Abstract
The Zhanguohong agate from Beipiao (Liaoning province, China), which occurs in the intermediate–felsic volcanic breccias of the Early Cretaceous Yixian Formation, generally shows massive and banded structures, with red, yellow, and/or white layers or zones. Little research has been done on its mineralogical and gemological characteristics or its genesis. In this study, we present petrographic and spectroscopic constraints on the mineral composition and micro-texture of the silica matrix, as well as the ferruginous inclusions within the agates, in order to deduce the origin of the Zhanguohong agate. According to the microscopic observations, sandwich-like interlayered micro-granular quartz, fibrous chalcedony, and jigsaw quartz bands are common in the banded agates. X-ray diffraction (XRD) and Raman spectroscopic analyses revealed that all of the samples were mainly composed of α-quartz and moganite, with minor hematite and goethite. The moganite content (17–54 wt%) of the silica matrix decreases by varying degrees from the outermost to the innermost part of the banded agates. The crystal defects and ferric iron in the microcrystalline silica grains probably contributed to the moganite crystallization. The red, yellow, and orange zones are rich in hematite, goethite, and their mixtures, respectively. The ore-forming fluids fluctuated between acidic and alkaline within a temperature range of 100–200 °C and at a sustained positive Eh. Combined with the field observations, these results suggest that the multiperiod precipitation of the agates probably resulted from the episodic volcanic activity during the Early Cretaceous lithospheric extension in eastern China.
Highlights
Agate, generally defined as banded chalcedony, is mainly composed of silica phases (i.e., α-quartz, moganite, opal-CT, opal-C, and opal-A) [1,2,3,4], with minor impurities, such as Fe compounds, sulfates, and carbonates [4,5,6]
The Raman spectroscopic analysis revealed that the moganite contents of the Zhanguohong with the findings that the moganite contents are significantly different for the different samples, and agates in the Beipiao district vary significantly (17–54 wt%, Figures 8–10), which is in agreement even for different zones of the same sample collected from other places [20,62,68,69]
The with the findings that the moganite contents are significantly different for the different samples, and moganite content decreases and fluctuates with different degrees in the banded agates during the even for different zones of the same sample collected from other places [20,62,68,69]
Summary
Generally defined as banded chalcedony, is mainly composed of silica phases (i.e., α-quartz, moganite, opal-CT, opal-C, and opal-A) [1,2,3,4], with minor impurities, such as Fe compounds, sulfates, and carbonates [4,5,6]. The different colors and patterns of agates make each specimen unique from the others. Agates have been found in igneous, metamorphic, and sedimentary rocks all over the world [7,8,9,10], their genesis is still not completely understood. Agates mainly exhibit microcrystalline fibrous, granular, and jigsaw textures with different contents of moganite [1,3,12] which belongs to the monoclinic group. The structure of moganite has been described as the alternate stacking of layers of left- and right-handed α-quartz corresponding to a Minerals 2020, 10, 401; doi:10.3390/min10050401 www.mdpi.com/journal/minerals
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
