The effects of grain size on the elastic properties of quartz through the
α–β
phase transition have been investigated by resonant ultrasound spectroscopy.
It is found that there are three regimes, dependent on grain size, within
which elastic properties show different evolutions with temperature. In
the large grain size regime, as represented by a quartzite sample with
∼100–300 µm
grains, microcracking is believed to occur in the vicinity of the transition point, allowing
grains to pull apart. In the intermediate grain size regime, as represented by novaculite
(1–5 µm grain size) and
Ethiebeaton agate (∼120 nm grain size), bulk and shear moduli through the transition follow closely the values expected
from averages of single crystal data. The novaculite sample, however, has a transition temperature
∼7 °C
higher than that of single crystal quartz. This is assumed to be due to the development of internal
pressure arising from anisotropic thermal expansion. In the small grain size region, agates from Mexico
(∼65 nm) and
Brazil (∼50 nm) show significant reductions in the amount of softening of the bulk modulus as the
transition point is approached from below. This is consistent with a tendency for the
transition to become more second order in character. The apparent changes towards second
order character do not match quantitative predictions for samples with homogeneous strain
across elastically clamped nanocrystals, however. Some of the elastic variations are
also due to the presence of moganite in these samples. True ‘nanobehaviour’ for
quartz in ceramic samples thus appears to be restricted to grain sizes of less than
∼50 nm.
Chalcedony and agates from a variety of world-wide hosts have been examined using cathodoluminescence (CL). Gaussian fitting of the experimental data shows that there are two dominant spectral emissions at ∼400 and ∼660 nm. A third subordinate peak is also found at ∼470, ∼560 or ∼620 nm. An age-related link is shown between the respective decreasing and increasing relative intensities of the 660 and 620 nm emissions. It is proposed that this change is due to a condensation reaction between neighbouring Si–OH groups eliminating water and forming a strained Si-O-Si bond.Agates from a variety of hosts and regions produced no clear demonstrable CL distinctions. However, a set of Western Australian agates was examined from host rocks that had been subjected to burial metamorphism. Cathodoluminescence produced different spectral emissions in the petrographic fibrous and granular regions of these agates. One agate shows a partial transformation of the petrographic fibrosity into granularity. This conversion is characterized by emission bands at 570 nm and 460 nm. Similar emission-band changes were produced by heating Brazilian agates for 35 days at 300°C. The identification of these changes in agate could serve as an indicator of palaeoheating within the parent rock.
Crystallite growth in natural agate samples has been investigated at
temperatures of 350—550°C and 100 MPa pressure in the presence of water vapour.
Initial crystallite coarsening is accompanied by the transformation of moganite to
α-quartz that is apparently inhibited by residual moganite when the crystallite
sizes reach ~50 nm. At 350—500°C 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 = 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,
= kt
with n = 6.5, only applies at 550°C. Limited growth was
obtained when small agate cubes were heated in an open furnace up to 122 d at
550°C, 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.
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