Formation conditions of agates
Agates are formed at various temperatures, which covers a wide range from those of basaltic and andesitic melts (above 1100 °С) to several tens of centigrade, but at rather low pressures. Their formation in liquid mafc magmas is indicated by correlation between orientation of amygdules and inclination of onyx layers in them and the lava fow layers, which fowed under diferent rate. The alkaline supercritical fuid forms gas vesicles in lavas and dissolves ambient silica producing a silica sol. This sol is coagulated on the vesicle walls forming concentric layers, if the fuid pressure provides exfltration from amygdules. If the pressure in amygdules is low, the coagulation leads to the formation of horizontal onyx layers. Multiple repetitions of various gel layers can be caused by overlapping of fresh lava layers on the cooling agate-bearing lava fow. In submarine conditions, phase separation of the fuid and formation of a gel flm between two phases stimulate osmotic processes, which result in growth of empty membrane tubes and branchy moss-like assemblages at the bottom of amygdules. Some agates often exhibit numerous channels, which were formed as a result of repeated squeezing of gel from inner zones to periphery of amygdules. Previously, such channels were considered the infux channels for silica supply. Periodic replacement of fuid exfltration from amygdules by compression requires no additional Si supply, because the volume of amygdules decreases proportionally to the fuid loss. The concentric and horizontal layering and moss textures of agates from lithophisae of felsic volcanic rocks were formed during active volcanism as well. The karst agates of carbonate rocks were formed under moderate temperatures of low-grade burial metamorphism, which is supported by lack of moss textures and onyx layering.
Figures 17. References 54.
Key words: amygdules, agate texture, volcanism, colloids, silica minerals, osmotic pressure, heterogenization of fuid, membrane (myeline) textures, druses, lithophisae.
МINERALOGY № 2 2017