wAbstract
@Crystallite growth in natural agate samples has been investigated
at temperature of 350-550 and 100 MPa pressure in the presence
of water vapour. Initial crystallite coarsening is accompanied
by the transformation of moganite to æ-quartz that is apparently
inhibited by residual moganite when the crystallite sizes reach
`50 nm. At 350-500 the coarsening kinetics can be described by
an empirical law developed to describe Zener pinning which incorporates
the maximum crystallite size prior to growth inhibition: (Co-Cs)/Cm
+ ln [(Cm-Co)/(Cm-Cs)] = kt. Co
= initial crystallite size, Cs = crystallite
growth after time t, Cm = the maximum size
achieved before inhibition and k is the rate constant that includes
the activation energy which was found to be 51 (}9) kJ mole-1.
A more conventional isothermal growth rate law, Csn
- Con = kt with n = 6.5, only
applies at 550. Limited growth was obtained when small agate
cubes were heated in an open furnace up to 122 d at 550, demonstrating
that water vapour was essential for continued crystallite coarsening.
The crystallite size and moganite content of agates formed under
normal earth surface conditions from hosts aged 13 Ma to 3.5 Ga
have also been determined. The high temperature crystallite growth
rate law does not describe natural agate growth quantitatively
but a qualitatively similar pattern is observed.
Keywords: agate; chalcedony; maganite; crystallite growth; XRD;
Zener pinning.x
Introduction
Agate samples
@Agate and chalcedony from igneous hosts
@Agates used for heat treatment
Experimental methods
@Hydrothermal heat treatment
@Heating in air
@X-ray diffraction (XRD)
@Characterization of water content
Results
@Crystallite size and moganite content of agate and chalcedony
samples
@Crystallite size and moganite content of heat treated Nebraska
Blue and Brazilian agate
@Thermogravimetric analysis
Discussion
@Crystallite growth kinetics in Nebraska Blue agate
@Correlation of crystallite growth with moganite content
@The role of water
@Crystallite growth in agate under normal earth surface conditions
Conclusions
Acknowledgements
References