Shen Mengmeng, Lu Zhiyun, He Xuemei
School of Gemmology, China University of Geosciences, Beijing 100083, China.
School of Earth Sciences, Zhejiang University, Hangzhou 310027, P. R. China.
ACS Omega. 2022 Jun 30;7(27):23858-23864. doi: 10.1021/acsomega.2c02538. eCollection 2022 Jul 12.
The phase composition and geochemical characteristics in banded agates with different structural sequences have been investigated in detail. The results reveal that the agate bands have a combination of a pseudo-granular silica → fibrous chalcedony → crystalline quartz (type I) sequence and a newly discovered pseudo-granular silica → crystalline quartz (type II) sequence. The banded agates mainly consist of α-quartz, moganite, and a minor amount of amorphous silica, goethite, hematite, kaolinite, illite, and carbonates. With the evolution of two structural sequences, the content of α-quartz and moganite increases and decreases, respectively. There is no moganite in crystalline quartz. The increased concentration of trace elements like Li, Na, Al, K, Ca, Ti, Mn, and Fe in different bands may correspond to the decrease in the water content in the mineral-forming fluid. The increased trace elements promote the structural transformation process of silica. With the evolution of the type I sequence, the thermal gradients between adjacent bands are 17 and 51 °C, respectively. In contrast, a significantly higher thermal gradient of 53-66 °C is exhibited when pseudo-granular silica transforms directly to crystalline quartz. It is inferred that a slightly increased thermal gradient between adjacent bands promotes the structural transformation process of the type I sequence. The sharply increasing thermal gradient between adjacent bands leads to the formation of the type II sequence from pseudo-granular silica to crystalline quartz. The formation process of different structural sequences in agate may be controlled together by trace element concentrations and thermal gradients.
对具有不同结构序列的条带状玛瑙的相组成和地球化学特征进行了详细研究。结果表明，玛瑙带具有假粒状二氧化硅→纤维状玉髓→结晶石英（I型）序列和新发现的假粒状二氧化硅→结晶石英（II型）序列的组合。条带状玛瑙主要由α-石英、莫来石以及少量的无定形二氧化硅、针铁矿、赤铁矿、高岭石、伊利石和碳酸盐组成。随着两种结构序列的演化，α-石英和莫来石的含量分别增加和减少。结晶石英中不存在莫来石。不同条带中锂、钠、铝、钾、钙、钛、锰和铁等微量元素浓度的增加可能与成矿流体中水分含量的降低相对应。微量元素的增加促进了二氧化硅的结构转变过程。随着I型序列的演化，相邻条带之间的热梯度分别为17和51℃。相比之下，当假粒状二氧化硅直接转变为结晶石英时，会表现出明显更高的53-66℃的热梯度。据推断，相邻条带之间略微增加的热梯度促进了I型序列的结构转变过程。相邻条带之间急剧增加的热梯度导致了从假粒状二氧化硅到结晶石英的II型序列的形成。玛瑙中不同结构序列的形成过程可能受微量元素浓度和热梯度的共同控制。
