Synchrotron Radiation μ-XRF Imaging Reveals Oscillatory Zoning of Freshwater Pearls
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摘要: 珍珠和贝壳是常见的生物矿物，也是广受人们喜爱的装饰品，其微量元素的组成和分布不仅与母贝的代谢活动密切相关，也在一定程度上反映母贝的生长环境。然而，有关珍珠和贝壳中元素分布的研究存在大量空白，这阻碍了我们对其矿化机理和生长过程的理解。本文采用同步辐射X射线荧光成像技术，对若干颗淡水珍珠和海水珍珠的元素时空分布进行了系统地研究。研究首次揭示淡水珍珠存在环带结构，如Mn、Fe和Ba韵律环带，这些环带与珍珠的生长轨迹耦合。测试的所有淡水珍珠(无核珍珠、Akoya珍珠和“爱迪生”珍珠)都有Mn环带，不同珍珠的Mn环带在数量、宽度、位置及Mn浓度变化等方面有所差异。部分珍珠表现Mn环带与Fe环带协同变化的现象。分析认为：淡水珍珠的韵律环带是一种耗散结构，其形成主要受控于母贝的新陈代谢速率的变化，而非环境因素的周期性变化。研究并未在海水有核珍珠中观察到类似的Mn环带，因而Mn环带有望成为区分淡水珍珠与海水珍珠的关键特点之一。Abstract: Pearls and shells are common biominerals characterized with a highly organized laminar structure that is constructed by inorganic aragonite platelets alternating with organic material films. Formed in the bodies of mollusks, the uptake of trace elements into pearls and shells is under both biological control and environmental control. Thus, the spatial and temporal resolution of the trace element distributions could reflect the life activities of the mother mollusks and the chemistry of the environments. However, elemental maps in pearls and shells have not received due attention from previous studies. In this study, the elemental profile in cultivated pearls is carefully investigated using synchrotron radiation μ-X-ray fluorescence imaging. A variety of trace elements such as Ti, V, Mn, Fe, Ni, Cu, Sr and Ba has been detected. Most elements are distributed evenly in these pearls, with exceptions of Mn, Fe and Ba that show oscillatory zoning in freshwater pearls, coupled with the pearl growth pattern. Mn zoning is ubiquitous in all the freshwater pearls, with different numbers, width, positions, and Mn concentrations, etc. in individual. In some pearls, the Mn zoning are cooperative with the Fe zoning. It is suggested that the oscillatory zoning in freshwater pearls is a dissipative structure, and its formation results from metabolic rate changes of the mother mollusks after pearl-mantle-inserting operation. The presence of the Mn oscillatory zoning is expected to be a key character of freshwater pearls, as no such zoning is found in seawater pearls. Seawater pearls are featured with extremely low Mn concentrations. It is thus, speculated that marine Pinctada martensii have different requirements for trace elements in the course of biomineralization in comparison with freshwater Hyriopsis cumingii, and could maintain relatively stable metabolism during pearl growth. In view that pearls and shells show highly similar crystal chemistry, it is reasonable to believe that the elemental profile in shells may also be affected by the mother mollusks' activities. So the geochemical signature of shells should be treated with caution to reconstruct environments.
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Keywords:
- biomineral /
- pearl /
- oscillatory zoning /
- dissipative structure
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图 1 危地马拉绿色翡翠(“玛雅绿”)代表性样品Figure 1. Representative Feicui samples from Guatemala ("Maya green")![]()
图 2 代表性翡翠样品的红外反射光谱Figure 2. Infrared reflection spectra of representative green Feicui samples致谢: 本研究中样品的微区X射线荧光测试在中国科学院高能物理研究所同步辐射装置4W1B线站完成，得到陈栋梁研究员的支持和帮助，李纯老师在数据处理方面提供悉心的指导，我们在此一并表示深深地谢意。 -
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图 2 淡水无核珍珠样品A21283横截面的显微照片(a)以及Mn(b)，Fe(c) 和Ba(d)的浓度分布呈环带图像
注：黑色矩形框显示μ-X射线荧光成像区域，白色虚线指示植入的外套膜小片
Figure 2. Micrograph of the cross section (a) and oscillatory zoning of Mn (b), Fe (c) and Ba (d) of freshwater non-nucleated pearl sample A21283
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图 3 淡水无核珍珠A24001横截面的显微照片(a)以及Mn的浓度分布呈环带图像(b)
注：黑色矩形框显示μ-X射线荧光成像区域
Figure 3. Micrograph of the cross section (a) and oscillatory zoning of Mn (b) of freshwater non-nucleated pearl sample A24001
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图 4 Akoya珍珠样品A21282横截面的显微照片(a) 以及Mn在珍珠层边缘呈条带状富集，与珍珠层的生长轨迹耦合(b)
注：黑色矩形框显示μ-X射线荧光成像区域
Figure 4. Micrograph of the cross section (a) and Mn concentrates at the margin of the nacre, coupled with the growth pattern of the pearl (b) of Akoya pearl sample A21282
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图 5 Akoya珍珠样品A21614横截面的显微照片(a)以及Mn(b)和Fe(c)的浓度呈环带分布，二者协同变化
注：黑色矩形框显示μ-X射线荧光成像区域
Figure 5. Micrograph of the cross section (a) and oscillatory zoning of Mn (b), coupled with that of Fe (c) of Akoya pearl sample A21614 that changes synergistically
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图 6 “爱迪生”珍珠样品A21284E横截面的显微照片(a)以及Mn(b)和Fe(c)的浓度呈环带分布，Mn环带与Fe环带协同变化
注：黑色矩形框显示μ-X射线荧光成像区域
Figure 6. Micrograph of the cross section (a) and oscillatory zoning of Mn (b), coupled with that of Fe (c) of Edison pearl sample A21284E that changes synergistically
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