6 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 445 



Lenses of fine-grained limestone are interbedded with the coal. Calcitic 

 shell fragments are also common. 



The very fine-grained groundmass, which makes up 40 to 60 volume per- 

 cent of the coal of Sample no. 1, is mainly composed of argillaceous limestone 

 but contains an estimated 2 to 40 volume percent of very finely dispersed coaly 

 material. 



Sample no. 2 (text figs. 1 and 2, and table 1) is from a 3 -inch "dark car- 

 bonaceous shale" (driller 1 s report). Only 0.8 inches (2.0 cm) of it had been 

 collected during routine sampling, and no orientation was noted. 



A band about 0.4 inches thick is darker, brighter, better laminated, and 

 contains more vitrinite than the rest of the coaly material (pi. 2, fig. 4). This 

 band is similar in composition to that of the laminated coal of Sample no. 1, but 

 the microstratification is not quite as well developed. The vitrinite lenses show 

 a higher degree of degradation. The spores are often broken. Less pyrite, mostly 

 as subhedral crystals rather than fine syngenetic grains, and a larger proportion 

 of fine-grained groundmass than in Sample no. 1 are characteristic of Sample no. 2. 



The amount of coal in the limestone decreases away from the coaly layer 

 in Sample no. 2. A thin transitional zone is composed mainly of closely packed 

 angular dolomite grains, limestone lenses, and many shell fragments. In the 

 interstices between these particles, coal and a fine-grained groundmass of coal, 

 pyrite, and limestone occur. The transition zone may represent reworking of 

 earlier deposited sediments (pi. 2, fig. 5). 



The brownish gray limestone in this coal is silty and contains scattered 

 pyrite grains and broken fossil fragments up to 4 mm long that are irregularly dis- 

 persed in a fine-grained calcitic matrix. A few thin carbonaceous partings occur 

 near the contact of the coal. Micros tylolites are very common along these partings 

 (pi. 2, fig. 6). The limestone is composed of fairly angular calcitic grains of a med- 

 ium to fine sand size. The grain boundaries are usually not sharp, and many of them 

 show secondary growth. 



Sample no. 3 is similar to Sample no. 2. The coaly layer, 0.75 inch 

 (1.9 cm) thick, is sharply delineated from the underlying and overlying light 

 gray to brown limestone. Shell fragments, which can be seen with the unaided 

 eye, are more common than in the first two samples. The silty, mainly fine-grained 

 limestone contains thin carbonaceous partings. 



COALIFICATION AND CHEMICAL ANALYSIS 



The rank of an Illinois coal is normally determined by analysis of its moist 

 but mineral matter-free calorific value expressed in Btu per pound. A high ash con- 

 tent, as in this Devonian coal, decreases the reliability of the mineral matter-free 

 calculated calorific value. Also, the coal has undergone considerable drying before 

 the analysis was made, so that the 0.9 percent moisture (table 2) may not represent 

 the true bedrock value needed to determine rank according to ASTM Standard D 388-66 

 One parameter of coalification, which is widely used among coal petrographers and 

 is applicable to this particular coal, is the reflectance as measured in oil immer- 

 sion (Rq) on a highly polished surface. Because vitrinite is the principal and most 

 homogeneous constituent of most bituminous coals, it is normally used for reflec- 

 tance measurements to determine the level of coalification. The reflectance of 

 vitrinite, in general, increases gradually with increasing rank of a coal. 



