Geology Reference
In-Depth Information
1600
T
1400
T = Tschermigite
G = Gypsum
M = Mascagnite
Q = Quartz
1200
1000
T
T
800
T
T
600
T
T
T
Q
T
M
400
G
T
200
0
2
10
20
Degrees 2
30
40
50
60
Θ
(Cu-K
α
1
radiation)
Figure 10.1.3. X-ray diffractogram of minerals encrusting a coal-fire gas vent in sandstone, Williams Fork
Formation, Colorado. The temperature ~5 cm (2 in) inside the vent on May 18, 2004 was 89 °C (193 °F).
Labeled peaks are strong line reflections for the major minerals in the sample. Peak positions and intensities
best match respective phases with the following International Centre for Diffraction Data Powder Diffraction File
numbers: tschermigite (7-22), mascagnite (7-2), gypsum (33-311), and quartz (33-1161). The minerals nucleated
from coal- fire gas with the exception of quartz, likely picked up from the Williams Fork sandstone during sample
collecting. Photo by Paul A. Schroeder, 2008
.
more enriched in Si, Fe, and Al and depleted in K, Ca, and Mg when compared to unweathered volcanic rocks.
Volcanic rocks associated with subduction zones versus those associated with spreading centers are composition-
ally differentiated, with the latter generally even more depleted in Si and Al and more enriched in Fe and Mg than
the former. Other factors likely influencing these differences include climatic, hydrologic, geomicrobial, and
temperature conditions. One example includes the observation that the Fe-rich mineral goethite is commonly
found in the Fe-rich hot springs of Iceland, hosted in the volcanic rocks of a spreading center (Konhauser and
Ferris, 1996, Geptner et al., 2005).
Sulfate minerals are also found in the fly ash from smoke stacks used to vent gas from burning coal into the
atmosphere. However, the mineral assemblages that occur in fly ash appear to be less hydrated and not as diverse in
the number and types of sulfate minerals. Ca-bearing sulfates commonly appear with smokestack vents. Also included
in Table 10.1.1 are mineral assemblages associated with burning coal heap piles (Dokoupilova et al., 2007). These are
dump piles sitting on the earth
'
s surface with more exposure to the atmosphere. It is interesting to note that there are
some minerals in common with volcanic and underground coal fire vents such as the Ca-, K-, NH
4
-, and Fe-bearing.
The notable difference is that the heap piles often contain Na- and Mg-bearing phases, the volcanic and underground
vents do not.
Electron Microprobe
E
MPA is one of many analytical techniques based on the principle of sample bombardment with an electron beam in
a vacuum and the subsequent detection of scattered electrons, light photons, and X-rays. Other commonly employed
analytical and imaging techniques include scanning electron microscopy (SEM), cathode luminescence (CL), and
transmission electron microscopy (TEM). The primary mission of EMPA is to generate and measure X-rays from a
small volume of a sample and to visualize compositional variations using electrons scattered from the sample.
Electrons emitted from a tungsten or lanthanum hexaboride filament in a vacuum column are accelerated through a
high potential difference of 10
25 keV.
The resulting beam is focused to a spot of ~1 µm in diameter (i.e., ~1/100th the thickness of a human hair). The
electron beam is then either rastered over the sample at varying magnifications to visualize sample surface
morphology and compositional variation or focused on a single spot to generate X-rays within the sample.
-
25 kV, so the energy gained or lost by the charge/electric field system is 10
-