3.1 Mineral structure characterization
-
Polarizing microscope characterization. As shown in Fig. 3, the control samples were thin layer of siltstone with multiple strip. Their touch feeling of surface and color were similar to the unearthed relics sample, which was grayish-green. Besides, the surface were wavy and uneven with visibly horizontal layer. It could be seen that samples 1–6 were composed of silt-grade crumb, and showed their basic skeleton via the polarizing microscope. The sorting of the clastic particles was moderate, and the roundness was not well. The Fig. 3 also indicated that mineral particles minerals contained quartz, sericite, chlorite and other minerals. They showed directionality, and their plastic deformation was caused by compaction [15].
-
XRD characterization. Figure 4 showed the XRD patterns of relics sample No.1 and control samples No. 2, 3, 4, 5, 6. It exhibited diffraction peaks at 2θ along 27.1, 29.2 corresponding to the sericite. It could be seen that there are common diffraction peaks at 2θ along 12.6, 25.7, which belong to chlorite. In addition, diffraction peaks at 2θ along 21.2, 36.9 were consistent with the quartz [16]. The XRD results demonstrated that the mineral types of the cultural relics samples are identical with the control samples.
-
SEM and EDS characterization. Firstly, the surfaces of samples were sprayed with gold before testing. Then their microstructures were observed by SEM (Fig. 5) and the distributions of chemical elements on their surfaces were obtained through EDS (Fig. 6). On the one hand, it was found that both the relic sample and the control samples mostly showed micro-squama textures, mainly containing elements such as Si, O, Al, Fe, K, Ca, C, Na, etc., and Si, O, Al elements were absolutely dominant. On the other hand, the compositions of samples suggested that inkstones mainly consist of Oxygen-containing silicate minerals, aluminosilicate minerals, and the intergranular pores are filled with carbonate minerals and clay minerals. Among them, the morphology of minerals were mainly composed of irregular flaky (such as goeschwitzite, mixed-layer illite/smectite) and thin flaky (such as montmorillonite), with fuzzy boundary and scattered distribution.
In addition, the SEM images suggested that there were many lamellar minerals on the inkslab surface and they were squama-like (Fig. 5) [17, 18]. As shown in Fig. 6, after superimposing individual elements through the EDS surface scanning, it was concluded that such minerals are mostly silicate minerals (quartz, sericite, chlorite, etc.). These minerals were arranged together, such as "blade" obliquely standing on the inkstone surface, known as “inkslab blade” [19, 20]. When grinding the ink, moving of the ink on the inkslab surface could be cut continuously by the inkslab blade to effectively meet the requirements of generating ink. Moreover, the arrangement of the fragment not only made the ink fine and symmetrical, but also effectively protected the inkslab and made the surface durable. Besides, irregular plate-like clay minerals between the minerals played a role in cementation, which not only ensured the strength requirements of the inkslab, but also made the surface smooth and mild.
3.2 Chemical element characterization
Table 1
Major (mass %) elements of relics sample (1) and control samples (2, 3, 4, 5, 6)
Chemical
composition
|
1
|
2
|
3
|
4
|
5
|
6
|
SiO2
|
64.38
|
58.14
|
57.80
|
53.09
|
61.75
|
63.65
|
Al2O3
|
15.68
|
15.86
|
16.08
|
20.37
|
14.04
|
13.18
|
MgO
|
2.17
|
2.35
|
2.33
|
2.58
|
2.07
|
1.90
|
CaO
|
0.38
|
4.17
|
4.65
|
2.41
|
4.55
|
4.73
|
TFe2O3
|
7.09
|
7.15
|
6.52
|
7.10
|
5.93
|
5.02
|
TiO2
|
0.81
|
0.71
|
0.70
|
0.84
|
0.67
|
0.64
|
Na2O
|
0.85
|
0.30
|
0.17
|
0.12
|
0.67
|
0.62
|
K2O
|
3.24
|
2.97
|
3.17
|
4.13
|
2.42
|
2.18
|
LOI(1000 ℃)
|
4.49
|
7.91
|
8.18
|
8.97
|
7.65
|
7.57
|
∑
|
99.09
|
99.38
|
99.6
|
99.61
|
99.75
|
99.49
|
There were many existential forms of iron element, and the TFe2O3 mainly included Fe2O3 and FeO. However, the content largely influenced by the material provenance. Additionally, aluminum, sodium and potassium were mainly found in clay minerals mica and other minerals. It was found that Al2O3 mainly existed in the crystal voids in the form of clay minerals via the EDS (Fig. 6) [22]. The chemical composition of major elements was presented in Table 1. In Fig. 7 are plotted the contents of major elements of the archaeological sample (1) and control samples. On the whole, the main elements of the two groups of samples have similar change rules, and quartz is taken as the main framework of rock, which is consistent with the observation results of polarizing microscope. In addition, the relics sample of contents of Al2O3, MgO, TFe2O3, TiO2 and K2O were all in the change ranges of the corresponding control group [23]. The ratio of Al2O3 and SiO2 (Al2O3/SiO2) represented the ratio of all aluminum-containing minerals (mica, clay minerals, etc.) to quartz [24, 25]. Therefore, the calculation results showed that the Al2O3/SiO2 of the relics sample was 0.24, while ranges of the control groups was 0.21 ~ 0.38 with an average of 0.27. The relics samples were in the variation range of the control group and close to the average value, indicating that the correlations were consistent between the aluminum-containing minerals and quartz. In a word, it is concluded that the cultural relics samples are well matched with the control samples from the perspective of major elements.
-
ICP-AES and ICP-MS characterization. What's more, in order to avoid the experimental error caused by rock weathering, trace elements and rare earth elements were analyzed. The trace elements and rare earth elements are relatively stable and have high homogenization characteristics, which determines that they can provide relatively accurate geochemical parameters for sedimentological provenance analysis. Incompatible elements, such as La, Zr, Hf, Co, Sc, Nb, Th (inert elements), show stable properties under various geological processes of surface and are less or not affected by the deposition process. Despite the transformation effect during the deposition process, geochemistry is still dominated by provenance, which is used as an indicator of the composition characteristics of crop sources. During the process of practice apply, a more stable ratio of trace elements (La/Sc, La/Co, Th/Sc, Th/Co, Th/Cr) is often adopted to indicate the source stability instead of the absolute values in order to exclude the influence of particle size and other factors. The data results of trace and rare earth elements were listed in Table 2.
Table 2
Contents of trace, rare earth (µg/g) elements in relics sample (1) and control samples (2, 3, 4, 5, 6)
element
|
1
|
2
|
3
|
4
|
5
|
6
|
La
|
39.4
|
36.4
|
36.3
|
48.4
|
34.4
|
34.9
|
Ce
|
79.0
|
73.8
|
72.7
|
97.1
|
70.4
|
71.5
|
Pr
|
9.10
|
8.23
|
8.54
|
10.65
|
7.94
|
8.29
|
Nd
|
34.8
|
32.8
|
33.2
|
39.3
|
31.4
|
32.9
|
Sm
|
6.61
|
6.21
|
6.47
|
6.85
|
6.35
|
6.90
|
Eu
|
1.27
|
1.40
|
1.37
|
1.44
|
1.26
|
1.52
|
Gd
|
5.16
|
5.23
|
5.82
|
5.03
|
5.17
|
6.24
|
Tb
|
0.90
|
0.92
|
0.87
|
0.85
|
0.83
|
0.85
|
Dy
|
4.67
|
4.75
|
4.88
|
4.53
|
4.51
|
4.77
|
Ho
|
1.00
|
0.99
|
1.08
|
0.97
|
0.96
|
1.06
|
Er
|
2.65
|
3.02
|
3.06
|
2.84
|
2.52
|
2.71
|
Tm
|
0.42
|
0.44
|
0.46
|
0.45
|
0.40
|
0.43
|
Yb
|
2.77
|
2.80
|
3.01
|
3.02
|
2.51
|
2.55
|
Lu
|
0.39
|
0.42
|
0.45
|
0.44
|
0.35
|
0.36
|
Rb
|
144.0
|
139.0
|
153.5
|
198.0
|
116.0
|
105.0
|
Ba
|
432
|
487
|
599
|
656
|
400
|
412
|
Th
|
14.00
|
13.50
|
13.70
|
17.50
|
12.20
|
12.00
|
U
|
3.01
|
3.24
|
3.20
|
4.04
|
2.74
|
2.74
|
Nb
|
14.1
|
13.4
|
13.7
|
16.2
|
12.6
|
12.2
|
Ta
|
1.00
|
0.98
|
0.90
|
1.08
|
0.84
|
0.82
|
La
|
39.40
|
36.40
|
36.30
|
48.40
|
34.40
|
34.90
|
Ce
|
79.00
|
73.80
|
72.70
|
97.10
|
70.40
|
71.50
|
Co
|
15.20
|
14.10
|
54.70
|
22.80
|
10.00
|
15.80
|
Cr
|
77
|
84
|
91
|
113
|
73
|
64
|
Pb
|
55.8
|
24.8
|
93.9
|
57.3
|
20.7
|
23.7
|
Pr
|
9.10
|
8.23
|
8.54
|
10.65
|
7.94
|
8.29
|
Sr
|
59.70
|
136.00
|
152.00
|
110.00
|
144.00
|
159.00
|
Nd
|
34.80
|
32.80
|
33.20
|
39.30
|
31.40
|
32.90
|
Zr
|
201.00
|
172.00
|
176.00
|
175.00
|
181.00
|
197.00
|
Hf
|
5.70
|
4.70
|
4.50
|
4.60
|
4.70
|
4.70
|
Sc
|
12.50
|
14.00
|
13.80
|
16.30
|
11.50
|
10.40
|
Sm
|
6.61
|
6.21
|
6.47
|
6.85
|
6.35
|
6.90
|
Eu
|
1.27
|
1.40
|
1.37
|
1.44
|
1.26
|
1.52
|
Ti
|
4855.95
|
4256.45
|
4196.5
|
5035.8
|
4016.65
|
3836.8
|
Gd
|
5.16
|
5.23
|
5.82
|
5.03
|
5.17
|
6.24
|
Tb
|
0.90
|
0.92
|
0.87
|
0.85
|
0.83
|
0.85
|
Dy
|
4.67
|
4.75
|
4.88
|
4.53
|
4.51
|
4.77
|
Yb
|
2.77
|
2.80
|
3.01
|
3.02
|
2.51
|
2.55
|
Y
|
26.7
|
28.6
|
30.1
|
28.1
|
28.0
|
28.6
|
Ho
|
1.00
|
0.99
|
1.08
|
0.97
|
0.96
|
1.06
|
Er
|
2.65
|
3.02
|
3.06
|
2.84
|
2.52
|
2.71
|
Zr
|
201.00
|
172.00
|
176.00
|
175.00
|
181.00
|
197.00
|
∑REE
|
188.14
|
177.41
|
178.21
|
221.87
|
169.00
|
174.98
|
LREE
|
170.18
|
158.84
|
158.58
|
203.74
|
151.75
|
156.01
|
HREE
|
17.96
|
18.57
|
19.63
|
18.13
|
17.25
|
18.97
|
LREE/HREE
|
9.48
|
8.55
|
8.08
|
11.24
|
8.80
|
8.22
|
LaN/YbN
|
1.34
|
1.23
|
1.14
|
1.51
|
1.29
|
1.29
|
δEu
|
1.02
|
1.15
|
1.05
|
1.15
|
1.03
|
1.09
|
δCe
|
0.99
|
1.01
|
0.98
|
1.01
|
1.01
|
1.00
|
La/Co
|
2.59
|
2.58
|
0.66
|
2.12
|
3.44
|
2.21
|
La/Sc
|
3.15
|
2.60
|
2.63
|
2.97
|
2.99
|
3.36
|
Th/Sc
|
1.12
|
0.96
|
0.99
|
1.07
|
1.06
|
1.15
|
Th/Co
|
0.92
|
0.96
|
0.25
|
0.77
|
1.22
|
0.76
|
Th/Cr
|
0.18
|
0.16
|
0.15
|
0.15
|
0.17
|
0.19
|
LREE = La + Ce + Pr + Nd + Sm + Eu |
HREE = Gd + Tb + Dy + Ho + Er + Tm + Yb + Lu + Y |
∑REE = LREE + HREE |
δEu = EuN∕ (SmN + GdN)*2 |
δCe = Ce ∕ (LaN + PrN)*2) |
The Primitive-mantle normalized incompatible element diagrams of samples 1–6 were shown in Fig. 8b. It could be seen that the results of relics sample (1) obtained in this study for most of the elements, such as Nb、Ta、Zr、Hf, are in good agreement with the positions overlap of control samples (2,3,4,5,6), indicating the similar overall characteristics between relics samples and the control samples [26, 27]. The calculated ratios of La/Sc (2.60 ~ 3.36), La/Co (0.66 ~ 3.44), Th/Sc (0.96 ~ 1.15), Th/Co (0.25 ~ 1.22) and Th/Cr (0.15 ~ 0.19) in Table 2 illustrated the source stability of samples 1–6, with the average of 2.91, 2.20, 1.05, 0.79 and 0.16 for each other. Obviously, the trace element ratios of the cultural relic sample 1 and control groups 2–6 were similar, and the average values of the former was close to the latter.
According to previous studies, the main factor controlling REE composition in sediments is provenance [28]. Therefore, the characteristics of REE partition model curve are one of the reliable indicators for provenance analysis [29]. The standardization of REE used in different rock types is distinguishing. North American Shales (NASC) could better reflect the geochemical characteristics of sedimentary rocks than other standardization. In this study, NASC was used as the standard to standardize the data of REE in sample 1–6 and the NASC standardized partition pattern of REE was obtained Fig. 8a [30]. The normalized REE patterns indicated that the overall correlation of samples 1–6 is consistent, and the regularity is evident, without the separation of curves. The control samples 2–6 were characterized by ∑REE, LREE and HREE contents (the average content being 184.29, 165.78 and 18.51, severally) [31, 32]. The results proved that changes of ∑REE, LREE and HREE contents were 169 ~ 221.87, 151.75 ~ 203.74 and 17.25 ~ 19.63, respectively.
As can be seen from Table 2, the average values of ∑REE, LREE and HREE of samples 1–6 were higher than the NASC standard values. Among them, sample 1 and samples 2–6 showed a similar trend, and the former was within the range of the latter [33, 25]. On one hand, the higher the LREE/HREE value is, the more concentrated the LREE are, while the HREE are relatively scarce [33, 34]. On the other hand, LaN/YbN value is the slope of the distribution curve in the standardized diagram of REE, showing the degree of inclination of the curve. When the LaN/YbN value exceeds 1 or not, the curve inclines toward the right or left, which belongs to LREE concentrated type or LREE deficit type; when the value is equal to 1, the curve is horizontal [35]. Higher REE concentrations suggested that the dominant REE carrier phase might be a detrital mineral, which was consistent with the results of SEM characterization [36–38]. There is a consistent value of about 1.34, 1.23, 1.14, 1.51, 1.29, 1.29 for LaN/YbN and LREE/HREE ratios for samples 1–6, indicating a slight LREE enrichment in relation to the average shale NASC [39].
Besides, δEu and δCe values, the third geochemical indices, were used to illustrate the degree of Eu-anomaly and Ce-anomaly [40, 41]. The consistent values of samples 1–6 about 1.03 ~ 1.15 for δEu and 0.98 ~ 1.01 for δCe (Table 2), indicating positive anomaly of Eu and no anomaly for Ce, which also suggesting the similar degree of anomaly between relics sample (1) and control samples (2,3,4,5,6) [42]. As the above discussions, the relics sample 1 and the control samples 2–6 exhibited strong identities from the aspects of slope the distribution-curve slope, differential degree and degree of differentiation and deficit degree. It could be demonstrated that the unearthed inkslab and the control samples come from the same geological provenance.