electrification of minerals

That minerals can be electrified, thus acquiring electrical charges, has been a well known property since ancient times:

the same term 'electricity' comes from the Greek elektron, ' amber ', since the ancient Greeks had already noticed that this mineral, after being properly rubbed, possessed the singular property of attracting other light bodies.

Amber (Adrian Pingstone)

Minerals, in the form of crystals, can take electrical charges also in a different way, for example after being subjected to heating or mechanical stress;

these properties are called, respectively, pyroelectricity and piezoelectricity - the latter phenomenon important industrial applications, especially in the electronics sector - and can be found with those particular types (classes) of symmetry in which the lack of a center is always detected.

An example of pyroelectricity is given by the tourmaline , a famous gem whose name derives, according to some, from an Indian term which means 'to attract ash':

the tourmaline when heated is electrified, thus attracting ash, dust or other small and light bodies.

Tourmaline (Marco Del Torchio)

Even a simple experiment demonstrates this.

Just heat a tourmaline crystal and then spray it with a powder consisting of a mixture of sulfur and of minio in equal parts:

Minio (Rob Lavinsky)

the sulfur, of a pale yellow color, will thus be attracted above all by those areas of crystal with a positive charge, while the bright red lead by those charged with negative electricity.

Furthermore, areas with positive electric charge will correspond, on the same crystal, to other equivalent ones with negative charge;

the overall charge, therefore, will be zero.

The phenomenon of pyroelectricity also manifests itself when some specimens preserved in a collection for a long time are enslaved:

la polvere, che non si riesce mai ad allontanare completamente, viene infatti attratta dai cristalli, soprattutto in zone come gli spigoli (maggiore densità di carica), creando in tal modo curiosi effetti visivi.

To give rise to this phenomenon, small variations in ambient temperature are sufficient.

Typical examples are those of the hemimorphite , which is precisely pyroelectric, and of the ra scawtite , a hydrated calcium carbonate silicate.

Hemimorphite Scawtite (David Hospital)

Piezoelectricity, as already mentioned, consists instead in the formation of electrical charges when the crystal is subjected to mechanical stress..

The most common case is that in which the stress corresponds to a compression along a certain direction and a typical example is that of the quartz :

for this reason, compressing the crystals of this mineral electrifies it.

this phenomenon also occurs in the 'reverse' way:

if a quartz crystal is subjected to an electric field, it compresses or expands;

if the electric field is oscillating, as in many electrical circuits, then the fragment will tend to vibrate with the same frequency of vibration, the crystal will help stabilize some characteristics of the circuit.

For these reasons, quartz turns out to be precious for building devices such as, for example, ultrasound generators ;

furthermore, suitably calibrated and cut fragments of this mineral are widely used in the construction of particularly stable oscillating electronic circuits.

It's well known, in fact, the use of the ' piezoelectric quartz ' in electronics, the supply of which, during the Second World War, had even become a strategic problem:

until a few years ago the residues of large quantities of Brazilian crystals could be found on the market, which had been purchased to guarantee adequate reserves of this precious mineral.

Various studies show that the phenomenon of piezoelectricity is limited only to crystals without a center of symmetry;

however, for pyroelectricity to be detected, the presence of symmetry classes without a center is not always sufficient;

in fact, the so-called ' polar symmetry axes' must be present, along which the properties of the crystal differ in both directions;

furthermore, the direction of the axes must be unique;

in other words, there must be no other equivalent directions in the crystal for symmetry.

From the external appearance of the best crystals it is easy to detect differences in symmetry between tourmaline and quartz , referring only to the 'unique' direction, not equivalent to others for symmetry, which is that of the ternary axis.

Both substances have no center of symmetry , but while in quartz the vertical ternary axis exits the crystal in two similar (equivalent) points, in tourmaline this does not happen, because the two ends are clearly different.

Especially in the latter, therefore, the biterminated crystals are interesting, because they also show the polarity of the axes in their appearance;

for these reasons tourmaline is pyroelectric, as well as piezoelectric .

Strictly speaking, however, quartz is only piezoelectric , although it shows secondary phenomena of pyroelectricity, due to internal tensions that can develop with non-uniform heating, which then triggers the piezoelectric phenomenon, developing charges.

Unfortunately, not all natural quartz crystals are suitable for use in electronics , both due to defects often present in natural material (inclusions, malformations, etc.). ), and because what looks like a single crystal is instead a geminato , useless for these applications.

The recognition of the geminates, as well as the useful directions for cutting the crystals starting from the natural material, is a difficult operation:

also for this reason the use of synthetic quartz, 'grown' along already established directions, has largely supplanted natural sources.

For instruments of lesser precision, other crystals are widely used in circuits instead of quartz:

among these, the so-called ' salt of Rochelle ' or ' salt of Seignette ', consisting of from sodium and potassium tartrate, easily obtainable, even in enormous crystals, from aqueous solutions at room temperature.

However, the mechanical properties of Seignette salt are rather poor and, furthermore, its crystals tend to absorb moisture, thus making the instruments easily perishable.