Tracing the continental diagenetic loop of the opal-A to opal-CT transformation with X-ray diffraction

Abstract

Opaline silica forms in a variety of continental deposits on the Earth's surface. The maturation from opaline silica to quartz is traditionally traced with X-ray diffraction that suggests a gradual phase transformation and structural ordering. However, the identification and categorization of the gradual opal-A to opal-CT transformation remains challenging due to potential mineral impurities and the transient structure of opaline silica. Here we use X-ray powder diffraction to identify peak parameters that characterize the maturation from opal-A to opal-CT in continental settings. We analyzed inclusion-free opals from non-sinter continental deposits, and the products of opal mixing and heating experiments. Twenty-five samples were analyzed from Australia, Ethiopia, Slovakia, USA, Mexico, and Madagascar, and represent opal from a range of host rocks and formation conditions. XRD analysis reveals that opal peak parameters constitute a diagenetic loop and that the structural order is decoupled from the low-tridymite/cristobalite ratio at large d-values. Systematic changes of the width and position of a subsidiary peak at ca. 2.5 Å accurately reflect the low-tridymite/cristobalite ratio and degree of ordering. Our results suggest that the identification of individual stages in the opal-A to opal-CT transformation sequence requires the application of multiple peak parameters that complement or replace existing maturation indicators. This information is essential for a precise silica identification and genetic reconstruction, especially of open continental systems with internal re-distribution and external input of aqueous silica.