MOGANITE AND WATER CONTENT AS A FUNCTION OF AGE IN AGATE: AN XRD AND THERMOGRAVIMETRIC STUDY

Abstract

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.