D. A. Spears

Mineral matter and trace elements in coals of the Gunnedah Basin, New South Wales, Australia

Abstract The concentrations of major and trace inorganic elements in a succession of Permian coals from the Gunnedah Basin, New South Wales, have been determined by X-ray fluorescence techniques applied to both whole-coal and high-temperature ash samples. The results have been evaluated in the light of quantitative data on the minerals in the same coals, determined from X-ray diffraction study of whole-coal samples using a Rietveld-based interpretation program ( Siroquant ™), to determine relationships of the trace elements in the coals to the mineral species present. Comparison of the chemical composition of the coal ash interpreted from the quantitative mineralogical study to the actual ash composition determined by XRF analysis shows a high degree of consistency, confirming the validity of the XRD interpretations for the Gunnedah Basin materials. Quartz, illite and other minerals of detrital origin dominate the coals in the upper part of the sequence, whereas authigenic kaolinite is abundant in coals from the lower part of the Permian succession. These minerals are all reduced in abundance, however, and pyrite is a dominant constituent, in coals formed under marine influence at several stratigraphic levels. Calcite and dolomite occur as cleat and fracture infillings, mostly in seams near the top and bottom of the sequence. The potassium-bearing minerals in the detrital fraction are associated with significant concentrations of rubidium, and the authigenic kaolinite with relatively high proportions of titanium. Zirconium is also abundant, with associated P and Hf, in the Gunnedah Basin coal seams. Relationships exhibited by Ti, Zr, Nd and Y are consistent with derivation of the original sediment admixed with the seams from an acid volcanic source. Pyrite in the coals is associated with high concentrations of arsenic and minor proportions of thallium; no other element commonly associated with sulphides in coals, however, appears to occur in significant proportions with the pyrite in the sample suite. Small concentrations of Cl present in the coal are inversely related to the pyrite content, and appear to represent ion-exchange components associated with the organic matter. Strontium and barium are strongly associated with the cleat-filling carbonate minerals. Ge and Ga appear to be related to each other and to the coals organic matter. Cr and V are also related to each other, as are Ce, La, Nd and Pr, but none of these show any relationship to the organic matter or a particular mineral component.

Geochemistry and origin of elements in some UK coals

Abstract Twenty-four UK coals ranging in rank with 4.6%–37.6% volatile matter were analysed for 46 major and trace elements. The samples were obtained from the UK Coal Bank and are representative of the major UK coal fields. The major element distributions are interpreted in terms of the mineralogical variations—quartz and kaolinite are largely responsible for the Si and Al, carbonates for Ca and Mg and pyrite for Fe. Also exerting an influence in some samples are siderite, Al-phosphate minerals and illite. Based on statistical relationships with the major elements, Rb, Cr, Th, Ce, Zr, Y, Ga, La, Ta, Nb and V are thought to be mainly present in the clay minerals, and As, Mo, Sb, Tl, Se and Bi and Pb are probably present in pyrite. Strontium and Ba are concentrated in a restricted number of samples related to the phosphate minerals. Germanium is the only element for which a major organic association can be demonstrated. Elements with an indirect association with the organic matter are Na, Cl, and Br in porefluids and possibly Te. The ash content is controlled mainly by the detrital input and the trace elements related to the ash content are therefore those elements associated with the clay minerals. Variations with rank would appear to be mainly related to the moisture content (porefluids). The trace elements associated with the quartz and clay minerals are thought to be dominantly detrital in origin. The non-detrital elements, essentially those contained in pyrite, are thought to have been incorporated in the depositional environment from waters with enhanced salinities through seawater ingress, hence there are positive relationships between S and trace element concentrations.

The breakdown of British Coal Measure rocks

Abstract The influence of geological structures, textural features and mineralogy on the breakdown of Coal Measure rocks are considered, particular attention being paid to the more common rocks that are intimately associated with coal seams. A geographical variation is recorded in the clay mineralogy and evidence is introduced which favours two dominant processes being involved in the immediate slaking properties of mudstones, shales and seatearths. These are capillarity and intra-particle swelling of the 10 A mixed-layer mineral. Disintegration in terms of fundamental grain size is believed to be brought about by inter-particle dispersive mechanisms which are of longer term. An incipient rank relationship is tentatively suggested by the slaking behaviour of the more extreme end-members associated with low-rank coals.

Titanium in some Carboniferous sediments from Great Britain

In shale, mudstone and siltstone samples (154) a positive relationship exists between the TiO2Al2O3 ratio and the quartz content. As the grain size of the sediment increases, so titanium oxides (rutile and anatase are present) increase in importance relative to the silicates, due to sorting. In sandstones the ratio reflects the maturity, but the degree of sorting must be taken into account. The ratio of TiO2 substituting in the clay minerals to the Al2O3 content is not greater than 0.025 based on the whole rock analyses. This is borne out by values close to 0.010 obtained following the method of Raman and Jackson (Amer. Mineral. 50, 1086–1094, 1965). The addition of volcanic material to the normal sediment may either increase or decrease the whole rock TiO2Al2O3 ratio depending on the ash composition. Volcanic contributions may thus be recognized and several examples are given from the British Carboniferous.

The distribution and origin of trace elements in a UK coal; the importance of pyrite

Coal samples were obtained from a multigravity separator at Harworth (UK) coal-preparation plant. The minerals identified by X-ray diffraction in low-temperature ashes include kaolinite, illite, mixed-layer illite-smectite, chlorite, quartz, pyrite, apatite and gypsum. The latter is an artefact of the ashing procedure. Based on X-ray diffraction and major element analyses the coal samples show a preferential separation of quartz and pyrite. Samples were analysed for trace elements using energy dispersive, polarised XRF and data obtained for 25 trace elements with detection levels of ∼1 ppm for most elements and sub-ppm for some. One group of trace elements shows highly significant correlations within the group and with the pyritic S%. The concentrations (in ppmw) of these elements in pyrite were determined, per 1 wt% pyritic S, as: Mo 2, Se 0.6, Ni 8.2, As 21, Pb 6.1, Sb 0.4, Cu 13.8 and Zn 8.8. There are other sources for these elements in the coal, particularly for Ni, Cu and Zn, but pyrite is the main location. The concentration levels determined for pyrite may be applicable to pyrite in other coals because of a common process of enrichment from a seawater source. Another group of elements, consisting of Rb, V, Cr, Ga, Sr, Y, Zr, Nb, Sn, Ba, La, Ce and Th is thought to be contained within the detrital minerals. The only member of this group with a significant non-detrital origin is V, which is probably contained within the organic matter and in a low ash coal this would appear to be the major source. Germanium and Br are the two elements which have a dominant organic association.

Mineral matter in coals: cleat minerals and their origin in some coals from the english midlands

Abstract In four coal seam profiles from the Cannock Coalfield, West Midlands, the cleat (joint) frequency is a function of lithotype (greatest in vitarin) and is inversely related to bed thickness. The cleat minerals were studied: (1) under the microscope (optical and electron); (2) by X-ray diffraction; and (3) chemically. The minerals present are pyrite, marcasite, sphalerite, galena (sulphides), quartz, kaolinite (silicates), ankerite, calcite (carbonates) and apatite. This sequence has features in common with diagenetic sequences recorded in other rock types and it is concluded that the cleat minerals in coal form in response to pore fluid evolution and movement during burial diagenesis. The elements are thought to originate from both the coal and the associated sediments, mainly mudrocks. Similarities in diagenetic sequences are in part a reflection of the importance of organic maturation reactions in all the rock types in the sequence, whereas differences are attributed to the dominance of these reactions in the coal itself.

Towards a classification of shales

The problems relating to the classification of mudrocks, and in particular the role of grain size, are discussed. Classification based on silt-clay proportions is more realistic than a silt-clay-sand system. The grain size, as measured in mudrocks, is an uncertain quantity and the use of the quartz content is advocated. The method is applicable in the field using standard rocks of known quartz content. Attention is drawn to the property of fissility, which may in certain cases be shown to be due to the presence of laminae. The formation and preservation of laminae provide important sedimentological information.

MAS NMR study of surface-modified calcined kaolin

Abstract Calcined kaolins, modified by a silane coupling agent, were analyzed by magic angle spinning nuclear magnetic resonance (MAS NMR) and compared to unmodified samples. The results show no chemical shift for 29 Si, whereas there is a marked change for 27 Al. The chemical shifts of 5.44 and 65.69 ppm of 27 Al in unmodified samples are respectively moved to 3.8–4.4 and 54.6–59.9 ppm after modification. This is attributed to changes in the chemical environment around the surface Al ions on the calcined kaolin resulting from chemical bonding of the silane coupling agent molecules to the Al. The results demonstrate that MAS NMR is a useful technique for characterizing the surface modification mechanisms of minerals.

THE DETERMINATION OF QUARTZ IN SEDIMENTARY ROCKS USING AN X-RAY DIFFRACTION METHOD

An X-ray diffraction method for determining quartz in sediments is described which is both rapid and precise, with a coefficient of variation of 1·9 per cent. Samples are ignited at 950°C prior to X-ray analysis. This removes the interference of clay peaks, increases the relative intensity of the quartz peaks and reduces the initial matrix variation of samples. The peak area ratio of quartz (4·26Å) to an added standard boehmite (6·18 Å) is measured. Quartz content is obtained from a working curve constructed using similar rocks of known free silica content, which were analysed by the method of Trostel and Wynne (1940).RésuméOn décrit ici une méthode rapide et précise de diffraction par rayons X permettant de déterminer la présence de quartz dans les sédiments, avec un coefficient de variation de 1,9 pour cent. Les échantillons sont allumés à 950° avant l’analyse aux rayons X. Ceci détruit l’interférence des pics d’argile, accroît l’intensité relative des pics de quartz et réduit la variation initiale de matrice des échantillons. On mesure le rapport de la zone de crête du quartz (4,26 Å) par rapport à un boehmite standard d’apport (6,18 Å) La teneur en quartz est obtenue à partir d’une courbe de travail construite en utilisant des rocs similaires d’une teneur connue en silice libre, qui one été analysés par la méthode de Trostel et Wynne (1940).KurzreferatEine Röntgendiffraktionsmethode zur Bestimmung von Quartz in Sedimenten wird beschrieben, die sowohl schnell als auch präzise ist. Der Variationskoeffizient beträgt 1,9%. Die Proben werden bei 950°C vor der Röntgenanalyse geglüht. Dies beseitigt die Interferenz der Ton-peaks, erhöht die relative Intensität der Quartzpeaks und verringert die ursprüngliche Grundmassenvariation der Proben. Das Verhältnis der Peakflächen des Quartzes (4,26 Å) und eines zugesetzten Normalböhmiten (6,18 Å) wird gemassen. Der Quartzgehalt wird an Hand einer Arbeitskurve festgestellt, die unter Anwendung ähnlicher Gesteine mit bekanntem Gehalt an ungebundener Kieselerde entwickelt wurde; diese Gesteine wurden nach der Methode von Trostel und Wynne (1940) analysiert.РезюмеОписан быстрый и точный метод рентгенометрического определения кварца в осадочных породах с коэффициентом вариации 1,9%. Образцы предварительно прокаливаются при 950 °С, что исключает появление пиков от глинистых минералов, увеличивает относитеьл-ную интенсивность пиков кварца и уменьшает начальные фоновые различия образцов. Измеряется отношение площади пика кварца (4,26 А) к площади пика добавленного к образпу бемитового стандарта (6,18 А). Содержание кварца определяется по рабочей кривой, пост¬роенной с использованием аналогичных пород, содержащих определенное количество кремнезема; эти породы анализировались по методу Тростела и Уайнна (1940).

Geochemical and mineralogical characteristics of a power station feed-coal, Eggborough, England

Abstract Chemical and mineralogical analyses are presented for a power station feed-coal in the Yorkshire Coalfield, U.K. The mineralogy of the whole coal consists of quartz, kaolinite, illite, mixed-layer clay, chlorite, pyrite and carbonates. Pyrite, carbonates and some of the kaolinite are diagenetic. In separated, low-density fractions of the coal both pyrite and kaolinite are concentrated relative to the detrital minerals. Compared with the high density fractions the detrital clay mineralogy of these low density fractions contains less chlorite, and the mixed-layer clay to illite ratio is higher, which is consistent with a grain size control during sedimentation. The major element analyses are comparable with mudrocks in the stratigraphic sequence, but there is evidence of Na 2 O enrichment related to connate brinnes and the Fe 2 O 3 content shows significant enrichment, particularly in the ashes of the low density fractions. The dissolution of oxhydroxide grain coatings associated with detrital sediment in the coal, and also in the adjacent strata, are probable sources of the iron in the diagenetic pyrite. There is evidence of iron mobility in the sedimentary sequence and introduction into the coal itself during diagenesis. Within the coal, trace elements are also enriched (on an ash basis) relative to the mudrocks and their origin is also probably very similar to that of iron. A noteworthy feature of the trace element enrichment is that the enrichment factors for a number of elements are comparable with those determined for marine black shales in the sequence. This suggests a common origin for the elements during the diagenetic processes. Analysis of the coal density fractions, after correcting for detrital mineralogy, suggests Pb, Cu, Ni, Zn and possibly Mn are associated with pyrite and V, Sr, Ba, Zr, and Nb are either directly or indirectly associated with organic components. Possibly Zr and Nb originate from dispersed volcanic ash.