Interfacial angle is the angle between adjacent faces of the crystal lattice. Because crystal faces have a direct relationship to the internal structure, it follows that the faces have a definite relationship to each other. This relationship is expressed ”law of constancy of interfacial angles”, which states that ”the interfacial angles between corresponding faces are constant for all crystals of a given mineral”.
Interfacial angles are measured either with a contact goniometer or a reflecting goniometer. A contact goniometer is a simple device, which consists of a pivoted metal arm, called a pointer. This slides over a semicircular scale and measures the interfacial angle between two adjacent faces of a crystal as shown in the following figure.
In a contact goniometer, two metal rules pivoted together at the centre of a graduated semicircle are used. The instrument is placed with its plane perpendicular to an edge between two faces of the crystal to be measured, and the rules are brought into contact with the faces. The angle between the rules, as read on the graduated semicircle, then gives the angle between the two faces. The rules are slotted, so that they may be shortened and their tips applied to a crystal partly embedded in its matrix. The instrument illustrated is employed for the approximate measurement of large crystals.
The reflecting goniometer is an instrument of far greater precision and is used for the accurate measurement of the angles when small crystals with smooth faces are available. Such faces reflect sharply defined images of a bright object. By turning the crystal about an axis parallel to the edge between two faces, the image reflected from a second face may be brought into the same position as that formerly occupied by the image reflected from the first face. The angle through which the crystal has been rotated, as determined by a graduated circle to which the crystal is fixed, is the angle between the normals to the two faces.