Use of the Etch Technique for Determining Orientation and Twinning in Quartz Crystals

11 12 BELL SYSTEM TECHNICAL JOURNAL of quartz are: (1) by optical effects (birefringence and rotatory power), (2) by X-ray reflections from atomic planes, and (3) by the use of etch pits which are developed when the quartz surface is etched in fluorine compounds. Other methods are or may be used in rather special cases. For example, in finished plates of known orientation types, the electrical axis direction is distinguished from other directions by electrical polarity tests (on tension or compression), or a plate known to be one of several types may be tested in an electric circuit for activity, frequency and temperature-coefficient, to determine which type it is. The selective fracture characteristics of quartz offer another method of determining orientation. Microscopic fractures resulting from grinding a quartz surface may be used for determining orientation. Thus unetched, ground, Z-cut surfaces of quartz give a hexagonal figure, when examined by pinhole illumination, which may be used to determine the approximate orientation (but not sense) of the electric axes.2 By optical methods (see Chapter II) it is possible to determine the orientation of a quartz body relative to only one direction of the structure, the optic or Z axis. Thus optical methods are limited to determining the angle between the optic axis and a line or surface of the body (but not the rotation of that line or surface about the optic axis). Twinning of the "optical" variety may be detected optically, even when located internally, but the determination of its location in depth is approximate.