Water in microcrystalline quartz of volcanic origin: Agates

Abstract

Agates of volcanic origin, containing the different quartz species, fibrous, length-fast chalcedony (CH), granular fine quartz (FQ), and fibrous, length-slow, to lepidospheric quartzine (QN), have been investigated to evaluate possible relations between microstructure, i.e. crystallite size and texture, refractive indices, densities, contents of trace elements and of “water”, as well as dehydration behaviour. By means of near infrared spectroscopy, total “water” contents \(c_{({\text{H}}_{\text{2}} {\text{O}}){\text{tot}}} \), could be differentiated quantitatively into contents of molecular “water”, \(c_{{\text{(H}}_{\text{2}} {\text{O)mol}}} \), and silanole-group “water”, \(c_{{\text{(H}}_{\text{2}} {\text{O)SiOH}}} \). Despite the low total “water” contents of the agates studied (\(c_{({\text{H}}_{\text{2}} {\text{O}}){\text{tot}}} \) between 1 and 2 wt.%), near infrared spectroscopy results in reliable data on \(c_{{\text{(H}}_{\text{2}} {\text{O)mol}}} \) and \(c_{{\text{(H}}_{\text{2}} {\text{O)SiOH}}} \).