Mean Maximum Vitrinite Reflectance Research Articles

Chemical functional characterization of immature and mature coals at the nanoscale by atomic force microscopy-based infrared spectroscopy (AFM-IR)

Understanding the composition and molecular structure of kerogens and coals is a key issue in geochemistry, as these parameters control their chemical evolution and their by-products generated in sedimentary basins. As they are chemically heterogenous, investing their structure and composition at various spatial scales is essential and relies on a variety of micro-analytical tools. Conventional infrared (IR) microscopy has proven to be a powerful and promising approach to characterize these heterogeneous materials, but due to the diffraction limit, the spatial resolution cannot get below ∼1 μm, and cannot measure individual submacerals, tiny mineral inclusions, and more generally cannot access to chemical variations at the nanometer scale. In contrast, atomic force microscopy coupled to infrared spectroscopy (AFM-IR) attains a much higher spatial resolution, down to tens of nanometers. In this paper, AFM-IR measurements were collected on three immature and mature coals, with mean-maximum vitrinite reflectance R0 of 0.33%, 1.16%, and 2.8%. Measurements were collected in both tapping and contact modes, on samples prepared as sulfur embedded ultrathin sections and pressed on diamond windows. These spectral data were compared to spectra collected with conventional micro-IR microscopy (μ-FTIR). Spectra with a high quality could be obtained, which point to chemical heterogeneity at the sub-micrometer scale, and they display similar bands than those observed in spectra collected by means of conventional micro-IR. The signal-to-noise ratio was better in spectra collected in contact mode compared to tapping mode. The spatial resolution was around ∼100 nm in contact mode, while a resolution of at least 20 nm was achieved in tapping mode. Kaolinite was detected as a ∼ 500 nm grain included in organic matter. These results show that sulfur embedded ultrathin sections are as a suitable sample preparation for AFM-IR. However, systematic differences are reported compared to spectra collected by conventional μ-FTIR, possibly due to an instrumental artifact that increases the absorption signal as the wavenumber is decreased. Similarly, systematic differences are observed between data collected in tapping and contact modes, along with a much broader chemical heterogeneity. These differences are not fully understood, and should be investigated further in future studies.

Fractal characteristics of pulverized high volatile bituminous coals with different particle size using gas adsorption

To better understand the effect of particle size on fractal characteristics of pores in coal, three high volatile bituminous coals with mean maximum vitrinite reflectance of 0.82–0.90% collected from the Naryksko-Ostashinskaya coalbed methane field in Kuznetskiy Basin in Russia were crushed and sieved to five progressivelydecreasing particle size fractions: 1.0–0.5 mm, 0.50–0.25 mm, 0.250–0.125 mm, 0.125–0.063 mm and 0.063–0.032 mm. For all particle size fractions, particle size distribution measurement, proximate analysis, maceral analysis and low-pressure N2 adsorption and CO2 adsorption analyses were conducted. Fractal dimensions D1 (pore surface roughness) and D2 (pore structural irregularity) were determined from relative pressure intervals of 0–0.45 and 0.45–1 of N2 adsorption isotherm with the application of the fractal Frenkel-Halsey-Hill theory, respectively. The results demonstrate significant differences in fractal dimensions in different particle size fractions. As particle size of coal samples decreases, D1 decreases, while D2 increases. Some constricted mesopores are transformed into non-constricted mesopores during crushing. The non-constricted mesopores increasingly concentrate in smaller pore sizes, which results in smoother pore surface (decreasing D1) and more heterogeneous pore structure (increasing D2). Mesopores with pore width between 2 nm and 5 nm are the key to fractal dimensions. Individual mesopore structure parameters closely associated with particle size have universal correlations with fractal dimensions. The increasing mesopore specific surface area and volume and decreasing average mesopore width lead to a decrease in D1 and an increase in D2, which further confirms the effect of particle size on fractal dimensions.

Intelligent Estimation of Vitrinite Reflectance of Coal from Photomicrographs Based on Machine Learning

The accurate measurement of vitrinite reflectance (especially for mean maximum vitrinite reflectance, MMVR) is an important issue in the fields of coal mining and processing. However, the application of MMVR has been somewhat hampered by the subjective and the time-consuming characteristic of manual measurements. Semi-automated methods that are oversimplified might affect the accuracy in measuring MMVR values. To address these concerns, we propose a novel MMVR measurement strategy based on machine learning (MMVRML). Considering the complex nature of coal, adaptive K-means clustering is firstly employed to automatically detect the number of clusters (i.e., maceral groups) in photomicrographs. Furthermore, comprehensive features along with a support vector machine are utilized to intelligently identify the regions with vitrinite. The largest region with vitrinite in each photomicrograph is gridded for further regression analysis. Evaluations on 78 photomicrographs show that the model based on random forest and 15 simplified grayscale features achieves the state-of-the-art root mean square error of 0.0424. In addition, to facilitate the usage of petrologists without strong expertise in the machine learning domain, we released the first non-commercial standalone software for estimating MMVR.

Organic matter network in post-mature Marcellus Shale: Effects on petrophysical properties

Shale samples of the Marcellus Shale from a well drilled in northeastern Pennsylvania were used to study diagenetic effects on the mineral and organic matter and their impact on petrophysical response. We analyzed an interval of high gamma ray and anomalously low electrical resistivity from a high thermal maturity (mean maximum vitrinite reflectance > 4%) part of the shalehgas play. A suite of microanalytical techniques was used to study features of the shale down to the nanoscale and assess the level of thermal alteration of the mineral and organic phases. The samples are organic rich, with total organic carbon contents of 3–7 wt. %; the vast majority of the organic matter was identified as highly porous pyrobitumen. Matrix porosity is also present, especially within the clay aggregates and at the interface between rigid clasts and clay minerals. Mineral- and organic-based thermal maturity indices suggest that during burial the sediment had been exposed to temperatures as high as 285°C (545°F). Under these conditions, the residual, migrated organic matter assumed a partially crystalline habit as confirmed by the identification of turbostratic structures via electron microscopy imaging. Experimental dielectric measurements on organic matter–rich samples confirm that the anomalous electrical properties observed in the wire-line logs can be ascribed to the presence of an electrically conductive interconnected network of partially graphitized organic matter. The preservation of porosity suggests that this organic network can contribute not only to the electrical properties but also to the gas flow properties within the Marcellus Shale.

Nanostructure of cokes

Twenty eight cokes made from Australian coals and prepared under a range of experimental conditions were examined using Small Angle Neutron Scattering (SANS) and Ultra Small Angle Neutron Scattering (USANS) to determine the size distribution of all of their pores and also pores inaccessible to deuterated toluene (closed pores) in the size range 1 to 1000 nm radius. The main findings were that in all cokes, even those made from relatively inertinite-rich coals, nearly all pores of <5 nm radius were closed and >70% of the pores of 1000 nm in radius or larger were open. At intermediate pore sizes, the fraction of open pores was highly variable, with cokes made from inertinite-rich coals having more open pores. For all cokes made from coals sourced in Queensland, Australia, the number of open pores increased linearly with increasing inertinite content of the original coal and was not strongly affected by rank over the range investigated (0.9–1.3% mean maximum vitrinite reflectance). In contrast, all of the cokes made from the coals from the Illawarra region, New South Wales, Australia, had less than half the number of total or open pores than the other cokes. The number of pores of <5 nm radius decreased with increasing rank of the starting coal and was not strongly affected by the starting maceral composition or region of origin. The ratio of number of pores in cokes made from inertinite-rich concentrates to those made from the corresponding vitrinite concentrate from the same coal varied with pore size and was greatest at around 100 nm pore radius. The number of pores in cokes at all sizes below 1000 nm was largely unaffected by the bulk density of the charge, oven size or method of quenching. The insensitivity of numbers of nanometre-sized pores in coke to coke preparation conditions can explain why increasing the bulk density of a coal charge may increase coke strength but will have much less effect on coke reactivity to carbon dioxide.

Multifractal analysis of Hg pore size distributions of tectonically deformed coals

To get deep insight into the variability and heterogeneity of pore size distributions (PSDs) in different tectonically deformed coals (TDCs) (mean maximum vitrinite reflectance Ro,max ranging from 1.720% to 1.857%) collected from Hancheng Mine, Weibei coalfield, the multifractal analysis using generalized dimensions Dq was employed to study PSDs of TDCs based on mercury intrusion porosimetry. The mercury intrusion data under pressure over 10MPa was corrected in combination with gas adsorption data. The results show that, for the test coals, tectonic deformation (TD) mainly increases seepage-porosity while maceral composition (MC) is the main control on adsorption-porosity. The variations of Dq versus q show that the PSDs of TDCs exhibit multifractal behavior, however, differ in their multifractality suggested by the extracted parameters from PSDs including information dimension D1, the Hurst exponent H, the width of the right side D0–D10 and the left side D−10–D0 of Dq spectra. TD and MC have great influence on the variability and heterogeneity in the inner distribution of seepage-porosity and adsorption-porosity respectively, as demonstrated by the change of D1, H and D0–D10 for moment q>0 and D−10–D0 for moment q<0. Both D1 and H increase with TD but D0–D10 shows a reverse trend. Consequently, TD leads to narrower distribution with higher fluctuation, lower pore connectivity and greater complexity in the distribution of seepage-pores, which may be due to the presence and uneven distribution of microfracture and inter-granular pores formed through deformation process. Increasing vitrinite and decreasing inertinite contribute to a heterogeneous distribution of adsorption-porosity, as observed by the high correlation between D–10–D0 and MC. The aforementioned results confirm that the multifractal approach is useful to characterize the internal heterogeneity of PSDs and to detailedly distinguish between PSDs of TDCs.

Production and characterization of ash-free coal from low-rank Canadian coal by solvent extraction

In this work, ash-free coal (AFC) was extracted from low-rank Canadian coals with mean maximum vitrinite reflectance (MMVR) in the range of 0.38–0.69 using non-polar organic solvent, organic solvent combination (polar–nonpolar solvent mixture), and with and without hydro-treated heavy aromatic hydrocarbon solvents from coal–tar industry to study the effect of type of coal and solvent type on the production yield of AFC. High temperature solvent extraction was carried out in 0.5L autoclave in the temperature range of 473 to 723K. It was observed that 1-methylnaphthalene (1-MN), a non-polar solvent did not give any significant difference in yields [~30% (daf)]. However, an extraction yield of 73% (daf) AFC was achieved using hydrotreated aromatic hydrocarbons at 673K. The performance of extraction yields was correlated by the vitrinite content and MMVR of the coal and it was observed that higher proportions of vitrinite and a lower MMVR value of coal produced higher extraction yield. Proximate and ultimate analysis, FTIR, ICP-MS, 13C CP/MAS NMR, thermogravimetric analysis and particle size distribution were used to characterize AFC. The heating value of the AFC was estimated to be in the range of 36–37MJ/kg and a substantial decrease of sulfur content (ca. 12.5–61.1%) is also observed in AFC. AFC showed a narrower particle size distribution with a d50 of 7.0μm.

Improving coke strength prediction using automated coal petrography

A range of Australian and overseas coals with a mean maximum vitrinite reflectance (Ro, max) range of 0.68–1.71% were carbonized in a test coke oven. Coal properties were characterized using XRF, XRD and automated imaging of polished sections of discrete coal grains. The Full Maceral Reflectance (FMR) parameter was calculated from the reflectance data and was correlated with coal as well as coke properties. The Ro, max values and the vitrinite estimates from the semi-automated microscopic technique indicated a good correlation with similar data based on manual point count analysis. The FMR parameter is shown to increase with increasing carbon content, structural ordering of carbon and decrease with increasing volatile matter. The FMR parameter of coal was related to cold coke strength DI15150 and coke strength after reaction (CSR). The FMR parameter was modified by diluting the contribution of high reflectance coal grains as well as incorporating the effect of ash contribution to propose a combined coal index (CCI). The new coal index is shown to improve the accuracy of coke strength prediction. The combined coal index provides a promising objective measurement based alternative for predicting coke strength.

Hartshorne coal rank applied to Arkoma Basin coalbed methane activity, Oklahoma, USA

Coal rank, generalized for all coal beds at or near the surface, in the eastern Oklahoma coalfield has been known since 1915. Near the surface (depths <305m, <1000ft), coal rank increases from high volatile bituminous to low volatile bituminous from west to east in the Arkoma Basin in Oklahoma. The rank of subsurface (depths >305m, >1000ft) coals in Arkoma Basin coalbed methane (CBM) prospects was previously unknown. A new Hartshorne coal (Middle Pennsylvanian) rank map is based on mean maximum vitrinite reflectance values of 0.76% to 2.41% Rmax from 70 coal samples from vertical depths of 0 (surface) to 1327m (4355ft). Hartshorne coal rank in the subsurface increases from high-volatile bituminous to semianthracite from west to east in the Arkoma Basin in Oklahoma. Bituminous rank boundaries changed considerably from earlier surface rank maps (primarily by increasing the area of medium volatile bituminous rank coal in the subsurface) and revealed a previously unknown semianthracite rank area. This discrepancy is significant because the rank of deep (>305m, >1000ft) coal resource assessments and CBM exploration projects have been based on shallow coal rank assignments.A total of 2635 Hartshorne (Hartshorne, Lower Hartshorne, and Upper Hartshorne) CBM wells have been completed in Oklahoma since 1988. Most (1610) wells are horizontal with lateral lengths ranging from 4 to 1498m (14 to 4914ft; an average of 669m [2195ft]). Coal in most Hartshorne CBM wells is medium volatile bituminous rank. Hartshorne coal is semianthracite rank in about 160 CBM wells in Le Flore County. The four Hartshorne CBM wells with the highest initial potential (IP) gas rates (48 to 65 thousand cubic meters per day, Mcmd; 1.7 to 2.3 million cubic feet per day, MMcfd) are medium volatile bituminous rank from horizontal wells in Haskell and Pittsburg counties. IP gas rates<28.3 Mcmd (<1 MMcfd) in low volatile bituminous and semianthracite rank Hartshorne horizontal CBM wells may be low due to complications in drilling into high rank coals with cleat spacing <1cm, to stimulation differences, or to gas migration into adjacent sandstone channels.

Fractal characterization of seepage-pores of coals from China: An investigation on permeability of coals

To better understand the characteristics of seepage-pores (pore radius larger than 100 nanometers) and their influence on the permeability of coals, we have conducted fractal analyses for 34 fresh coal samples (mean maximum vitrinite reflectance Ro,max from 0.43% to 4.21%) from North, Northwest and Northeast China. Mercury porosimetry data indicate that the coals are fractal, with pore radius ranging from 0.1 to 50μm. Calculated fractal dimensions of these coals range from 2.61 to 2.98, higher than those from other kinds of rocks such as sandstone, shale, and carbonate. The data suggest that the coals have more complicated and inhomogeneous pore structures than other rocks.The fractal dimension has a negative correlation with the petrologic permeability of coals, particularly for higher rank coals (with 1.47–4.21% Ro,max), from which a strong negative linear correlation (R2=0.85) between fractal dimension and permeability is observed. A ‘U-shaped’ trend between fractal dimensions and coal ranks is observed, with the minimum fractal dimensions occurring at 1.1–1.3% Ro,max. The sub-bituminous, high volatile bituminous, semi-anthracite, and anthracite have higher fractal dimensions. The effects of coal rank upon fractal dimensions are mainly due to the variety of micropore contents and aromaticity of coals with coalification.

A Comprehensive Model for Evaluating Coalbed Methane Reservoirs in China

Abstract: Coalbed methane reservoir (CBMR) evaluation is important for choosing the prospective target area for coalbed methane exploration and production. This study aims at identifying the characteristic parameters and methods to evaluate CBMR. Based on the geological surveys, laboratory measurements and field works, a four‐level analytic hierarchy process (AHP) model for CBMR evaluation is proposed. In this model, different weights are prioritized and assigned on the basis of three main criteria (including reservoir physical property, storage capacity and geological characteristics), 15 sub‐criteria, and 18 technical alternatives; the later of which are discussed in detail. The model was applied to evaluate the CBMR of the Permo‐Carboniferous coals in the Qinshui Basin, North China. This GIS‐based fuzzy AHP comprehensive model can be used for the evaluation of CBMR of medium‐high rank (mean maximum vitrinite reflectance &gt;0.5%) coal districts in China.

CO2 storage and gas diffusivity properties of coals from Sydney Basin, Australia

Abstract Measurements of CO 2 adsorption and diffusion properties of coals are reported for various coalfields within Sydney Basin, New South Wales (NSW), Australia. Adsorption measurements were undertaken using a gravimetric method. Measurements carried out on 27 coals show that Sydney Basin coals at CO 2 sub-critical conditions, namely gas pressures below 6 MPa and temperatures below 39 °C, can adsorb a maximum volume (Langmuir volume) of 40 to 80 m 3 of CO 2 per tonne of coal on a dry ash free basis (daf). The coals used in this study are of sub-bituminous to bituminous rank, ranging from 0.66 to 1.45% mean maximum vitrinite reflectance, and are from depths ranging from about 27 m to 723 m. The highest adsorption capacity applies to the highest rank coal, which is also the deepest coal. The standard deviation between Langmuir modeled and measured values is less than 1.5 m 3 /t, corresponding to a relative error of less than 2.7% for all except one coal. Based on adsorption isotherms, the CO 2 storage capacity for in-situ seam pressure conditions range from about 6 to 51 m 3 /t. CO 2 diffusion properties of 15 of these coals, determined using a newly developed system capable of accurately measuring diffusivity of gases in solid coal indicate that CO 2 diffusivity (diffusion coefficient) in the Sydney Basin coals varies from 1.2 × 10 − 6 to 10.2 × 10 − 6 cm 2 /s. The diffusivity does not show any discernable trend with the variation in depth and rank. Porosity measured by a mercury injection method varies from 4 to 10% and decreases with increase in coal depth and rank. For some of the coal samples adsorption measurements for pure CH 4 , CO 2 and N 2 indicate that the Sydney Basin coals can store twice as much CO 2 as CH 4 and six times more CO 2 than N 2 (volume basis). Also, measurement of diffusivity in solid coal samples shows that CO 2 diffuses twice as quickly as CH 4 . The data obtained from this study and the estimated coal resources in the state of New South Wales, allow CO 2 sequestration potentials to be calculated.

The influence of tectonic deformation on some geochemical properties of coals—a possible indicator of outburst potential

Nearly all outburst-prone coal is known to have been tectonically deformed in fault and fold zones which underwent one or more episodes of strong structural deformation. The physical properties of deformed coal, such as strength coefficient and rate of gas desorption, have been broadly studied and used to predict outbursts in the past. However, it can be difficult to reliably determine these physical coefficients from core sampling operations because the coal can be inadvertently pulverized during drilling. The development of a new index which predicts the outburst potential of coal seams and is not influenced by sampling procedures could be beneficial to mining safety technology. Tectonic deformation may not only affect the original physical structure of a coal but also may bring about changes in chemical properties. Any chemical property with the potential to identify outburst-prone coal must also be stable and independent of the physical changes imposed by drilling. Most of the chemical characteristics of coal display no obvious difference between normal undeformed coal (which is not subject to outbursts) and deformed coal. However, recent studies have shown that the chloroform extract yield is a very good geochemical index for differentiating these coals. Nineteen coal samples from five mines experiencing outbursts in the Pingdingshan coalfield, Henan, China, were analyzed. Their mean maximum vitrinite reflectance in oil ranges from 0.97% to 1.28%. All of the samples were prepared to −180 μm (−80 mesh US) and subsamples were extensively extracted with chloroform in a Soxhlet apparatus at 75 °C. The mean extract yield of the 10 deformed coals is 1.45% whereas that of the nine normal coals is 0.44%. We conclude that deformed coal has a higher chloroform extract yield than equivalent coal which has not been tectonically disturbed. Chloroform extraction, therefore, can be used to recognize deformed coal and the potential to undergo further deformation. Chloroform extract yield is an accepted measure of the hydrocarbon-generating potential of coals and other rocks. The results presented here, therefore, suggest that tectonic deformation may modify the hydrocarbon generating potential of coals. The adsorption of soluble organic matter expelled during and following tectonic deformation onto enhanced surface areas may have contributed to the higher extract yields of the deformed coal.

Vitrinite reflectance as a possible indicator of metamorphic grade and stratigraphic position of formations: a study of Oligocene metapelites in NE Taiwan

A total of 47 samples of Oligocene metapelites collected from the northeastern (NE) Hsuehshan Range, Taiwan, were studied by measuring vitrinite reflectance. The vitrinite reflectance of the three studied stratigraphic units, from top to bottom, are as follows: the Tatungshan Formation: R ̄ max (mean maximum vitrinite reflectance) =1.35–2.76% and R ̄ m (mean random vitrinite reflectance) =1.18–2.56%; the Tsuku Formation: R ̄ max =2.89–3.51% and R ̄ m =2.37–3.26%; and the Kankou Formation: R ̄ max =4.06–5.97% and R ̄ m =3.04–5.07%. Either R ̄ max or R ̄ m changes continuously along the studied geologic sections, and the variation serves well as a metamorphic grade indicator. In the studied area, each stratigraphic formation has its own characteristic vitrinite reflectance value range. It is postulated that, in the future, vitrinite reflectance can also serve as an indicator of stratigraphic position of formations to aid stratigraphic identification and correlation in geologic mappings.

Biaxial vitrinite reflectance in coals of the Elk Valley coalfield, southeastern British Columbia, Canada

Ninety-nine grab coal samples collected from ten different diamond drill-cores from the Jurassic-Cretaceous Mist Mountain Formation in the Elk Valley coalfield in southeastern British Columbia were analyzed petrographically for mean maximum vitrinite reflectance ( R max ), mean random vitrinite reflectance ( R m ) and the lengths of the maximum, intermediate and minimum axes of the reflectance indicating surface or RIS of vitrinite ( R max R int and R min). Coal rank in the coalfield ranges from high-volatile A to low-volatile bituminous. Values of R max in the core samples range from 0.95 to 1.50%. R max , R m , R max , R int and R min are all strongly intercorrelated ( r>0.89 in all combinations). The vitrinite in all samples is biaxial, in most cases biaxial negative. As a consequence, R max is always less than R max, the relationship being R max = 1.044 R max . There is no relationship between degree of vitrinite reflectance anisotropy, as represented by the RIS shape, and rank. The results indicate that a secondary stress field, not perpendicular to bedding, existed during coalification.

A progress report on the Alpern Coal Classification

The Alpern Coal Classification has already been largely utilized and successfully applied, both to Laurasian (North Atlantic) and Gondwana coals. It was accepted on basic principles by the International Committee for Coal Petrology, and, as the official contribution of France, it has been recently also partly adopted by the Economic Commission for Europe of the United Nations in Geneva (1988). This paper deals with the improvement of the previous versions of the classification, mainly by introducing new statistical data in order to establish more accurately proposed rank limits. A ternary division for RANK is maintained with the categories: low-rank coal or lignite (instead of brown coal because high-rank lignites may be black not brown), medium-rank coal or bituminous coal, and high-rank coal or anthracite. The prefixes hypo-, meso- and meta-, are used for rank subdivision, thus avoiding non-coherent previous prefixes such as sub-(bituminous) and semi-(anthracite). The selected rank parameters are: moisture holding capacity (ash free) and calorific value (moist, ash free) for lignites, mean random reflectance for bituminous coals and anthracites. Correlations for bed moisture (ash free) and calorific value (dry, ash free) are given for lignites. Correlations for volatile-matter content, mean maximum vitrinite-reflectance, and hydrogen content are also given for bituminous coals and anthracites. TYPE is determined from maceral data and is divided into: vitric (vitrinite (V) > 60%), fusic [V < 60%; inertinite (I) > liptinite (L), and liptic (V) < 60%; L >. I). The ash content and the washability potential at 10% ash level are used to determine GRADE (or FACIES) and allow the distinction between transportable clean coals and non-washable coals. The run-of-mine ash content (dry basis) permits the division between: coal, middlings, and shale. If the type is liptic, the subdivisions “sapropelic coals” and “oil shales” are used. These are not washable but are solid fuels with a positive energy potential. If the coals to be classified are clean (or washed) the vertical axis becomes free and can be used, for example, for swelling properties in bituminous coals. The system has been recently computerized and allows for data to be displayed effectively for all relevant fields of application. Computer-output examples are presented for coals from a Euramerican and a Gondwana basin.

Natural weathering of coal

The effects of weathering on the mean maximum vitrinite reflectance, moisture content, and low-temperature ash mineralogy of the low volatile bituminous, Lower Mississippian Price Formation coals of southwest Virginia were determined on samples from depth profiles of three weathered coal beds. The results show that the outermost ≈50 cm of each seam is intensely weathered showing humic acid formation, higher moisture content and physical disintegration. Beyond this depth (which corresponds to the average frost depth of this area), the amount of weathering decreases sharply. Freezing and thawing appear to be important in weathering the outermost zone, and, together with wetting and drying, disintegrate the coal creating reactive surfaces that oxidize readily. Reflectance values from weathered coal were found to be reliable indicators of rank if measurements were taken away from identifiable weathering features. The low-temperature ash mineralogy (silicates) showed no change with weathering.