Grain size dependence of elastic anomalies accompanying theα–β phase transition in polycrystalline quartz

Abstract

The effects of grain size on the elastic properties of quartz through theα–β phase transition have been investigated by resonant ultrasound spectroscopy.It is found that there are three regimes, dependent on grain size, withinwhich elastic properties show different evolutions with temperature. Inthe 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, allowinggrains to pull apart. In the intermediate grain size regime, as represented by novaculite(1–5 µm grain size) andEthiebeaton agate (∼120 nm grain size), bulk and shear moduli through the transition follow closely the values expectedfrom 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 internalpressure arising from anisotropic thermal expansion. In the small grain size region, agates from Mexico(∼65 nm) andBrazil (∼50 nm) show significant reductions in the amount of softening of the bulk modulus as thetransition point is approached from below. This is consistent with a tendency for thetransition to become more second order in character. The apparent changes towards secondorder character do not match quantitative predictions for samples with homogeneous strainacross elastically clamped nanocrystals, however. Some of the elastic variations arealso due to the presence of moganite in these samples. True ‘nanobehaviour’ forquartz in ceramic samples thus appears to be restricted to grain sizes of less than∼50 nm.