Raman and Luminescence Images Applied to Study Internal

CORALS II Conference (2011)
RAMAN AND LUMINESCENCE IMAGING APPLIED TO STUDY THE INTERNAL TEXTURES OF
MINERALS AND GEOLOGICAL SAMPLES. L. Nasdala, Institute for Mineralogy and Crystallography, University
of Vienna, Althanstr. 14, A–1090 Wien, Austria (lutz.nasdala@univie.ac.at).
Minerals are typically characterized by deviations
from their perfect structural state and ideal chemical
composition. Such deviations from perfectness, which
are commonly described as real structure, include not
only point defects (i.e., substitutions of an atomic species
by another element, vacancies, or interstitials) but
also domain or twin boundaries, dislocations, stacking
faults, inclusions of other phases, strain, and many
more. For an overview see [1].
If such real-structure properties show a marked
heterogeneity pattern within a mineral, this is referred
to as internal texture. Note that, in view of the definitions
of the terms, the formerly more common description
as “internal structures” should better be avoided
(cf. [2]). Common internal textures of minerals include,
among others, primary growth and sector zoning,
diffusion zoning, patchy or other alteration patterns,
stress textures, and exsolution patterns.
The investigation of such patterns of heterogeneity
may provide detailed information on the primary formation
and post-growth alteration of crystals and minerals.
An increasing number of analytical techniques is
therefore applied to visualize internal textures of minerals
and other geological samples. Recent needs,
however, are in many cases not met anymore by the
mere visualization of internal distribution patterns, but
require (i) a more quantitative evaluation of the variable
parameter under discussion, and (ii) better understanding
of what exactly is visualized. To give an example
for the latter, SEM–CL imaging of zircon grains
has become a routine technique prior to U–Pb isotopic
measureements in a SHRIMP or LA–ICP–MS system.
Here, the mere knowledge of the extension of interior
regions and zones and the location of their boundaries
is sufficient for placing carefully single measurements
within zones, and hence to avoid biased results. For
sound conclusions on the sample’s history, however,
knowledge of the causes of the variations in CL emission
among zones would be most useful.
There are two principally different ways to generate
images from luminescence or Raman scattered
light (the same is true for the application of other analytical
techniques). First, images can be obtained directly:
The detector of the analytical system collects an
either panchromatic or monochromatic image of the
area to be studied. This technique is used commonly to
obtain CL or PL images whereas direct Raman imaging
is rarely applied in the Earth sciences. To obtain a
direct image, the area of interest is either illumi-
Fig.1: Pairs of BSE and CL images of polished zircon
grains. A, reference zircon 91500 from Kuehl Lake,
Ontario (cf. [3]). B, synthetic, un-doped zircon grown
using the Li-Mo flux technique (cf. [4]).
nated with a defocused beam (e.g., UV-excited PL
images or CL images obtained in an optical microscope),
or scanned by a focused beam (e.g., CL images
obtained in a SEM). Two examples for the latter, demonstrating
the sensitivity of luminescence phenomena
to minute amounts of defects in the sample, and hence
the particular powerfulness of luminescence images to
reveal even weak internal textures, are shown in Fig. 1.
The BSE image of a chip of zircon 91500 (Fig. 1A,
left) reveals solely moderate primary growth zoning,
whereas the CL image of the very same grain (Fig. 1A,
right) is strongly affected by striations due to stressinduced
luminescence, which in turn is caused by
stress introduced during the mechanical preparation.
Figure 1B shows a synthetic ZrSiO4 crystal. The BSE
is virtually homogeneous. The CL image, in contrast,
shows a clear sector zoning. The emission is partly due
to slightly enhanced concentrations of defects (presumably
contaminant Dy in the raw ZrO2 used) that
were incorporated preferentially upon the growth of
the pyramid crystal faces.
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CORALS II Conference (2011)
Fig. 2: Color-coded Raman map of a polished agate
from Saxony, Germany, generated from 54,096 single
analyses (step width 2.5 �m). Strong zoning of the intensity
ratio of the main bands of moganite (502 cm �1 )
and �-quartz (464 cm �1 ) indicates a strongly rhythmic
growth.
Second, images can be generated mathematically
from hyperspectral data sets: A full emission or Raman
spectrum is recorded per individual sampling point
(i.e., per pixel of the future image). The whole data set
is then processed (i.e., background correction and band
fitting), and colour-coded images visualizing the lateral
distribution of virtually any user-defined spectral
parameter across the mapped area can be produced.
In contrast to luminescence imaging, the hyperspectral
mapping technique is applied comparably
rarely in the study of luminescence emissions. Mapping
is applied mostly in cases where more detailed
spectral information is needed [5,6], or if the information
cannot be obtained in direct images. For instance,
FLIM (fluorescence lifetime imaging microscopy)
images show color-coded the distribution of luminescence
decay times across the area visualized [7].
During the past few years, the hyperspectral Raman
mapping technique has been applied successfully
in virtually all sub-disciplines of the Earth sciences.
The primary zoning of crystals is detectable in Raman
maps because the inhomogeneous incorporation of
non-formula elements during primary growth causes
heterogeneous lattice strain [8,9]. At least as informative,
for the understanding of secondary processes in a
sample’s post-growth history, is the application of
highly-ressolved Raman maps in the study of altera-
tion textures in naturally formed minerals [10] or
technical products [11]. Raman mapping is increasingly
used to investigate non-destructively gem mate-
rials and precious art objects, and in the study of geological
samples of extraterrestrial origin, such as star
dust and meteorites [12,13]. Another increasing field
of application are studies dealing with mineral formation
at high pressure, both in the Earth’s interior (i.e.,
primary growth and metamorphism) and near the
Earth’s surface (i.e., due to impact events) [14,15].
An example for the latter is presented in Fig. 3:
The Raman map of this diamond crystal containing a
graphite inclusion, showing the distribution of the
width of the main LO=TO band of diamond near 1332
cm –1 , is affected by two features. First, there are minute
but still well-detectable variations among primary
growth zones, perhaps due to small variations in trace
elements and/or structural defects. The map shows that
the graphite is located in the center of the diamond
zoning. This led to the conclusion that the graphite
inclusion must be protogenetic in nature (i.e., the
graphite was the primary phase and was overgrown by
diamond; [16]). Second, compressive stress still acting
on the inclusion (caused by heterogeoeus volume expansion
of the diamond–graphite couple upon uplift to
the Earth’s surface) results in strong upshift and
broadening of the LO=TO band [17]. The circular
Fig. 3: Diamond crystal containing a single-crystal
graphite inclusion, from the Panda kimberlite, Canada.
A, photomicrograph. B, color-coded Raman map
(52,074 single analyses; step width 5 �m) showing the
heterogeneous broadening of the main diamond band.
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CORALS II Conference (2011)
black feature in Fig. 3B hence indicates an area of particularly
extensive compression of the diamond lattice.
To finish the – still incomplete – list of geoscientific
fields in which Raman mapping has recently received
particular attention, applications in the study of minerals
that suffered radiation damage (Fig. 4; e.g., [18]),
investigation of biominerals such as ivory, bones, and
pearls, and studies of microfossils [19,20] are mentioned.
Fig. 4: Radiohaloes in cordierite from the Strangways
Range, Australia. A, photomicrograph. B, composite
Raman map (7,134 point analyses; step width 3 �m),
showing the locations of radioactive inclusions (red,
monazite; green, zircon) and the broadening of a cordierite
band (blue) due to the irradiation damage.
The various sorts of imperfectness of minerals, and
their spatial distribution patterns, constitute a record of
a sample’s geological history. This is because minerals
are formed mostly in a non-homogeneous process,
consisting of the primary growth and possible secondary
alteration, both occurring in natural and hence
“polluted” milieus. Correspondingly, the study of internal
textures of minerals may provide a wealth of
valuable information. Images or distribution patterns
generated from luminescence emissions or Raman
scattered light have proven to be most versatile tools in
the study of internal textures of minerals and geological
samples. The powerfulness of the two techniques
discussed in detail in this plenary lecture is due to the
particular sensitivity to minute variations of defects
(luminescence) and the ability to detect and visualize a
broad range of physical parameters (Raman), respectively.
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