SEDIMENTATION IN VESICLES: INTERPRETATION OF GEOPETAL FABRICS IN AMYGDALOIDAL AGATES

Polished agates derived from lava vesicles are a common commodity at rock shows. Vesicles may be infilled by silica in several habits; opal, chalcedony and quartz. In this paper we investigate these various fabrics under light and electron microscope using etched thin sections. It is inferred that silica diffuses into the vesicles from the surrounding lava via capillary water. Microcrystalline chalcedony (length fast) occurs as thinly laminated isopachous, radiaxial fringes throughout vesicles and indicates multiple seeding in capillary water fringes within the vadose zone. Drusy quartz (length slow) is similarly radiaxial but occurs as late fillings on cavity walls. Crystals are larger, unlaminated, with fewer nucleation sites and appear to have grown in less concentrated solutions probably below the water table. Most interesting fabrics are deposits of geopetal agate occupying the bottoms of cavities. These are derived from both colloidal precipitates (opal) and fibrous crystals (chalcedony) in vesicles only partly filled by water. Colloidal layers show sedimentary structures, fibrous crystals are cement. Thin sections reveal that horizontal laminae are mantled by botryoidal chalcedony, sometimes growing both up and down from the surface. Small scale druses commonly occur between laminae. The botryoidal chalcedony is continuous with radiaxial fringes on vesicle walls but seeded intermittently. The fabric suggests that crystals were growing at the interface between liquid and gas either supported by surface tension or possibly seeded on rafts of bubbles. This interpretation is reinforced by the occurrence of unusual palisade structures associated with fluctuating water levels. It is inferred that chalcedony is original cement that precipitated as a thin meniscus at the interface between liquid and gas on cavity walls and in geopetals. The permeability of this cement allowed water to penetrate the vesicle but diminished over time, especially after the precipitation of drusy quartz.