A05
COMBINE APPLICATION OF SCANNING ELECTRON MICROSCOPY (SEM)
AND ENERGY DISPERSIVE X-RAY SPECTROSCOPY (EDX) IN IDENTIFYING
CONSTITUENT MINERALS IN GEOARCHAEOLOGY ARTIFACTS
Hamzah Mohamad, Abdul Mutalib Abdullah, Mokhtar Saidin & Muhammad Ikhwan Harun
Centre for Global Archaelogical Research
Universiti Sains Malaysia
11800 USM Pulau Pinang
Email: [email protected]
This paper illustrates how the combination of scanning electron microscopy technique (SEM) and energy
dispersive X-ray spectroscopy technique (EDX) may be utilized to determine the constituent minerals of
geoarchaeological artifacts. The studied materials are soil for brick making from the archaeology site of
Sungai Batu, Kedah and suevite, a rock produced by meteorite impact collected from Bukit Bunuh,
Lenggong, Perak. Three approaches have been employed, i.e. (1) images produced by electrons from the
sample, which display the crystal form, which is one of the determining characteristic of mineral, (2)
chemical composition --- a specific characteristic of mineral by EDX, and (3) the elemental distribution of
the sample, which is capable of differentiating minerals and their locations in the sample, using EDX.
Kaolinite and quartz have been used to illustrate the high accuracy characterization. The instruments used are
Quanta FEG 650 scanning electron microscope (Fei, Holland) and its attachment, i.e. X-Max 50 energy
dispersive X-ray spectrometer (Oxford Instrument, England). The main experimental condition applied is
low vacuum mode. The resolution of the SEM is 1 ηm. The EDX analysis is associated with high accuracy,
as shown by the result of analysis performed on Standard Reference Material (CRM). For example, a
standard albite mineral (produced by MAC, Cambridge shire, UK) gives the following recommended and
observed values, respectively for five elements analyzed: Si (31.79%, 32.17%), Al (10.48%, 10.99%), Na
(8.38%, 7.82%) and O (48.45%, 49.08%). The experimental conditions for kV, WD and magnification are
shown on the result page. Air-dried sample has been pulverized to individual grain size of 20 - 30µm and
oven-dried for an hour at 105ºC. Approximately 0.1g of the sample was put on the sample stub using liquid
form carbon tape. The quartz image depicts hexagonal cross section (six-sided but sometimes subhedral in
nature) and the planes which make its columnar form (see Fig. 1A). Kaolinite image also shows hexagonal
cross section normal to its c-axis, but in contrast to quartz it shows parallel plates normal to its c-axis and the
kaolinite plates staking to each other, forming the well-known book feature (see Fig. 1B). The six-sided
minerals can be further distinguished using their chemical compositions at several tiny spots on the minerals.
Quartz is characterized by the occurrence of Si and O only, in elemental ratio of 33.33:66.66, leading to its
formula SiO2 (see Fig. 2A1 and Fig. 2A2). In addition to Si and O, kaolinite shows the occurrence of Al (see
Fig. 2B1). If the crystal water in pure kaolinite with formula Al2O3.2SiO2.2H2O is neglected, the volatile-free
formula for kaolinite is 2Al.2Si.9O or, 15.5% Al, 15.5% Si and 69 % O. The kaolinite analysis shown in Fig.
2B2 is very close to the theoretical formula of typical kaolinite. The elemental mapping of adjacent quartz
and kaolinite has shown clearly the different composition of these minerals --- quartz shows high intensity of
Si + O, while kaolinite is made up of Si + Al + O but the intensities of Si and O are lower than in quartz.
Kaolinite also contains small amount of K and Mg (see Fig. 3). The individual grain size of kaolinite is <
1µm and 50µm for quartz.
c
A
B
Fig.1 (A) A tiny quartz crystal showing chop-off portion perpendicular to the mineral’s c-axis, exposing four of the six
outlines of the six-sided plane, as well as planes making the column (3,000X). (B) Plates of six-sided (hexagonal
system) of kaolinite, or “book kaolinite” (132,000X). The c-axis is towards observer.
Quartz EDX Spectrum
2A1
Element
O
Si
2B1
Kaolinite EDX Spectrum
Spectrum
2A2
Quartz EDX
Wt %
46.74
53.26
100.00
Atom %
Quartz Atom %
66.67
33.33
100.00
66.67
33.33
100.00
2B2
Kaolinite EDX
Element
O
Al
Si
Wt %
60.73
18.17
21.10
100.00
Atom %
72.71
12.90
14.39
100.00
Kaolinit Atom %
69.24
15.38
15.38
100.00
Fig. 2 (2A1) EDX spectrum of analyzed quartz. (2A2) EDX analysis of quartz. Apart from weight percentages and
atomic percentages, the atomic percentages of typical quartz are shown as reference. (2B1) EDX spectrum of analyzed
kaolinite. (2B2) EDX analysis of kaolinite. Apart from weight percentages and atomic percentages, the atomic
percentages of typical kaolinite are shown as reference.
K
Q
Fig.3 X-ray intensity of elements from a sample containing quartz (Q) and kaolinite (K) in intimate association
(Spectrum 1). Oxygen (O) and silicon (Si) occur in both quartz and kaolinite but with lower intensity in area occupied
by kaolinite. Aluminum (Al) and potassium (K) are strictly confined to area where kaolinite is located. Small amount
of magnesium (Mg) may present in kaolinite.
Reference
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Gribble, C.D. 1988. Rutley’s elements of mineralogy. 27th Edition. London: Unwin Hyman.
Mokhtar Saidin. 2012. Historic discovery of meteorite impact at Bukit Bunuh, Lenggong, Perak. Abstract
International Conference on Archaeogeology of Meteorite Impact at Bukit Bunuh Area, Lenggong,
Perak, Malaysia: 1-2.
Schneider, R. 2011. Energy-dispersive X-ray spectroscopy (EDXS). New York: John Wiley.