Dear PARE members,
Snow has fallen yesterday and the temperature is getting low. Everywhere we look at, it is always something
covered by white colour. Then Sapporo city becomes brighter than the day before
yesterday, even though sun does not appear yet. It is because the effect of white
colour.
Generally speaking, colours are usually represented qualitatively and
individually in our daily life. Therefore, in this session I will write down my experiment about color and its relation to my field study. As for one of my study course in rock engineering,
quantitative parameters are essential. However,
colour is not related with the rocks strength. Black rock does not mean that
the unconfined comprehensive strength (UCS) of rock is higher than the reddish
rock. Sometimes, reddish rock is stronger than black rock. The colour of the
rock is related to the composition of the constituent minerals. Minerals
composition of these rocks is strongly influenced by the composition of the
magma origin so that it can be seen from the colour of its constituent magma
types, except for rocks that have a glass texture. Brightly colours of igneous
rocks are generally acidic rocks composed of felsic minerals, such as quartz,
potash feldspars and muscovite. Igneous rocks are generally dark to black
igneous rock intermediate in which the amount of felsic minerals and mafic
minerals are nearly same.
Moreover, the rocks strength is closely
related with the weathering process. The weathered rocks commonly have small
unconfined compressive strength value, comparing with fresh rocks or unweathered
rocks. Weathering is the breakdown of rock into smaller pieces through exposure
to wind, water, heat, and cold. There are two types in weathering i.e. mechanical
and chemical. Due to the weathered process, some minerals are separated and
react. Based on Nagano and Nakashima (1989), the colour of rocks is considered
to be one of the few parameters which can determine weak degrees of weathering
of rock due to small changes in their chemical or mineralogical properties. The
oxidation process due to rocks weathering generally makes rocks colour become
reddish.
According to the comprehensive understanding of the relationship
between colours and properties of soils and rocks, there are siltstone samples
taken in various location (see figure 1). Although, the rock type of the
samples are similar (siltstone), each sample has different colour (see figure
2).
 |
| Figure 1. Sample location |
 |
| Figure 2. Siltstone in various colours |
By using colorimeter, we measured quantitative colours of the rock samples.
The measurement data are descripted by L*a*b*, which presented in Table 1,
Table 2, Table respectively. Rocks
become red due to increasing of a* value and become yellow due to increasing b*
value.
Table 1. Data of L - value
No. Measurement
|
L
|
Sample 5
|
Sample 6
|
Sample 7
|
Sample 8
|
Sample Origami
|
1
|
29.15
|
53.59
|
71.08
|
58.25
|
52.15
|
|
2
|
27.83
|
51.42
|
53.88
|
56.44
|
53.83
|
|
3
|
36.64
|
58.92
|
68.86
|
56.76
|
52.64
|
|
4
|
28.17
|
46.05
|
54.31
|
57.24
|
52.17
|
|
5
|
28.66
|
47.48
|
66.63
|
53.44
|
53.66
|
|
AVERAGE
|
30.09
|
51.492
|
62.952
|
56.426
|
52.89
|
|
Standard Deviation
|
3.695
|
5.131
|
8.238
|
1.804
|
0.807
|
Table 2. Data of a* - value
No. Measurement
|
a*
|
Sample 5
|
Sample 6
|
Sample 7
|
Sample 8
|
Sample Origami
|
1
|
1.21
|
0.42
|
4.52
|
3.95
|
57.21
|
|
2
|
-0.03
|
0.36
|
11.8
|
3.34
|
58.03
|
|
3
|
8.64
|
2.78
|
6.98
|
3.33
|
56.64
|
|
4
|
2.96
|
9.82
|
13.09
|
3.56
|
57.96
|
|
5
|
2.87
|
12.39
|
7.14
|
3.38
|
56.87
|
|
AVERAGE
|
3.13
|
5.154
|
8.706
|
3.512
|
57.342
|
|
Standard Deviation
|
3.321
|
5.594
|
3.597
|
0.262
|
0.630
|
Table 3. Data of b* - value
No. Measurement
|
b*
|
Sample 5
|
Sample 6
|
Sample 7
|
Sample 8
|
Sample Origami
|
1
|
2.86
|
12.51
|
15.71
|
13.64
|
30.86
|
|
2
|
-0.23
|
11.61
|
24.48
|
12.5
|
30.23
|
|
3
|
12.17
|
19.41
|
23.02
|
12.21
|
29.17
|
|
4
|
3.02
|
16.91
|
20.63
|
12.9
|
30.02
|
|
5
|
5.83
|
23.13
|
20.64
|
11.76
|
29.73
|
|
AVERAGE
|
4.73
|
16.714
|
20.896
|
12.602
|
30.002
|
|
Standard Deviation
|
4.679
|
4.801
|
3.331
|
0.714
|
0.623
|
By
input the average value of a*b* from each samples, scatter diagram can be
created by excel program. The scatter diagram shows that a* and b* values of
sample 5, 6, and 7 gradually increase.
Then, the value of sample 8 decreases.
In addition, b* values significantly increase during weathering compared
by a*. The scatter diagram is presented in Figure 3.
 |
| Figure 3. Diagram for the various colour of Siltstone |
Based
on the L*a*b* value from colorimeter measurement, it can be conclude that the
colour of the rocks determine the degree of weathering rocks. The most
indicator colour is yellow due to the significant change in b* value.
Reference:
Nagano T. and Nakashima S., (1989), Study of colors and degrees of weathering of
granitic rocks by visible diffuse reflectance spectroscopy, Geochemical
Journal, Vol. 23, pp. 75- 83
Best Regards,
Shofa R. Haq
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