Characteristics and formation processes of (Ba, K, NH4)-feldspar and cymrite from a lower Cambrian black shale sequence in Anhui Province, south China

• Published in: Mineralogical magazine Vol. 82; no. 1; pp. 1 - 21
• Main Authors: Chang Chao, Chang Chao, Hu Wenxuan, Hu Wenxuan, Fu, Qi, Cao Jian, Cao Jian, Wang Xiaolin, Wang Xiaolin, Wan Ye, Wan Ye, Yao Suping, Yao Suping
• Format: Journal Article
• Language: English
• Published: London Mineralogical Society 01.02.2018; Cambridge University Press
• Subjects: alkali feldspar; alkaline earth metals; Anhui China; Asia; Barium; black shale; Cambrian; carbonate rocks; chemical composition; chemically precipitated rocks; Chert; China; clastic rocks; crystal chemistry; Crystals; cymrite; dolostone; exhalative processes; extension tectonics; Far East; feldspar group; fluid phase; framework silicates; Geological time; hydrothermal conditions; infrared spectra; Investigations; K-feldspar; Lithology; metals; mineral deposits, genesis; Mineralogy; Minerals; mudstone; Paleozoic; Quartz; Raman spectra; sedimentary rocks; Shales; sheet silicates; Silicates; spectra; Spectrum analysis; stoichiometry; Stone; tectonics; China
• ISSN: 0026-461X; 1471-8022
• DOI: 10.1180/minmag.2017.081.017

Barium-rich silicates were recently found in the lower Cambrian black shale sequence of Anhui Province, South China. The Ba silicates can be divided into Ba-poor K-feldspar, Ba-rich K-feldspar and cymrite, on the basis of occurrence and composition. The Ba-poor K-feldspar have low BaO contents (<1.00 wt.%) and occur as sporadic grains in mudstone. In contrast, the Ba-rich K-feldspar have higher BaO contents (1.36-20.51 wt.%) and occur mainly as rims around grains of Ba-poor K-feldspar in mudstone, as void fillings in dolostone, and as grains dispersed in chert. The cymrite is observed only in chert and occurs as dispersed euhedral tabular crystals. Analyses of the Ba-poor K-feldspar and Ba-rich K-feldspar provide compositions that total <100 wt.% and show non-stoichiometry characterized by (Na + K + Ba + Ca)apfu less than 1.00 and (Al + Si)apfu slightly higher than 4.00. Raman and infrared (IR) spectroscopy has revealed the presence of NH4+S and H2O in the Ba-rich K-feldspar, which accounts for the unusual composition of these feldspars. We suggest that the formation processes of the Ba-poor K-feldspar, Ba-rich K-feldspar and cymrite were closely associated with multi-stage hydrothermal fluids enriched in Ba and NH4, consistent with an extensional tectonic setting during the early Cambrian. The Ba-poor K-feldspars might have formed through interactions between pre-existing K-feldspars and hydrothermal fluids. The Ba-rich K-feldspars in mudstone and dolostone formed mainly by precipitation from hydrothermal fluids that infiltrated the sediments, and we suggest the Ba-rich K-feldspars and cymrite in the chert are of exhalative hydrothermal origin.