Piezoelectric effects in quartz-rich rocks

Piezoelectricity, a polarization of charge produced by an applied stress, occurs in many minerals. It is particularly strong in quartz. Aggregates of piezoelectric grains are themselves piezoelectric if the grains are suitably aligned. Such aggregates may be said to have a piezoelectric fabric. Thus quartz-rich rocks may possess a piezoelectric fabric and this paper discusses the various possible fabrics. To test whether a piezoelectric fabric might be detected in a quartz-rich rock, apparatus was built that hydraulically applied a sinusoidal stress to cubic specimens. The three resulting orthogonal polarizations of charge were measured via a charge amplifier. A specimen of pure quartz was used to verify the experimental method and to ensure that absolute piezoelectric moduli were being measured. Rocks with and without preferred orientation were tested. Of the latter types, those containing little or no free quartz (marble, basalt) did not exhibit measurable piezoelectric effects. However, all quartz-rich rocks (quartzites, granites, gneisses, mylonites) did show piezoelectric effects when stressed. These effects were in two categories (1) effects due to piezoelectric fabrics, called true piezoelectric effects

(2) effects due to random distributions of the piezoelectric vectors, called statistical effects.

To distinguish between these two effects, three criteria were used. Firstly, the measured effects were compared with the expected statistical effect for a rock of that grain size and composition. Secondly, where possible, multiple specimens were cut from the one rock sample, all specimens with the same orientation. Specimens from a rock with a piezoelectric fabric should show similar results. Thirdly, the optically observed c-axis distribution and orientation was compared with the piezoelectrically predicted fabric and orientation. This paper shows that while most rocks gave results consistent with statistical effects from a non-polar or random distribution, some rocks exhibited a true piezoelectric effect due to fabric. This effect may be used, with some imprecision, to locate the a-axes and c-axes of quartz in the aggregate. The polarities of the a-axes are also obtained.