Water and moganite participation in agates from Bou Hamza (Morocco)

Basalt-hosted monocentric agates from Bou Hamza (Morocco) were examined to unravel a discrete relationship between moganite content, water content, and the abundance of various types of chalcedony/quartz microtextures, so that the agate-forming process is refined. Herein, moganite content is noticeably low in length-slow chalcedony (quartzine, up to 18 wt.%), as compared with the co-occurring length-fast (LF) chalcedony (up to 31 wt.%). Such a type of disparity reflects a stabilization of the crystallization front during the formation of chalcedonic bands. Recrystallization-related quartz types (i.e. feathery and mosaic/jigsaw puzzle) show low and high moganite content (0 and up to 34 wt.%, respectively) since they have likely originated from different silica precursors (i.e. opaline and chalcedonic, respectively). Meanwhile, the initial formation of mosaic quartz can be exclusively witnessed in the so-called zebraic variety of length-fast chalcedony (i.e. with twisted fibres) that should be hence regarded as a ,,metastable" variety of silica though. The range of total water content remains quite stable across chalcedonic bands (0.5-0.6 wt%) and reaches the highest values (0.7 wt%) in feathery quartz. However, there is rather a general negative relationship between the content of molecular water and silanol-group water. This can stem from the recycling of water species following the condensation reaction of silanols within the agate geodes. The agates originated from Fe-Ti-bearing solutions that co-precipitated goethite, hematite, and rutile, but also contain minor low-grade carbonaceous material.