Geology Reference
In-Depth Information
A consequence of rapid formation is that the crystals are tiny (
10 µm), with the possible exception of long
acicular growths and therefore are difficult to study using traditional optical microscopes or a hand lens. There are
numerous ways to characterize the mineral assemblages comprised of such tiny crystals; however, each approach
varies in expense, time, and the need for specialized equipment.
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The purpose of this chapter is to introduce common methods (i.e., techniques generally available to most geology
departments) that can be used to identify minerals that nucleate from coal-fire gas. A brief overview of the practice
and principles of (1) X-ray powder diffraction (XRD), (2) electron microprobe analysis (EMPA), and (3) short-
wave infrared spectroscopy (SWIR) is introduced. Other techniques can be used to provide added insight about
the crystal chemistry of vent minerals and the micro-Dumas carbon
-
nitrogen method (MDCN) is briefly
discussed as just one example.
It is important to point out that no single method provides all the information about these gas-vent mineral
assemblages. A good analogy for knowing how to accurately characterize a mineral assemblage that is beyond
examination by the unaided eye is like trying to identify an elephant using only the feel of your hand (i.e., your
eyes, ears, and nose are closed). It is not until you collectively sense enough parts (e.g., the trunk, ears, hide, and
tail) that you begin to get the whole picture and realize you are not studying a hippo or giraffe! The more analytical
methods one can employ to characterize a mineral assemblage, the more evidence available for understanding the
nucleation process. Examples of XRD, EMPA, and SWIR data from gas-vent mineral assemblages that occur
around the world will be presented to learn what minerals are common to coal-fire vents, volcanic vents, and
smokestacks, and what minerals are unique to coal-fire vents alone.
X-Ray Diffraction
E
ach analytical method used for the detection and quantification of an unidentified mineral partly relies on the
interaction between electromagnetic radiation and atomic planes in the mineral. XRD is a mineral identification
technique based on a particular and essential feature for which all wave-interference phenomena are based. This is
the situation where the distances between the atomic planes in a mineral are similar to the wavelength of the waves
being scattered. This same principle applies to the constructive and destructive interference of all waves including
ocean waves, seismic waves, light waves, and sound waves.
Minerals by definition have a generally uniform composition and a repeating orderly arrangement of atoms, i.e., a
crystal structure. Repeat distances of basic crystal motifs (unit cells) are on the order of angstroms (1Å= 10
-
10
m)
and nanometers (1 nm= 10
-
9
m). A unit cell is the smallest volume of a mineral that can exist and still retain all
the physical and chemical properties of that mineral.
X-rays are part of the electromagnetic spectrum and have wavelengths on the order of angstroms. X-rays for
diffraction experiments are generated by applying a large potential energy difference called voltage (usually around
40 kV) across electrodes in a vacuum tube. Electrons in the tube stream from a filament and hit a metal target.
When these electrons collide with the electrons of atoms that are in the metal target, the electrons in the target atoms
move to higher energy levels. When the displaced electrons fall back to their original energy level, a
“
monochro-
matic
source of X-rays is generated. Filtering X-rays of higher and lower energy by using energy-sensitive
detectors, curved monochromatic crystals, and/or metal-foil absorption windows results in a single wavelength of
X-rays used to analyze the mineral sample of interest.
”
One commonly used target that is bombarded with electrons to generate X-rays is copper, which generates X-rays
with a 1.540 59 Å wavelength, referred to as Cu-K
α
1
radiation. This single wavelength of radiation then con-
structively and destructively interferes with a mineral sample ground to a fine powder. The powdered sample is
pressed into a disk shape before it is exposed to the beam of X-rays. Once exposed, the X-ray beam is diffracted
from the mineral
s atomic planes in all possible crystal orientations in the powder, and it is then detected by
positioning an electronic detector in the hemisphere surrounding the sample. This is done by either moving a
detector through the hemisphere in an arc-like path or having a position-sensitive detector on an arc of the
hemisphere. A goniometer is a device that makes it possible to orient something at a specific angle in space
while measuring the angle. Therefore, the motorized device that orients the X-ray source, sample, and detector is
called an X-ray goniometer. Using either a moving detector or a detector that simultaneously senses all angles
allows analysts to observe the unique diffraction pattern scattered into space by a crystal structure (Figure 10.1.2).
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