Atomic and microstructural origin of banded colours in purple-blue variety of agate from Yozgat Province, Turkey

IF 1.2 4区 地球科学 Q4 MATERIALS SCIENCE, MULTIDISCIPLINARY
Roberto Lorenzi, Andrea Zullino, Valentina Gagliardi, Loredana Prosperi, Alberto Paleari, Ilaria Adamo
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Abstract

Agates from Yozgat province are appreciated on the gem market for their white and purple-blue banded colours. In this study, we present a detailed investigation aimed at the identification of the atomic and structural origin of this peculiar colouration of chalcedony. X-ray diffraction and Raman spectroscopy revealed the presence of fine grains of quartz and moganite with a preferential accumulation of the latter in the blue bands. Near-infrared diffuse absorption spectra show overtones of hydroxyls vibrations at 1425, 1900, and 2250 nm. In the visible, the broad absorption at about 500 nm, as well as its behaviour at low temperatures, is compatible with the optical activity of iron impurities in quartz matrices, such as that observed in amethysts. Peak intensities and shapes are very similar for spectra collected in blue and white bands. Accordingly, trace-element composition from laser ablation inductively coupled mass spectrometry confirmed that the two regions have similar Fe content. The perceived changes in band colours are indeed originated by differences in microstructural arrangement and size of the grains visualised through scanning electron microscopy. White and blue stripes have grains of about 5 µm and 300 nm in size, respectively, resulting in an accentuated scattering component for the white bands. Therefore, the unique purple-blue shades typical of Yozgat agates are a combination of iron-related colour centres and scattering effect.

Abstract Image

土耳其尤兹加特省紫蓝色玛瑙带状颜色的原子和微观结构来源
来自尤兹加特省的玛瑙因其白色和紫蓝色条纹颜色而在宝石市场上受到赞赏。在这项研究中,我们提出了一项详细的调查,旨在鉴定这种特殊颜色的玉髓的原子和结构起源。x射线衍射和拉曼光谱显示石英和莫干石的细颗粒存在,后者在蓝带中优先堆积。近红外漫射吸收光谱在1425,1900和2250nm处显示羟基振动的泛音。在可见光下,约500 nm处的广泛吸收,以及它在低温下的行为,与石英基体中铁杂质的光学活性相容,例如在紫水晶中观察到的。在蓝色和白色波段收集的光谱的峰强度和形状非常相似。因此,激光烧蚀电感耦合质谱的微量元素组成证实了两个区域具有相似的铁含量。所感知到的波段颜色变化确实是由扫描电子显微镜观察到的晶粒的微观结构排列和大小的差异引起的。白色条纹和蓝色条纹的晶粒尺寸分别约为5µm和300 nm,导致白色条纹的散射分量增强。因此,尤兹伽特玛瑙特有的紫蓝色色调是铁相关色心和散射效果的结合。
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来源期刊
Physics and Chemistry of Minerals
Physics and Chemistry of Minerals 地学-材料科学:综合
CiteScore
2.90
自引率
14.30%
发文量
43
审稿时长
3 months
期刊介绍: Physics and Chemistry of Minerals is an international journal devoted to publishing articles and short communications of physical or chemical studies on minerals or solids related to minerals. The aim of the journal is to support competent interdisciplinary work in mineralogy and physics or chemistry. Particular emphasis is placed on applications of modern techniques or new theories and models to interpret atomic structures and physical or chemical properties of minerals. Some subjects of interest are: -Relationships between atomic structure and crystalline state (structures of various states, crystal energies, crystal growth, thermodynamic studies, phase transformations, solid solution, exsolution phenomena, etc.) -General solid state spectroscopy (ultraviolet, visible, infrared, Raman, ESCA, luminescence, X-ray, electron paramagnetic resonance, nuclear magnetic resonance, gamma ray resonance, etc.) -Experimental and theoretical analysis of chemical bonding in minerals (application of crystal field, molecular orbital, band theories, etc.) -Physical properties (magnetic, mechanical, electric, optical, thermodynamic, etc.) -Relations between thermal expansion, compressibility, elastic constants, and fundamental properties of atomic structure, particularly as applied to geophysical problems -Electron microscopy in support of physical and chemical studies -Computational methods in the study of the structure and properties of minerals -Mineral surfaces (experimental methods, structure and properties)
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