Chapter 6 - Modes of occurrence of elements in coal

Abstract

This chapter focuses on the modes of occurrence of all elements that occur in coal with the exceptions of organically associated C, H, O, and N. What follows is a comprehensive review of the literature on the modes of occurrence of 58 elements, rare earth elements and Y, platinum group elements, and rare gases in coal. At the end of each review of elements, we offer our opinion as to the most likely modes of occurrence of the element in coal. The modes of occurrence of elements in coal are classified into native elements, sulfides, selenides and tellurides, halides, oxides, hydroxides and oxides containing hydroxyl, carbonates, sulphates, phosphates, silicates, organic compounds and organically associated, moisture/pore water, and others (chromates, vanadates, arsenates). The estimation of “degree of certainty” of each mode of occurrence of elements was based on modes of occurrence determined by direct or indirect analysis. Each element with multiple modes of occurrence has been given a corresponding different degree of certainty, i.e., five levels of confidence including very high, high, moderate, low, and very low. Based on how often the mode of occurrence of an element occurs in coal, each “occurrence” of elements is classified into ubiquitous, common, uncommon, rare, and nonreported. Although analytical methods certainly play critical roles in determining the modes of occurrence of elements in coal, in-depth understanding of the nature of the coal and the geological background of coal formation is particularly useful in investigating when and how these modes of occurrence of elements were formed.