Abstract
Iron oxides/hydroxides are important magnetic minerals to provide information about changes in the forming environment. However, the magnetic behavior in agate has been rarely investigated. In this study, the magnetic behavior of the Xuanhua-type agate with intense yellow to red colors from the Xuanhua District (China) was investigated by temperature dependence of magnetic susceptibility, hysteresis loop, isothermal remanent magnetization and the analysis of remanent coercivity components from the gradient acquisition plot. Yellow goethite and red hematite can be quantitatively identified by XRD and Raman spectroscopy due to their relatively higher content. Results showed that the red, yellow and orange Xuanhua-type agate had different magnetic behavior, and magnetite existed in the yellow and orange ones. Fluid inclusions in such agate had the homogenization temperature of ~168 °C to 264 °C. All results suggested that the dehydration of goethite to form hematite was the main reason for the high remnant coercivity (above 1000 mT) of hematite in the red agate. The co-existence of magnetite and goethite in the yellow and orange agate reflects the transformation from Fe2+ to Fe3+, indicating the change in the redox property of the environment. Unique patterns mainly formed by hematite and goethite make it a popular gem-material with high research value.
Highlights
Agate, formed by hydrothermal fluid filling the pores or cavities in igneous rocks, is mainly composed of α-quartz with minor impurities
For red Xuanhua agate samples whose color are caused by the presence of hematite, the Néel point of χ-T curve near 700 ◦ C and the characteristics of the hysteresis loop are similar to natural hematite
The magnetic behavior of iron oxides/hydroxides with a relatively higher content in yellow to red Xuanhua-type agate can provide some information about the formation condition, which along with the simple mineral composition makes such agate an excellent material for the study of a hydrothermal environment
Summary
Agate, formed by hydrothermal fluid filling the pores or cavities in igneous rocks, is mainly composed of α-quartz with minor impurities. The range from yellow to red is usually caused by iron oxides/hydroxides (especially hematite and goethite) [1,2] which show weak magnetic properties at ambient conditions. Because the main mineral component is diamagnetic quartz and a few kinds of Fe-bearing minerals, yellow to red agate can be regarded as an excellent material for studying the magnetic behavior of iron oxides/hydroxides formed in the hydrothermal environment. Due to their low content under the detection limit of XRD, iron oxides/hydroxides in most agate are usually identified by a laser Raman spectrometer which cannot provide quantitative data of their content. There is a need for more investigation to find the relationship between magnetic behavior, the variety and the content of iron oxides/hydroxides in such an agate
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