Mineralogical weathering sequences in sediments overlying lignite beds were investigated in a core (27 m deep) from Calvert, Robertson County, northeast Texas. Weathering trends were evaluated based on the properties and relative distributions of both the expandable and non-expandable minerals. The sulfide minerals in these sediments are the most susceptible to weathering and were only observed in the unaltered (reduced) zone below 7 m. Oxidation of the sulfides has resulted in the formation of jarosite and gypsum in the upper 7 m of the core (oxidized zone). The oxidized zone is further characterized by reddish brown colors (high chroma), a greater quantity of dithionite-extractable iron, and absence of chlorite. Although the major clay minerals in these sediments (smectite, kaolinite, mica) are largely detrital, weathering has resulted in an increase in the content of kaolinite and a decrease in the content of mica towards the surface. The mica appears to have altered to a high-charge smectite characterized by basal spacings of 32 Â after intercalation with octadecylammonium cations. The high-charge smectite is most abundant in the soil horizons at the top of the core and gradually decreases with depth. A low-charge smectite is the most abundant species in the un weathered parent sediments and increases with depth. Key Words—Lignite, Oxidation, Reclamation, Smectite, Soil, Weathering.

The mineralogy of partially kaolinized strata interbedded with lignite at the San Miguel mine, Atascosa County, Texas, was investigated by X-ray powder diffraction and optical and scanning electron microscopy. The San Miguel lignite occurs in the lower Jackson Group (late Eocene) of southern Texas. Based on mineralogical and micromorphological data, some of these clay partings are probably volcanic in origin and were exposed to variable degrees of in situ kaolinization in a swamp environment. Coexistence of kaolinite, clinoptilolite, and opal-CT in several of these strata suggests that the partially kaolinized volcanic layers were subjected to a subsequent resilication process following burial. Kaolinite is the dominant mineral in the oldest and most kaolinized volcanic layer (underclay) below the lowest lignite bed (seam D). The kaolinite exhibits a well-developed vermicular morphology. The youngest volcanic layer, which occurs stratigraphically above the uppermost lignite seam, is characterized by pseudomorphs of volcanic glass shards and consists mainly of clinoptilolite. Movement of siliceous ground water from this layer to the underlying strata apparently provided silica-rich solutions from which opal-CT and large (as long as 300 μm) euhedral crystals of clinoptilolite precipitated in the fossilized plant roots, veinlets, and fractures within the underlying strata. Micromorphological relationships between the Sirich (opal-CT and clinoptilolite) and sulfide (marcasite and pyrite) minerals in the fossil roots and fractures suggest that the marcasite formed before and pyrite after the resilication process.

A kaolinite-rich bed (tonstein) and an associated bentonite in the upper part of Yegua Formation at College Station, east-central Texas, were formed by in situ weathering processes in a late Eocene swamp. X-ray powder diffraction, infrared spectroscopy, petrographic studies, and scanning and transmission electron microscopy not only show that dioctahedral smectite and coarsely crystalline kaolinite are the dominant minerals in the bentonite and tonstein, respectively, but that cryptocrystalline halloysite and kaolinite are localized along the weathering front (transitional zone) between the tonstein and the bentonite. As weathering progressed, the cryptocrystalline minerals gradually recrystallized to yield the coarse books and vermicular growths of kaolinite characteristic of the tonstein.

Small amounts of cristobalite, sanidine, and euhedral zircon crystals with liquid or gaseous inclusions accord with the formation of the bentonite by alteration of volcanic ash. Clinoptilolite in the lignitic layer and sandstone below the bentonite probably formed from ions that were released during alteration of the volcanic materials to smectite, but clinoptilolite in the tonstein and overlying strata appear to have formed after kaolinization of the bentonite.

Most previous studies of the kaolin deposits in the southeastern United States have focused on their mineralogy and petrology to understand better the depositional and diagenetic environments of the kaolins. Many studies suggest, however, that much of the information held within the minerals was changed during extensive post-depositional groundwater and microbial alteration. Organic δ13C and biomarker analyses were used, therefore, to provide further information on the nature of the original sediments, the depositional environment(s), and the amount of diagenetic alteration that has occurred in Georgia kaolin deposits.

Two different types of kaolin can be discerned, based on their total organic carbon contents: organic-lean kaolin and lignitic kaolin. The bulk organic δ13C in the Georgia kaolins ranges from ~−26 to −19% (VPDB, Vienna Pee Dee Belemnite standard), with a noticeable enrichment in 13C with decrease in organic carbon concentration. The lean kaolins are by far the more dominant types, with an organic-matter composition primarily of C16–C22n-alkanes, C16 and C18 fatty acids, and unresolved complex mixtures. Lignitic kaolin has a distinctly different organic matter (OM) composition. The lignitic material is primarily C15–C33n-alkanes with a greater abundance of C23–C31n-alkanes and lesser amounts of resinous and microbial constituents along with the oxidized forms of the saturated lipid fractions.

Biomarker data suggest that the lignitic material is primarily terrestrially derived from conifers with minor input from microbial lipids. The OM in both types of kaolin shows strong signs of microbial decomposition that yield the organically lean kaolins. The oxidation of the detrital organic matter would subsequently yield organic acids that would have exerted significant influence on the mineralogy and metal mobility.

The Georgia, USA, kaolins contain three major occurrences of Fe-sulphide minerals, namely (1) in burrow structures, (2) as nodules associated with lignite and (3) as finely disseminated material. We have used sulphur isotope analysis and X-ray powder diffraction to understand these three occurrences in order to gain insight into the diagenetic conditions and microbial influences affecting the kaolin mineralogy. Burrows and nodules are dominated by corroded marcasite with thin pyrite overgrowths. The δ34S composition of each sulphide sample has internal fractionations (δ34Shigh – δ34Slow) ranging from 22‰ to 67‰. The secondary pyrite phase tends to be the most 34S depleted, with a low δ34S value of –47‰, whereas marcasite displays a δ34S range from 30‰ to +33‰. Early acidic environments from organic acids could have facilitated the initial marcasite crystallization, with a pH increase during later diagenesis producing pyrite overgrowths. Sulphide formation would have continued until burial restricted the diffusion of sea-water sulphate. At this point, the metabolism of sulphate-reducing bacteria would have been retarded, producing δ34S values ranging from –30‰ (open sulphate supply) to +40‰ (highly restricted sulphate supply). Under the nutrient-starved conditions of later-diagenesis, disproportionate sulphur-reducing bacteria would likely have become more dominant, yielding additional δ34Ssulphide fractionation and producing the observed fractionation (δ34Ssulphate – δ34Ssulphide) values of +67‰.

The up to 60 m thick Neurath Sand (Serravallian, late middle Miocene) is one of several marine sands in the Lower Rhine Basin which were deposited as a result of North Sea transgressive activity in Cenozoic times. The shallow-marine Neurath Sand is well exposed in the Garzweiler open-cast mine, which is located in the centre of the Lower Rhine Basin. Detailed examination of three sediment profiles extending from the underlying Frimmersdorf Seam via the Neurath Sand and through to the overlying Garzweiler Seam, integrating both sedimentological and palaeontological data, has enabled the depositional setting of the area to be reconstructed.

Six subenvironments are recognised in the Neurath Sand, commencing with the upper shoreface (1) sediments characterised by glauconite-rich sands and an extensive biota (Ophiomorpha ichnosp.). These are associated with the silt-rich sands of a transitional subenvironment (2), containing Skolithos linearis, Planolites ichnosp. and Teichichnus ichnosp. These silt-rich sands grade up to the upper shoreface subenvironment (1), which is indicative of an initial regressive trend. The overlying intertidal deposits can be subdivided into a lower breaker zone (3), characterised by ridge-and-runnel systems, and the swash zone (4) where the surge and backwash of waves resulted in the deposition of high-energy laminites. The intertidal deposits were capped by aeolian backshore sediments (5). Extensive root traces present in this latter subenvironment reflect the development of the overlying peatland (i.e. Garzweiler Seam). Within the Garzweiler Seam, restricted sand lenses indicate a lagoonal or estuarine depositional environment (6). Regional correlation with adjacent wells establishes that shallow-marine conditions were widespread across the Lower Rhine Basin in middle Serravallian times. The shoreline profile, characterised by both tidal and wave activity and influenced by fluvial input from the adjacent Rhenish Massif, is indicative of the complexity of the coastal depositional setting within the Lower Rhine Basin.

The Cenozoic-age Lower Rhine Basin is located in the NW part of the European Cenozoic Rift System. In Miocene times, a combination of warm climatic conditions and basin subsidence resulted in the deposition of up to 100m of lignite (i.e. Main Seam of the Ville Formation). The Main Seam can be subdivided into the Morken, Frimmersdorf and Garzweiler seams, separated by two intercalated transgressive sand units, namely the Frimmersdorf and Neurath sands, deposited in a shallow-marine, tide-dominated environment. The lignite seams of the Ville Formation are currently worked by RWE Power AG, in the Garzweiler II open-cast mine. In the Frimmersdorf Seam (between the Frimmersdorf Sand and the Neurath Sand), the presence of small-scale sand bodies, together with their variable dimensions, affects the industrial exploitation of the seam. Moreover, their irregular distribution complicates their precise and early recognition. Indeed, so-called barren lignite (≥ 17% of sand) and completely clean units can occur within a few metres of each other. Initial classification of these highly variable sand bodies suggests a variety of both pre- and post-depositional causal mechanisms, providing evidence of an extremely complex depositional and post-depositional system. Syn-depositional sand bodies were deposited in a swamp area that was located in the fluvial-dominated sub-environment of an extended tidal estuary. The post-depositional formation of sand bodies is related to the intrusion of fluidized sands from the underlying Frimmersdorf Sand. These sand injectites within the Frimmersdorf Seam are considered to be linked to seismic activity within the Lower Rhine Basin.