire skeleton was assembled, mounted and put on display at the Philadelphia Academy of Sciences where it is still available for public viewing. This is just one great example of how important the greensands of the inner coastal plain of New Jersey were to the history of American paleontology and will continue to do so in the future. The famous “bone wars” of Cope vs. Marsh were also initially largely centered on the Jersey greensand “marls”. Of course terrestrial finds such as dinosauria are certainly rare with very isolated occurrences of usually fragmented bone and teeth material. The preservation of Hadrosaurus foulkii, a nearly complete skeleton, is extremely rare and would have required the hadrosaur to bloat and float far away from its living environment to the marine environment in which it was deposited. Fortunately for the paleontologist, the process in which glauconite forms is especially good for the fossilization process preserving even the most delicate bones of aves.
In the mid 1800's New Jersey's greensand industry grossed over one-half of a million dollars every year, but by 1910 the gross was less than five-thousand dollars annually. Greensand application was especially good for forage type crops as it slowly released potash and many trace element nutrients, but when potash became readily available the industry was reduced to next to nothing. In the later part of the 1800's glauconite was used in brick making and also for making green glass (Dooley, 2006). This was done locally in central and southern New Jersey and were short lived having never developed into any type of mainstream practices. In the early 1900s glauconite had begun to be used as a water softener because of its base exchange properties. This exchange involves the taking of magnesia or lime in the water and releasing sodium ions. Only the outer surface area of the glauconite is involved in the exchange and once filled to its limit it must be regenerated by treatment of sodium chloride solution. The simple process called backwashing consists of using the sodium chloride to replace the absorbed hard water elements with sodium. In order to increase the exchange capacity of glauconites an increase in surface area is made by increasing porosity. The only caveat being that by increasing porosity you inherently increase the amount of salt needed for regeneration (Dooley, 2006). This coupled with the fact that the porosity inducing processes will reduce the life span of the pellets. In the late 1940s phenol formaldehyde resin largely replaced greensand having twice the water softening capacity. A short time later styrene resins replaced phenol formaldehyde resin having an even greater capacity (Dooley, 2006).
The Inversand Co. mine located in Sewell, NJ first began their glauconite mining operations in 1925 and is now the only commercial greensand producer in the United States. Today, the Cretaceous to Paleocene age greensands are extracted and processed into what is called manganese greensand. In short, the glauconite is treated with manganese oxide after being hardened and stabilized using a number of solutions including sodium aluminate, sodium silicate and aluminum sulfate (Dooley, 2006). This treatment in which coats the pellets gives glauconite its special chemical oxidation-reduction properties in order to efficiently remove manganese, iron and hydrogen sulfide (Augustine, 2009). Other than water treatment, glauconite is currently not being used for anything. Soil conditioning is done locally by gardeners and small scale farmers, but besides the spoil piles available for purchase from the Inversand Co., there is no market for this use. Throughout Monmouth County many land owners simply dig their own pits out of tributaries that cut through their property for personal use.
Discussion – Mining for Rare Earth Elements?
Investigations into the presence and concentrations of REE's in glauconite have recently been conducted with conflicting results. In 2000, Jarrar determined from his studies of cretaceous glauconies that the more highly evolved pellets contained a greater amount of REE's in comparison to those which are less evolved. This suggests a trend of increasing REE content with increasing maturity. In 2008, Toth demonstrated that the trend is just the opposite as the REE content decreases during glauconitisation. Toth pointed out that Jarrar's samples contained between 0.08 – 0.2 wt. % of P2O5, and in thin section likely substrates were identified by the phosphatic grains and biotite flakes. Because biotite commonly contains REE concentrating inclusions of zircon, apatite, or monozite as well as the phosphatic grains being REE carriers, Jarrar's main element analysis was likely camouflaged by the accompanying phases. Using two series of glauconitisation, Toth showed that the interlayered REE's that substituted for Ca in the layer silicates are removed as the smectitic materials become more mica-like (Toth, 2008). Based on their works we can see that REE content percentages vary throughout the maturity stages and hold the highest concentrations when there is an abundance of accessory minerals. In order to determine the REE content of glauconite you must eliminate all accessories by employing high-sensitivity, spatially-resolved techniques such as LA-ICP-MS (Toth, 2008). Regardless of whether the REE's are within the submicroscopic apatite and phosphate accessories or the glauconite itself , there is currently no interest found in mining glauconite as a source of REE's.
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