Mean Maximum Reflectance Research Articles

A Petrologic Study of a Thermally Altered Coal from the Purgatoire River Valley of Colorado

A column of coal from a seam thermally altered by an underlying lamprophyric sill has been studied petrographically, chemically, and mineralogically. Megascopically there are four major lithotypes present within the limits of the seam. They are: carbonaceous shale, natural coke, altered coal, and a layer of quartz grains. Megascopic evidence indicative of thermal alteration is the highly contorted bedding, natural coke with evidence of flowage, and prismatic fracturing within the highly altered coal zones. Micrinoids and fusinoids are common, with the cell cavities of the fusinoids commonly filled with calcite. Vitrinoids are all altered to a dark-brown color in thin section. Fungal derived bodies are common in the upper part of the seam and appear to be the most thermally resistant petrographic entities. No evidence of spore or cuticular material remains. Mosaic structure and vesiculation are additional common microscopic evidences for the thermal alteration. Sulfur distribution apparently has no relation to the presence of the intrusion. Ash values are highest at the top and bottom of the seam and are related to shale-rich zones. Volatile matter (d.a.f.) is high at the seam margins and increases as the distance from the intrusion decreases. This is attributed to one, or both, of two factors. Either the volatiles were released and readsorbed or they were never released. Only quartz, kaolinite, siderite, calcite, and Fe 2 (SO 4 ) 3 were identified by X-ray examination. No evidence of the presence of cristobalite, mullite, or pyprophyllite was detected. Mean maximum reflectance values range from 1.00 percent to 3.00 percent and increase toward the intrusion. Evidence based on mineralogy, chemistry, reflectance, melting point, and electron spin resonance indicates that a temperature gradient did exist across the seam from greater than 375° C at the top to less than 600° C at the base, and also that the temperature of the underlying shale at the contact with the sill was not more than 600° C.

Carbonization of semi-anthracitic vitrinite by an analcime basanite sill

The sill is an analcine basanite with phenocrysts of labradorite, two clinopyroxenes, and olivine. The coal is from a bore drilled in the Dirty seam of the Baralaba Coal Measures in the Bowen Basin, Queensland. The intrusive rock was penetrated in the bore from 1514 to 1532 /sup 1///sub 2/ ft depth, directly below the Dirty seam. The phenocrysts in the narrow chilled margins indicate a temperature of intrusion of not more than 1160/sup 0/C. The coal was metamorphosed from semi-anthracite (92% C) to anthracite, meta-anthracite (97% C), and vesicular natural coke towards the contact. The vitrinite shows a progressive increase in mean maximum reflectance from 2 to 12%. Particularly in the meta-anthracites, two distinct types of vitrinite can be distinguished on the basis of reflectance. These are considered to express the different behaviour on carbonization of Vitrinite A and Vitrinite B.