CL as a versatile tool with the CamScan SEM

CL as a versatile tool
with the CamScan SEM
CL SEM on zircon samples
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Charge neutralisation
4 segment backscattered detector, cathodoluminescence detector
Background
Characterisation of crystal growth has been an issue since the first crystals had been explored. Nature is an elaborate producer
of crystals, which have been investigated for their properties for more than a century. 1803 Dalton proposed elements were
composed of identical atoms. Not much later the concept of a material consisting of the same elements in repetitive units
was born. Ubiquitously occurring, but nevertheless most excitingly there are minerals called Zircons, mostly found in naturally
occuring and reworked (i.e. metamorphosed) igneous rocks that can be used for various characterisation purposes in geology.
Zircon is a mineral that comprises of zirconium, silicon and oxygen, also called zirconium silicate, chemical composition ZrSiO4.
It is often used as a replacement for diamonds, largely appreciated as a gemstone due to its high refractive index. Zircon has
suffered a decline in reputation due to the introduction of the diamond simulant cubic zircon, which is zirconium oxide, chemical
formula ZrO2, one of the most refractory materials known.
Methodology
As for any cathodoluminescent measurements the limiting factors are given by the detector but not the beam size itself,
resolution limits will be determined by sample characteristics and signal to noise ratio of the photomultiplier tube combined with
amplifier electronics. Crucial for the respective sample will be the cathodoluminescent electron beam interaction volume as well.
A randomly created accumulation of phosphorus particles has been used to test resolution and sensitivity ranges of the detector
(Gatan MiniCL) used on the Camscan SEM.
Challenge
Zircon poses as one of the key minerals in geochronology. Its characteristics of zoning observable through comparison of
backscattered and cathodoluminescent images deliver distinct knowledge for research in geology. High resolution imaging
provides the possibility of matching interrelated patterns or detecting various levels of contamination. Therefore high quality
imaging for luminescent material remains essential.
Solution
Figure 1: phosphorus particles embedded in silver epoxy, magnification factors of 1000 up to 10000x
The produced limits of cathode luminescence measurements comprise of the actual detector and the given layout.
To demonstrate the possibilities of the Gatan MiniCL detector, a phosphorus containing sample was chosen. As visible in Figure 1 a high depth of focus can be
achieved as well as high resolution down to tens of nanometers for these grains. The phosphorus particles exhibit a high internal contrast variation and great
clarity of their boundaries.
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Figure 2: zircon samples of a natural source mounted in epoxy resin, magnification factor up to 400x
In Figure 2 two examples of zircon particles measured by the cathodoluminescent detector are shown. The zircon particles were embedded in resin and the
samples were measured at low vacuum conditions, 15 kV, 125 µA emission current, 15 Pascal. Backscattered images with a 4 segment solid state detector
were taken for comparison simultaneously. (Zircon grains provided by: ARC National Key Centre for Geochemical Evolution and Metallogeny of Continents
(GEMOC) Department of Earth and Planetary Sciences, Faculty of Science, MACQUARIE UNIVERSITY NSW 2109 AUSTRALIA. GEMOC
website: http://gemoc.es.mq.edu.au/GEMOC/)
For zircon crystals the luminescence is attributed to dysprosium contamination of various levels in the crystal. euhedral or
concentric zoning is common in zircon that crystallises in magma. Since zircon contains trace amounts of uranium it can be used
for radiometric dating. Zircon typically contains trace amounts of the radioactive materials hafnium, uranium and thorium. Some
zircons undergo a process that is called metamictisation. It describes a procedure of radiation damage, whereas the single crystal
structure is partially destroyed by internal radiation leading to a partially amorphous crystal structure. Nevertheless, zircon
persists in sedimentary deposits due to its durability and relative chemical inertness since more than 4 billion years.
The two grains shown in Figure 2 both present highly oscillatory zoning. On the left side in Figure 2 the cross like shape happens
to be the centre of euhedral prismatic zoning, showing so called radial sector zoning, which is caused by growth with slightly
different compositions on crystallographically different faces. In this case the growth rates of adjoining faces are not constant,
which is why the sector boundaries are curved. The prismatic zoning morphology is elongated to the left side, showing small
contrast flip patches along a centre string. Unzoned metamorphic overgrowth is visible on the upper left part of the zircon grain.
The zircon shown on the right side of Figure 2 exhibits radial zoning with a large slightly oval-shaped prismatic core, whereas
zoning expands concentrically. A uranium-rich rim encompasses the oval shaped zircon crystal.Very thin dark lines throughout
the crystal coincide with lines in the backscattered image and show therefore most likely material inclusions representing a
material with less mass than zirconium silicate.
Zircon has been in the limelight since evolution of radiometric dating lead to cores of zircon crystals being dated at 4.4Ga which
is the oldest known substance on earth..
Conclusion
The Camscan SEM equipped with a 4 segment
backscattered and a CL detector delivers an ideal
tool to image profiles of luminescent structures,
in particular difficult samples with complex zoning
characteristics. The use of zircon and high-res CL
will improve our ability to date and understand
multiple events recorded in a single crystal.
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CamScan
Superior Electron Optics By Design
Obducat CamScan Limited
CamScan House, Pembroke Avenue, Waterbeach, Cambridge. CB25 9PY UK
Tel: +44 (0)1223 861 066 Fax: +44 (0)1223 861 077 [email protected]
www.camscan.com
A member of the Obducat Group
Local Distributor
LOT-Oriel GmbH & Co. KG
contact:
www.lot-oriel.com
[email protected]
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