CONDENSATE MINERALS FROM A BURNING COAL-WASTE HEAP IN AVION, NORTHERN FRANCE

Abstract

A coal-waste heap in Avion, northern France, that has been undergoing subsurface combustion for several decades, is marked by the occurrence of efflorescences at combustion sites where gas and water vapor escape from surficial cracks and vents. Analysis of the efflorescences show that twenty different species of mineral are present, comprising various sulfates, as well as halides and native sulfur. The constituents needed for the formation of these minerals are ultimately derived from the heap of coal-mine waste, composed mainly of pyrite-bearing carbonaceous shale, coal and minor amounts of sandstone. They may be partly released by acid weathering of waste-heap materials prior to combustion, but largely by thermal decomposition and oxidation of those materials during combustion processes. Formation of the efflorescences near gas-discharge zones indicates transport of constituent to those zones from the inner part of the waste heap, which can occur in the gas phase for some elements (S, N, H and Cl), but would require droplets of water vapor as carriers for most cations. Interaction between condensate solutions and the local debris at the surface of the waste heap may also contribute to cations input prior to the formation of secondary minerals. Most minerals formed from a liquid phase along the rock-debris surface at low temperatures (T ≤ 100°C), as confirmed by the occurrence of fluid inclusions. Only salammoniac, native sulfur and mascagnite (where crystallized as a monocrystalline phase) were formed directly from a gas phase. The coal-waste heap shows great variations in mineral assemblages among sampling sites, which must be related to variations in waste composition and in conditions at the level at which the elements are released and where mineral formation takes place.