The crystallite size, moganite, molecular surface water and internal water content of agates from volcanic host rocks ranging in age from 38 → 1 100 Ma have been measured. Molecular water was found to be independent of age but maturation produced a general decrease in both the moganite and internal water. Water is shown to be involved on the transformation of moganite into chalcedony and this change is responsible for an internal growth in chalcedony crystallites. The maximum content of moganite found in agate was 14% but after ∼ 410 Ma it is only present in trace amounts. Moganite has not been found in agates from any pre Silurian hosts. After the moganite transformation, the internal water in agate has been found to be constant at ∼ 0.4%. The change in composition of internal water is proposed as a method for approximate dating of agates that are from hosts younger than 410 Ma.

Changes in the degree of crystallinity of agates from 10 host rocks < 412 Ma have been determined. The growth in the crystallites from 8 regions suggests that agate genesis would be typically penecontemporaneous with the formation of the host rock. Evidence is offered for hydrothermal activity from a late second volcanic event to account for the formation of agates in the other 2 regions. It is proposed that hydrothermal solutions are the source of the silica in all cases of agate genesis in an igneous environment. With minimal moganite in hosts ≥ 412 Ma, crystallites from these older regions are shown to have achieved their maximum size. Furthermore, measurement of crystallite size, density, internal water and moganite can provide evidence of a later palaeoactivity.

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