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First Edition: Jun. 2021 01-00107-EN
Fig. 3 Phase Diagram
Conclusion
Elemental mapping of rocks can reveal the physical and chemical
processes of crystal growth and metamorphism of contained
minerals. Furthermore, the combination of elemental mapping
and phase analysis is effective for identifying minerals. With these
methods, it is possible to evaluate gemstones and precious stones
in jewelry and even to identify their geographic origin.
Phase Analysis
A phase analysis was carried out based on the results of the
elemental mapping of the stones. Fig. 2 shows the scatter plots
created by selecting two or three elements (the wt% of their
oxides).
Clusters were observed in each scatter plot. These clusters
could be classified as epidote from the scatter plot of the two
variables FeO-Al
2
O
3
, hornblende from that of the three
variables CaO-MgO-FeO, and plagioclase and K-feldspar from
that of the three variables K
2
O-Na
2
O-CaO. In the plagioclase
cluster, there was a further cluster of Ca-rich components.
Fig. 3 shows the phase diagram obtained from these results.
From these elemental mapping and phase analysis results, it
was found that the epidote has low Fe concentration in the core
of each crystal and higher Fe concentration at the boundary or
rim of the crystals. Furthermore, plagioclase was found to have
more Ca-rich components mixed inside. As this analysis shows,
the distribution of minerals in a rock can be easily visualized
through phase analysis.
FeO
Al
2
O
3
Epidote
(Fe-rich)
Epidote
CaO
MgO FeO
Hornblende
K
2
O
CaO
Na
2
O
CaO
K-feldspar
Plagioclase
Ca-rich
Fig. 2-4 Enlarged Section in Fig. 2-3
Fig. 2-3 Three-Variable Scatter Plot
of K
2
O-Na
2
O-CaO
Fig. 2-1 Two-Variable Scatter Plot
of FeO-Al
2
O
3
Fig. 2-2 Three-Variable Scatter Plot of
CaO-MgO-FeO
Epidote Epidote (Fe-rich)
Hornblende Plagioclase (Ca-rich)
Plagioclase K-feldspar
500 μm
(wt%) (wt%)
300 μm
COMPO 15.0 kV 1.5 u 1.1 mm
300 μm
Fe Ka 15.0 kV 1.5 u 1.1 mm
5.000
0.000
4.375
3.750
3.125
2.500
1.875
1.250
0.625
300 μm
Cr Ka 15.0 kV 1.5 u 1.1 mm
2.400
0.000
2.100
1.800
1.500
1.200
0.900
0.600
0.300
Fig. 4-1 Detailed Elemental Mapping of Epidote
(wt%)
(wt%)
500 μm
COMPO 15.0 kV 3.0 u 2.2 mm
500 μm
Cr Ka 15.0 kV 3.0 u 2.2 mm
2.000
0.000
1.750
1.500
1.250
1.000
0.750
0.500
0.250
500 μm
Fe Ka 15.0 kV 3.0 u 2.2 mm
6.000
0.000
5.250
4.500
3.750
3.000
2.250
1.500
0.750
Fig. 4-2 Detailed Elemental Mapping of Epidote
<References>
・ Y. Kuroda and K. Suwa: Polarizing Microscope and Rock Minerals(Second
Edition), Kyoritsu Shuppan (1983)
・ A. Miyashiro and I. Kushiro: Petrology I: Polarizing Microscope and Rock-
forming Minerals, Kyoritsu Shuppan (1972).
EPMA is a trademark of Shimadzu Corporation or its affiliated companies in Japan and/or other countries.
Mapping of Fe and Trace Cr in Epidote and
Hornblende
Mapping analysis was carried out for Fe and Cr in epidote and
hornblende, which are present in epidote containing stones. The
concentrations were obtained as weight % of the oxide of each
element, as above. Fig. 4 shows the results. The concentration of
Fe in epidote was higher at the rim and boundary of the crystals,
while the concentration of Cr was higher in areas with less Fe and at
the boundary. Furthermore, Fe distribution in hornblende was
found to be almost homogeneous, while Cr was heterogeneous in
relatively large crystals, with higher concentrations in the center
area. However, in some fragmental hornblende, the concentration
of Cr was higher at the rim and boundary.
It is known that epidote is produced in metamorphic rocks and
that the amount of Fe
3+
increases with higher metamorphic
temperatures, suggesting that the reactions proceeded at the
boundary and rim of the crystals during metamorphism. It can
also be assumed that hornblende assimilated some Cr-rich
substances during metamorphic process, or came into contact
with Cr-rich components in case abundant Cr was found at the
boundary.
