The formation of agate structures: models for silica transport, agate layer accretion, and for flow patterns and flow regimes in infiltration channels

Cathodoluminescence (CL) features of the Anatolian agates, hydrothermally deposited in different volcanic hosts from Turkey

“…It is interpreted that these spectral changes after higher energy charges, such as 24 keV (Fig. 6A and B), are due to a maturation reaction [32,13,24] in the microcrystalline quartz structure between neighboring silanol (Si-OH) groups, eliminating water and developing a strained Si-O-Si bond [6,36,9,11]. Therefore, this can be interpreted with some maturation reactions depending on the eliminating silanol water after higher energy charges.…”

“…The high defect density in agates points to rapid growth of silica from a strongly supersaturated solution probably with a non-crystalline precursor [17,19,41]. Moxon and Reed [26] stated that the identification of CL changes in agate could serve as an indicator of palaeo-heating within the host rock.…”

“…Generally, the abundance of silicon and oxygen vacancies in the microcrystalline agates is remarkably higher than in the crystalline quartzes [6,12,18]. The high defect density in agates points to rapid growth of silica from a strongly supersaturated solution probably with a non-crystalline precursor [17,19,41].…”

“…Agates occur in acidic and basic volcanic rocks, and they either form during hydrothermal processes (vein agates) or in sedimentary/diagenetic environments. Each of these types has its own characteristics [12,6,24]. In both cases, because of rapid accumulation and solidification of the silicic acid-rich solutions in different kinds of spaces of host rocks [19,30,27,38], the agates have a cryptocrystalline structure which is formed from microscopic particles, mainly fibrous and partially granular silica particles which are main chalcedonic silica components ( [22,17,18]).…”

“…In both cases, because of rapid accumulation and solidification of the silicic acid-rich solutions in different kinds of spaces of host rocks [19,30,27,38], the agates have a cryptocrystalline structure which is formed from microscopic particles, mainly fibrous and partially granular silica particles which are main chalcedonic silica components ( [22,17,18]). Thus, the agates (SiO 2 ) are classified as microcrystalline quartz variety, which has been recognized as aggregates of submicron-sized quartz crystals [31,6,7,9,38].…”