Geology Reference
In-Depth Information
discussed below, may occur during the crystallization of minerals derived from coal-fire gas. The first,
sublimation, is strictly a condensation process. The second involves condensation to a liquid followed by
solidification (freezing).
Sublimation
occurs when a gas component exhaled from a coal-fire vent rapidly cools below the liquid to solid
transformation temperature (supercools) and subsequently condenses to a solid without an intervening liquid state
(Hawley, 1971, p. 832). Supercooling occurs in response to a temperature gradient between the gas and the
substrate (heat sink) encountered by the gas at the surface. Heat sinks include rock, sediment, vegetation, the
atmosphere, etc. (Stracher, 1995). During sublimation, the gas isochemically condenses to a mineral. The process is
represented by the exothermic reaction
Sublimates
Supercools
X
X
ð
g
Þ !
ð
s
Þ
ð
9
:
1
:
1
Þ
where X(g) and X(s) designate an element or compound in the gas and solid states, respectively. If a mineral
encrusting a vent formed by sublimation, then the chemical composition of the mineral and the substrate on which
it sublimated are independent of one another. In other words, no mass transfer occurs between the sublimating gas
and the substrate as solidification occurs. Evidence for sublimation includes mineral encrustations whose elements
are contained in the gas and euhedral crystals as well as crystal faces that do not touch, suggestive of growth in a
nonrestrictive environment (Stracher et al., 2005a).
A variety of minerals are reported as the sublimation products of coal-fire gas including hydrous sulfates (Lausen,
1928), downeyite (SeO
2
) (Finkelman and Mrose, 1977), orthorhombic sulfur (Stracher, 1995), coccinite (HgI
2
)
(Witzke, 1997b), and salammoniac (NH
4
Cl) (Stracher et al., 2005a). At first glance, minerals encrusting gas vents
may appear to be sublimation products of the gas, especially if the minerals appear powdery and the substrate from
which the minerals were collected appears unaltered. This was the case in a recent study by Stracher et al. (2005a)
of mineral assemblages collected from the Wuda coalfield, Inner Mongolia. The gas-vent assemblages occurred on
quartzofeldspathic sand and sandstone that appeared chemically unaltered in the field (Figure. 9.1.1). However,
X-ray diffraction, energy dispersive spectrometry, gas chromatography, and micro-Dumas analyses coupled with
Figure 9.1.1. Alunogen, coquimbite, voltaite, and an unidentified phase comprise the white-colored mineral
assemblage at a gas vent in the Wuda coalfield, Inner Mongolia. The underlying substrate is quartzofeldspathic
sand and sandstone that appeared chemically unaltered in the field. Sample analyses and SEM imaging suggested
otherwise. The distribution of the assemblage suggests mineralization occurred in response to a gas moving toward
the upper right in the photo. From Stracher et al., 2005a, with permission of the Mineralogical Society of America.
