SPECIFYING QUARTZ CRYSTAL ORIENTATION 231 



The I.R.E. Angles for a Few Standard Piezoids 



GT -90 38° 52' ±45° 



MT 6° 40' 50° 28' 79° 36' 



NT 9° 25' 40° 40' 77° 40' 



2.2 Orientation by Natural Faces 



With well faced material one can do an accurate job of orienting without 

 X-rays if he knows the faces of quartz thoroughly. 



The quartz rhombohedral faces are highly perfect and polished, the 

 major often more so than the minor. With two such faces a device like 

 that illustrated in Fig. 2.9 could be used to give an orientation accurate to a 

 minute or two. An adjustable base, symbolized here as a ball and socket, 

 is adjusted so that the eye centers the lamp filament image on the cross 

 hairs, first for one face, and then, turning the base about on the reference 

 table it is adjusted for another face. When the images all pass through 

 center as the base is turned on the table the optic axis is perpendicular to 

 the table. When any one image is centered, the electric axis is perpendicu- 

 lar to the plane of the paper. This with the already mentioned fact that 

 AT plates are cut near a minor face and BT's near a major, allows us to 

 cut the crystal accurately. 



Although the rhombohedral faces are highly perfect the prism faces never 

 are. On the prism face, true prism faces that are very short in the 2 direc- 

 tion alternate with short rhombohedral faces to give the general contour 

 a slant. These " steps" give the face a striped look. The stripes are known 

 as growth lines or striations. They are parallel to x and can.be used to 

 find -v to a degree or so. If we sight on striations on two adjacent faces 

 we can locate the optic axis to nearly the same accuracy since the optic 

 axis is perpendicular to the striations on all faces. 



There are several indications that help us find, from the prism, where 

 the major rhombohedron would be in the absence of such faces. Some 

 crystals grow in the form shown in Fig. 2.10. They are symmetrically 

 doubly terminated and a very narrow prism is found under the major 

 rhombohedron, a wide face under the minor. Hence given a portion like 

 that enclosed in the dotted line we could deduce the complete orientation. 



Some crystals grew out at right angles to a wall and because they grew 



