Aluminium in quartz as an indicator of the temperature of formation of agate

An 'Al-thermometer' was applied to agate samples of volcanic parent rocks t o provide information about the temperature of agate formation. The tempera tures were calculated from concentrations of [AlO4](0) centres determined b y electron paramagnetic resonance (EPR) measurements. The calculations for agate bands in some cases yield temperatures of up to 655 degreesC, which a re assumed to be invalid; they are thought to be artifacts resulting from n on-equilibrium crystallization processes. In contrast, the formation temper atures (60-198 degreesC) estimated for associated quartz incrustations with in the agate geodes are in good accord with those calculated from oxygen is otope data. Direct estimation of the formation temperatures of agates is pr oblematic. The best results will be obtained by analyses of associated phan erocrystalline quartz incrustations.