Carbonization of semi-anthracitic vitrinite by an analcime basanite sill

The effects of intrusion of a sill, some 13 feet thick, under a 5-foot thick semi-anthracite seam are described. The sill is an analcime basanite with phenocrysts of labradorite, two clinopyroxenes, and olivine. The phenocrysts in the narrow chilled margins indicate a temperature of intrusion of not more than 1,160° C. The marginal zone of alteration along the contact with the coal is relatively narrow. The coal was metamorphosed from semi-anthracite (91-92% C;-42% vol. matter) to anthracite, meta-anthracite (96-97% C, 2% vol. matter), and vesicular natural coke towards the contact. The simultaneous change in optical properties of the macerals was studied by measurement of the reflectance in plane-polarized light in grain mounts of subsections of the seam. Semifusinite is hardly affected but the vitrinite shows a progressive increase in mean maximum reflectance from 2 to 12 percent, with minimum reflectance initially rising to 2\ percent and then falling to below 1 percent. Limiting maximum temperatures of metamorphism are given on the basis of reflectance of heated coal samples. The vitrinite of the high-rank and meta-anthracite is characterized by narrow resinite-like bands with higher maximum reflectance and stronger anisotropy than the surrounding vitrinite, and by a wide range of maximum reflectance values irrespective of distance from the contact. Particularly in the meta-anthracites two distinct types of vitrinite can be distinguished on the basis of reflectance frequency histograms. These types are considered to express the different behavior on carbonization of vitrinite types A and B.