If an electric charge is developed on the surface of a crystal by exerting pressure at the ends of a crystal axis, the crystal is said to show piezoelectricity. Only those minerals crystallizing in symmetry classes that lack a symmetry center and thus have polar axes can show this property.
Quartz is probably the most important piezoelectric mineral, for an extremely slight pressure parallel to an "electric axis" (any of the three a axes) can be detected by the electric charge set up. Because of this property, quartz is used extensively in carefully oriented plates to control radio frequency.
Tourmaline has also been used to a limited extent in a similar manner, but it is more important in the construction of pressure gauges. Today several synthetic crystalline substances that have strong piexoelecric response are manufactured.
<em>Pyroelectricty</em>
The simultaneous development of positive and negative charges of electricity at opposite ends of a crystal axis under the proper conditions of temperature chance is called pyroelectricty. Only those crystals that belong to the ten crystal classes having a unique polar axis are considered to show "true" or primary pyroelectricity.
For example, tourmaline has a single polar axis, <em>c</em>, and falls within this group whereas quartz with its three polar <em>a</em> axes does not. However, a temperature gradient in all other crystals lacking a symmetry center, such as quartz, will produce a pyroelectric effect.
In such crystals the polarization is the result of the deformation resulting from unequal thermal expression that produces piezoelectric effects. If quartz is heated to about 100°C, it will develop on cooling positive charges at three alternate prismatic edges and negative charges at the three remaining edges. These charges have been called secondary pyroelectric polarization.
<em>Magnetism</em>
Those minerals that, in their natural state, will be attracted to an iron magnet are said to be magnetic. Magnetite, Fe3O4, and pyrrhotite Fe(1-<em>x</em>)S, are the only two common magnetic minerals. Lodestone, a variety of magnetite, itself has the attracting power and polarity of a true magnet.
In the magnetic field of a powerful electromagnet many other minerals, especially those containing iron, are drawn to the magnet. Because of this, the electromagnet is an important means of separating mixtures of mineral grains having different magnetic susceptibilities.
