Introduction

In 2020, the Sichuan Provincial Institute of Cultural Relics and Archaeology, in cooperation with other institutions, conducted a comprehensive excavation of sacrificial pits K3 to K8 at the Late Shang Dynasty site of the Sanxingdui (三星堆) (ca. 1200–1000 BC). This excavation unearthed numerous valuable relics, including bronze, gold, jade, and ivory1,2,3. A wide variety of jade artifacts were uncovered, including zhang (璋), ge (戈), Zao (凿), yuan (瑗), jin (斤), Cong (琮)4. Some jade artifacts had transformed into a white powdery state and lost their original shape.

Lu’s research revealed evidence of a mineral thermal phase transition from tremolite to diopside in the jade artifacts from the K3–K8 sacrificial pits, thereby substantiating the occurrence of jade burning during sacrificial activities at the Sanxingdui site5. Guo’s analyses of the chromaticity and magnetic properties within the pits support the hypothesis that these pits were used for the burial of burnt sacrificial offerings6. The phenomenon of jade burning can be traced back to the late Neolithic period in China. Scientific analyses indicate that jade burning occurred at various sites across different periods, such as the Liangzhu Sidun (良渚寺墩) site7 and the Yinxu Fuhao tomb (殷墟妇好墓) of the Shang Dynasty8, where burnt jade artifacts have been discovered.

The presence of notable whitening features in burnt jade artifacts has been a consistent subject of study in ancient jade research. This phenomenon has prompted scholars to propose various explanations9. Wang’s research on the Raman spectra of whitened jade artifacts from the K3-K8 sacrificial pits revealed a characteristic hump-shaped fluorescence signal. In an oxygen-deficient environment, the crystal-enclosed organic matter within the jade undergone a high-temperature polymerization reaction in which the carbonyl groups are conjugated, resulting in fluorescence. The crystal-enclosed organic matter is derived from the magnesian carbonate, the parent rock of jade, and is mainly composed of waxes or resins and their corresponding ammonium or magnesium salts. In the imitation experiment, the jade rock was heated to 700 °C, thereby removing the adsorbed organic matter it contained. The jade rock was then immersed in a hydrochloric acid solution to facilitate its decomposition. Finally, the crystal-enclosed organic matter can be extracted using an organic solvent10.

At present, most research on Sanxingdui jade artifacts has focused on mineral identification4, chemical composition11, and origin traceability12, with comparatively few studies addressing the jade structure and conservation. This study selected several typical tremolite jade artifacts and attempted to identify their deterioration mechanisms by examining morphology, compositional analysis, and phase analysis.

Methods

Samples

After a systematic observation of the excavated jades, three representative samples were selected (Fig. 1). Samples K7QW-Y-32 and K7QW-Y-112 were chosen from pit K7, and sample K3QW-557 was chosen from pit K3. Sample K7QW-Y-32 is a fragment of a yuan (瑗), with only three segments remaining. After cleaning, it exhibits a pale yellow coloration with a waxy luster, and its texture is compact and dense. K7QW-Y-112 is a fragment of a zhang (璋), showing a gray-brown coloration with slight surface powdering. K3QW-557 exhibits severe powdering and whitening, and its original shape is unrecognizable.

Fig. 1
figure 1

Photographs of jade artifacts from the Sanxingdui (a K7QW-Y-32; b K7QW-Y-112; and c K3QW-557).

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Analytical techniques

The samples studied in this paper were small fragments of jade. These samples were inlaid with epoxy resin, sanded, and polished before analysis.

Ultra-depth-of-field microscope

The profile shape of the jade samples was observed using an ultra-depth-of-field microscope (KEYENCE Digital Microscope VHX-7000). Equipped with a 3.19-million-pixel CMOS image sensor, it utilizes a high-resolution HR lens that combines a large depth of field with high resolution, offering magnification from 20× to 2000×.

Scanning electron microscope–X-ray energy dispersive spectrometer (SEM–EDS)

A scanning electron microscope (Thermo Scientific, Quattro S) equipped with an X-ray energy dispersive spectrometer (Bruker QUANTAX EDS) was used to characterize the microscopic morphology and elemental composition of the jade samples. The analyses were conducted in a low-vacuum environment using backscattered electron imaging, with an operating voltage of 15 kV, a working distance of 10 mm, and a sampling time of 60 s. The oxide data were normalized to one decimal place.

Laser microscopic confocal Raman spectrometer (Raman)

The material composition of the jade samples was characterized using a laser microscopic confocal Raman spectrometer (RENISHAW inVia), which was equipped with a research-grade Leica microscope offering a spatial resolution of less than 0.5 μm. The experimental conditions were as follows: an excitation wavelength of 785 nm, a scanning time of 10 s, and 30 scans.

Fourier transform infrared spectroscopy (FTIR)

FTIR (Thermo Fisher Scientific) was used to analyze the phase transitions of the samples. The system was equipped with a micro-infrared mainframe (Nicolet iN10), an attenuated total reflection (ATR) accessory, and a transmission accessory (Nicolet iS10/iZ10 auxiliary optics platform). For sample preparation, the jade and potassium bromide were mixed in an agate mortar at a ratio of 1:150–200. After thorough grinding and mixing, the resulting mixture was pressed into transparent circular slices with a thickness of 0.1–1.0 mm for testing in the sample chamber. The experimental conditions were as follows: the detector was a DTGS KBr with a scanning range of 4000–400 cm−1 and a resolution of 4 cm−1, using 32 background scans and 32 sample scans.

Results

SEM–EDS and Raman spectral analyses

The SEM image of sample K7QW-Y-32 is shown in Fig. 2, and the EDS results are in Table 1. The sample was well-preserved, and the jade matrix appeared greenish with fractured areas. The major elements in the jade matrix (Fig. 2c, EDS1 and EDS2) were close to the theoretical values for tremolite. Raman spectra confirmed that the sample was tremolite (Fig. 5a)13,14.

Fig. 2
figure 2

Sample image of K7QW-Y-32 (a, b ultra-depth-of-field microscopic image; c BSE image of the matrix and yellowish inclusions area; and d BSE image of the brownish-black inclusions area).

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Table 1 Results of EDS analyses of sample K7QW-Y-32 (wt/%)
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The elemental composition of the yellow-greenish area at the edge of the sample was similar to that of the greenish matrix. The main elements of the yellowish inclusions (Fig. 2c, EDS3 and EDS4) and brownish-black inclusions (Fig. 2d, EDS3 and EDS4) were Si, Al, and Na, which were preliminarily presumed to be albite (NaAlSi3O8), as identified by the Raman spectra shown in Fig. 5b.

The SEM image of sample K7QW-Y-112 is shown in Fig. 3, and the EDS results are in Table 2. The sample was brownish-yellow overall, with some areas showing signs of whitening. The whitened areas were porous, and acicular crystals were exposed within the holes. Brown attachments were found locally on the outermost part of the surface, corresponding to the dark grey areas in the SEM BSE images (Fig. 3d, EDS1 and EDS2). These attachments were primarily clay minerals from the soil, containing elements such as Si, Al, Fe, Mg, Ca, and K. Beneath the attachments lay the jade matrix (Fig. 3d, EDS3 and EDS4), as confirmed by both EDS and Raman spectroscopy (Fig. 5c).

Fig. 3
figure 3

Sample image of K7QW-Y-112 (a, b ultra-depth-of-field microscopic image; c BSE image of the area of Fig. 3b; d BSE image of the brown attachments; e BSE image of the acicular crystals; and f BSE image of the materials wrapped outside the acicular crystals).

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Table 2 Results of EDS analyses of sample K7QW-Y-112 (wt/%)
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The composition of a few acicular crystals within the holes (Fig. 3e, EDS1) was similar to that of the dense area. The Ca content in most of the acicular crystals (Fig. 3e, EDS2, EDS3, and EDS4) was significantly elevated, while the Mg content was noticeably reduced. Raman spectra (Fig. 5c) indicated that their main physical phase remained tremolite.

The regions shown in Fig. 3f, EDS3 and EDS4 correspond to the materials enveloping the acicular crystals within the corrosion holes. Their elemental compositions were similar to those observed in the outermost thin layer, though with marginally elevated Mg and Ca contents. The Raman spectra (Fig. 5d) of the acicular crystals (Fig. 3e, EDS2), the brown attachments (Fig. 3d, EDS1), and the materials within the holes (Fig. 3f, EDS3) in the range of 0–3200 cm−1 all exhibit a characteristic strong hump-shaped fluorescence background.

The SEM image of sample K3QW-557 is shown in Fig. 4, and the EDS results are in Table 3. The sample was pale yellow and opaque, and it had completely powdered and whitened. This type of sample exhibited a loose structure, which allowed the epoxy resin to seep through during embedding. The BSE image showed that the broken crystals consisted of irregular, small, lumpy, granular light grey crystals, as well as some dark grey materials. The components of the light grey crystals (Fig. 4d, EDS1, EDS2, EDS3, and EDS4) were similar to diopside (CaMgSi2O6), with the theoretical components as follows: CaO: 25.9%, MgO: 18.5%, SiO2: 55.6%. Raman spectra (Fig. 5e) confirmed that the jade phase was predominantly diopside15,16. In the area in Fig. 4d, EDS2 detected the presence of both tremolite and diopside by Raman spectroscopy. The dark grey areas (Fig. 4d, EDS5 and EDS6) were amorphous and predominantly contained Si and Mg elements, with elemental ratios resembling those of enstatite (Mg2Si2O6). The Raman spectra of these areas (Fig. 5e) closely matched the pyroxene Di40En6017.

Fig. 4
figure 4

Sample image of K3QW-557 (a, b ultra-depth-of-field microscopic image; c BSE image of the area of Fig. 4b; and d partially enlarged BSE image in Fig. 4c).

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Table 3 Results of EDS analyses of sample K3QW-557 (wt/%)
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Fig. 5
figure 5

Raman spectral image (a sample K7QW-Y-32 matrix—tremolite; b Sample K7QW-Y-32 inclusions—albite; c sample K7QW-Y-112 material—tremolite; d sample K7QW-Y-112 fluorescent background—hump-shaped curves; and e sample K3QW-557 material—diopside).

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Infrared spectral analyses

An appropriate amount of powder from samples K7QW-Y-32 and K3QW-557 was scraped and subjected to rigorous testing by using the IR-KBr method. The results are shown in Fig. 6. The infrared spectra indicated that sample K7QW-Y-32 was identified as tremolite18,19.

Fig. 6
figure 6

Infrared spectra of K7QW-Y-32 and K3QW-557.

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For sample K3QW-557, the bands at 1073 cm−1, 953 cm−1, 920 cm−1, 860 cm−1, 672 cm−1, and 635 cm−1 exhibited characteristics of diopside, which were associated with Si–O vibrations20. The infrared spectroscopy results are consistent with those obtained from Raman spectroscopy and can corroborate each other.

Discussion

The preservation states of jade artifacts unearthed from different positions in the K3–K8 sacrificial pits at the Sanxingdui site exhibited considerable differences. The three representative samples discussed in this paper showed distinct deterioration features. The jade artifact K7QW-Y-32, which had not been subjected to heat, remained in relatively good condition. It is still dense, with no significant deterioration observed in the jade matrix. The phase of sample K7QW-Y-112 is still identified as tremolite, but its structure is loose, with numerous internal holes. The acicular crystals exposed within these holes showed a tendency to transform into pyroxene. Additionally, attachments with soil components were present both on the surface and inside the holes of the sample. Sample K3QW-557 had almost completely decomposed into pyroxene (diopside and enstatite), dominated by diopside. The artifact was severely whitened and powdered, with a highly fragmented internal structure and significantly reduced mechanical strength. The jade artifacts unearthed from the K3–K8 sacrificial pits at the Sanxingdui site were burned before being buried5. The deterioration was influenced by the placement of the artifacts and their distance from the heat source.

The pyroxene formed by the heating of tremolite was characterized by fragmented crystals with low crystallinity, making it more prone to destruction21. Pyroxene jade was more susceptible to dissolution and leaching in the burial environment, leading to the loss of elements such as Ca, Mg, and Si. This gave rise to the fracture of crystals within the jade artifacts, leading to a loose structure with numerous cracks and holes. Soluble components in the soil, carried by water, infiltrated the jade, accelerating its further weathering. The deterioration process of tremolite jade is shown in Fig. 7.

Fig. 7
figure 7

Schematic deterioration of jade artifacts from the Sanxingdui sacrificial pits - profiles.

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In conclusion, this study reveals that the deterioration of jade artifacts unearthed from the sacrificial pits at the Sanxingdui site is related to the sacrificial activities prior to their burial. Heating exposure at high temperatures can lead to the thermal decomposition of tremolite into a pyroxene group dominated by diopside. Through comprehensive analyses of the jade artifacts’ structural composition and mineral phases, this study elucidates that tremolite jade artifacts within the sacrificial pits have transformed to varying degrees of pyroxene. Some tremolite artifacts retain their original structure, while others have locally transformed into pyroxene, and some have completely transformed into pyroxene.

Compared to tremolite jade artifacts, pyroxene jade artifacts were more prone to environmental influences, resulting in more severe deterioration. Attachments containing external soil components can be observed on the surfaces of artifacts with localized pyroxenization, as well as within its deterioration holes. Jade artifacts with complete pyroxenization show the highest degree of deterioration, characterized by severe whitening and powdering. Both jade artifacts with localized pyroxenization and those with complete pyroxenization should be reinforced by using highly permeable materials to prevent further powdering and deterioration in practice.