with magnesium chloride, glycerol and potassium chloride until saturation, then all the
specimens were measured after air drying under ambient conditions. Potassium
specimens were additionally heated to 50
C, 400
C and 600
C. For quantification
of the bulk mineral composition, Rietveld software (Autoquan, Seiffert) was used, and
for quantification of the clay fraction, the software package Diffrac AT 3.3 (Siemens)
was utilized. X-ray fluorescence (XRF) was performed for total element analysis
using a Siemens SRS 200 instrument. Four quantification standards were used.
Scanning electron-microscopy (SEM) was carried out with a Leo 420 instrument.
This analysis was performed on sand-sized natural minerals or aggregates which had
been spattered with gold. These samples were also analyzed with energy dispersive
X-ray spectrometry (EDX) (Blume et al. 2011).
2.5.2 Limestone and Limestone Residue Mineral Composition
Paleozoic and Mesozoic limestones in the study areas were found to be very rich in
carbonates (98 % and over), and in all cases calcite was dominant, associated with
some dolomite in Laos (7 %) and Vietnam (up to 15 %). In the bulk samples only a
few non-carbonatic minerals were detected; the majority represented by mica of up
to 2 %, or quartz of up to 1 % (Table 2.11).
Analyzing the limestone residues gave a much more differentiated picture. In all
samples, primary minerals were detected (Table 2.11); however, the feldspar
content was always low, with 3–5 % albite and a maximum of 7 % anorthite.
This signifies the presence of some unweathered material input from the continent.
This finding was underlined by the rather significant presence of mica minerals in
the residue, ranging from 18 % in Vietnam up to more than 50 % in the older
limestones of Laos and Thailand. However, it is well-known that illitization takes
place when other 2:1 clay minerals or even kaolinite enter a marine environment.
Quartz was also found, which can be either derived from primary rocks or a residue
of strongly weathered material. Furthermore, iron oxides were found; goethite
without being significant, but hematite signifying a terrestrial environment. The
latter was found in Thailand and in minor amounts in Vietnam also. A significant
mineral found in the limestone residue was kaolinite, which is thought to be
unstable under a marine environment. Based on this assumption, kaolinite in
limestone residue has to have been imported from a terrestrial environment, with
carbonate precipitation rates being high in order to impede kaolinite transformation.
In particular, the observed 46 % kaolinite in the limestone from Muong Lum is a
clear indication of intense chemical weathering in the terrestrial source region
during formation. Also, the limestones from Laos and Thailand showed minor
components of kaolinite, indicating that limestone formation was “peri-continental”. The variable mineral composition of the limestone residue induced problems
in terms of understanding and, in particular quantifying the soil forming processes
at work, because conclusions should be based on very clear initial conditions.
80
K. Stahr et al.
specimens were measured after air drying under ambient conditions. Potassium
specimens were additionally heated to 50
C, 400
C and 600
C. For quantification
of the bulk mineral composition, Rietveld software (Autoquan, Seiffert) was used, and
for quantification of the clay fraction, the software package Diffrac AT 3.3 (Siemens)
was utilized. X-ray fluorescence (XRF) was performed for total element analysis
using a Siemens SRS 200 instrument. Four quantification standards were used.
Scanning electron-microscopy (SEM) was carried out with a Leo 420 instrument.
This analysis was performed on sand-sized natural minerals or aggregates which had
been spattered with gold. These samples were also analyzed with energy dispersive
X-ray spectrometry (EDX) (Blume et al. 2011).
2.5.2 Limestone and Limestone Residue Mineral Composition
Paleozoic and Mesozoic limestones in the study areas were found to be very rich in
carbonates (98 % and over), and in all cases calcite was dominant, associated with
some dolomite in Laos (7 %) and Vietnam (up to 15 %). In the bulk samples only a
few non-carbonatic minerals were detected; the majority represented by mica of up
to 2 %, or quartz of up to 1 % (Table 2.11).
Analyzing the limestone residues gave a much more differentiated picture. In all
samples, primary minerals were detected (Table 2.11); however, the feldspar
content was always low, with 3–5 % albite and a maximum of 7 % anorthite.
This signifies the presence of some unweathered material input from the continent.
This finding was underlined by the rather significant presence of mica minerals in
the residue, ranging from 18 % in Vietnam up to more than 50 % in the older
limestones of Laos and Thailand. However, it is well-known that illitization takes
place when other 2:1 clay minerals or even kaolinite enter a marine environment.
Quartz was also found, which can be either derived from primary rocks or a residue
of strongly weathered material. Furthermore, iron oxides were found; goethite
without being significant, but hematite signifying a terrestrial environment. The
latter was found in Thailand and in minor amounts in Vietnam also. A significant
mineral found in the limestone residue was kaolinite, which is thought to be
unstable under a marine environment. Based on this assumption, kaolinite in
limestone residue has to have been imported from a terrestrial environment, with
carbonate precipitation rates being high in order to impede kaolinite transformation.
In particular, the observed 46 % kaolinite in the limestone from Muong Lum is a
clear indication of intense chemical weathering in the terrestrial source region
during formation. Also, the limestones from Laos and Thailand showed minor
components of kaolinite, indicating that limestone formation was “peri-continental”. The variable mineral composition of the limestone residue induced problems
in terms of understanding and, in particular quantifying the soil forming processes
at work, because conclusions should be based on very clear initial conditions.
80
K. Stahr et al.