Geology Reference
In-Depth Information
9.1. The Origin of Gas Vent
Minerals
Glenn B. Stracher
Acicular Sulfur Crystals, Witbank Coalfield,
South Africa.
From Stracher (2007) with modifications, reprinted with permission of the Geological Society of America. Photo
by Glenn B. Stracher, 2004
.
Introduction
G
as vents associated with coal fires are circular to semicircular conduits in the earth or openings at its surface
through which the by-products of coal combustion are transported. Gas fissures associated with such fires are
elongate cracks in the earth or at its surface through which these by-products pass.
Near or at the surface, these vents and fissures are often encrusted with minerals commonly assumed to form by the
condensation of coal-fire gas. As the gas is exhaled at the surface, it cools and subsequently one or more minerals
may sublimate or a liquid may form which, when further cooled, solidifies to form a mineral assemblage (Lapham
et al., 1980; Stracher, 1995; Witzke, 1997a). These same reactions may also occur at volcanic gas vents (Stoiber
and Rose, 1974), at times resulting in the formation of mineral assemblages similar to those associated with coal-
fire gas. However, other complex thermochemical processes involving multiple reactions may be responsible for
the gas-related origin of minerals at either type of vent.
The purpose of this section is to delineate the processes by which mineral assemblages form at or in the vicinity of
coal-fire gas vents and ground fissures as a consequence of combustion. For the sake of brevity, reference herein is
made to gas vents only, since analogous processes occur at both vents and fissures.
Mineral-Forming Processes
I
sochemical and mass transfer processes occur in association with the crystallization of a mineral assemblage at a
gas vent. As described below, these processes may proceed along a variety of reaction paths.
An isochemical mineralization process herein refers to one in which the bulk composition of a mineral is
equivalent to that of the original (primary) gas phase from which it formed at a coal-fire vent. A mass transfer
mineralization process is one in which the bulk composition of the mineral is different than that of the primary gas
phase due to the transfer of mass into or out of this phase or a liquid condensed from the phase, via chemical
reactions. The reaction path followed during the crystallization of a mineral assemblage at a coal-fire gas vent
depends on one or more combinations of the following variables (Stracher, 1995; Stracher et al., 2005a): gas and
substrate chemistry, presence or absence of aqueous solutions, pressure and exhalation temperature of the gas, and
temperature of the substrate encountered by the gas during exhalation at the surface.
Isochemical Mineralization
C
ondensation is the isochemical process whereby a gas undergoes a change in state during cooling and
transforms directly into a liquid or a solid (Hawley, 1971, p. 231). Two condensation-related processes,
