Quantitative Study on Colour and Spectral Characteristics of Beihong Agate

Zhou, Yufei; Liu, Ziyuan; Zhao, Zitong; Guo, Ying

doi:10.3390/min12060677

Cited by 4 publications

(1 citation statement)

References 54 publications

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“…Therefore, DRS is widely applied to quantify the content of hematite and goethite in soil research [13,[17][18][19][20]. In addition, this method has been used in researching gemstones such as Beihong agates [21]. Based on the DRS, more data analysis methods have been proposed to improve the quantification of goethite and hematite.…”

Section: Introductionmentioning

confidence: 99%

The Color Formation of “Lumu Stone” in the Weathering Processes: The Role of Secondary Hematite and Goethite

Zheng

Chen

Han³

et al. 2023

Minerals

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Rocks and minerals buried in the earth’s surface usually undergo weathering processes and change color in the burying environment. A kind of yellow Chinese stamp stone named “Lumu stone”, which is buried in a yellowish weathering crust (yellowish soil), was selected to investigate its color changes in the weathering processes. In this study, the appearance features, mineral components, micromorphology, spectroscopy characteristics, and color causation of the “Lumu stone” were studied by using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), an electron probe microanalyzer (EPMA), a laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS), and a UV-Visible (UV-Vis) spectrum. The “Lumu stone” usually exhibits a darker yellow outer layer and a lighter yellow core, suggesting that yellow color permeated into the stone from the surface to the core gradually and the color is secondary forming. The results from XRD and SEM show the studied samples are mainly composed of dickite and illite. The individual particles of the dickite and illite are about 2–5 μm, randomly distributing in the three-dimensional space and constituting voids among the particles. The acid pickling experiments using HCl coupled with KSCN confirmed that the mineral phases that caused the yellow color of the matrix are iron oxide and hydroxide. On the other hand, goethite and hematite were observed gathering in the yellow and brown-red cracks on the “Lumu stone” by SEM study. However, iron oxide and hydroxide in the matrix were difficult to observe and detect among the dickite and illite aggregates by SEM and XRD methods. It indicates that they may be nanoscale in size and very low in content. According to the calculation of the second derivative of Kubelka-Munk (K-M) transformed diffuse reflection spectroscopy (DRS) curves obtained from UV-Vis, the characteristic peaks of goethite and hematite were found in the yellow matrix, and their contributions to the color were confirmed. The concentrations of goethite and hematite were calculated to be 0.32 to 1.87 g/kg and 0.22 to 0.93 g/kg in the studied samples, respectively. In this study, a series of methods were employed to detect very low levels of goethite and hematite in the samples undergoing weathering processes. Additionally, nanoscale goethite and hematite were considered newly formed minerals when buried in the weathering processes and may gradually move into the voids among phyllosilicate particles. Therefore, they turned the “Lumu stone” yellow.

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Section: Introductionmentioning

confidence: 99%

The Color Formation of “Lumu Stone” in the Weathering Processes: The Role of Secondary Hematite and Goethite

Zheng

Chen

Han³

et al. 2023

Minerals

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Origin of Color and Spectral Characteristics of Nanhong Agate from Northeast Yunnan, China

Song,

Chen,

Zuo

et al. 2024

Crystals

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Nanhong agate has attracted public attention as the most distinctive gemstone in China. Owing to the limited data on Nanhong agates from a new production area in northeastern Yunnan, this paper presents the first data from gemological studies on these agates. Complex investigations using Fourier-transform infrared (FTIR) spectrometry, ultraviolet–visible (UV–VIS) luminescence spectrometry, Raman spectroscopy, polarizing microscopy, scanning electron microscopy (SEM), electron microprobe (EPMA), and laser ablation–inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses were carried out to obtain basic data concerning the gemological properties, microstructure, and spectroscopic and geochemical characteristics of this interesting material. The results illustrate that Nanhong agates from northeastern Yunnan are mainly composed of α-quartz and also contain certain amounts of moganite, illite, goethite, and hematite. The typical red (yellow) and white appearance can be attributed to the microstructure of the agates, the contents of Fe and Al impurities, and trace elements. The Raman spectra, microstructure, and chemical composition of the unique “yellow-skinned” agates from northeastern Yunnan are significantly different from those of other types of Nanhong agate. This work lays a foundation for the future identification and research of this type of Nanhong agate.

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The Correlation of Colour and Iron Oxides in Yellow Seal Stones from Northern Laos

et al. 2023

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The yellow seal stone from northern Laos is one possible substitute for the Tianhuang Stone, the most famous Chinese seal stone, because of its similar yellow to orange-yellow appearance and the same main mineral composition. The colour causation of the yellow seal stone from northern Laos was studied. The samples’ phase, micro-morphology and chemical components were studied by Raman spectroscopy, and scanning electron microscopy (SEM) with energy disperse spectroscopy (EDS), respectively. The yellow seal stone from northern Laos is mainly composed of dickite, occasionally with minor impurity minerals, such as hematite, anatase, barite, diaspore and pyrite. Micro- to nano-sized iron oxides/hydroxides were observed and detected by SEM and EDS in the yellow to orange-yellow part of the samples. Moreover, these iron oxides/hydroxides were suggested to cause the yellow to orange-yellow in the seal stone from northern Laos. The UV-Vis spectrum and its second derivative, the Kubelka-Munk spectra, were used to identify and quantify hematite and goethite. The samples’ colour parameters were obtained with the Commission Internationale de l’Eclairage (CIE) 1931 standard space. According to the observation of the samples and the results obtained from experiments and calculations, the colour of the yellow parts (L* = 33.56~47.99, a* = 0.35~3.65, b* = 4.55~9.89) correlated with goethite (goethite is about 0.175~0.671 g/kg, the content of hematite was too low to be figured out in the yellow parts). In contrast, the colour of the orange-yellow parts (L* = 33.99~46.27, a* = 3.98~12.39, b* = 8.04~22.14) was more closely related with the content of hematite (goethite is about 0.096~0.691 g/kg, hematite is about 0.258~2.383 g/kg). The results of correlation analysis also support that the contents of iron oxides or hydroxides influence the samples’ colour. Therefore, it is suggested that micro- to nano-scaled hematite and goethite caused the colour of yellow and orange-yellow in the studied seal stone. Hematite can strengthen the red hue and change the colour from yellow to orange-yellow.

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Quantitative Study on Colour and Spectral Characteristics of Beihong Agate

Cited by 4 publications

References 54 publications

The Color Formation of “Lumu Stone” in the Weathering Processes: The Role of Secondary Hematite and Goethite

The Color Formation of “Lumu Stone” in the Weathering Processes: The Role of Secondary Hematite and Goethite

Origin of Color and Spectral Characteristics of Nanhong Agate from Northeast Yunnan, China

The Correlation of Colour and Iron Oxides in Yellow Seal Stones from Northern Laos

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