Elements of mineralogy, crystallography and blowpipe analysis from a practical standpoint .. . TWINNED CRYSTALS. 71 coid is the most frequent. Fig. 269 shows a twin of albite(albite law) the twinning plane shaded. In the so-called pericline twin of albite the macro axis is thetwin axis, Fig. 270. Twins also occur with the macro pinacoidas the twinning plane. CHAPTER IX.«MEASUREMENT OF CRYSTAL ANGLES. The instruments used in measuring the angles between crystalfaces are called goniometers. Hand Goniometers. The hand-goniometer consists essentially of two pivoted armsand a graduated arc. If the
Elements of mineralogy, crystallography and blowpipe analysis from a practical standpoint .. . TWINNED CRYSTALS. 71 coid is the most frequent. Fig. 269 shows a twin of albite(albite law) the twinning plane shaded. In the so-called pericline twin of albite the macro axis is thetwin axis, Fig. 270. Twins also occur with the macro pinacoidas the twinning plane. CHAPTER IX.«MEASUREMENT OF CRYSTAL ANGLES. The instruments used in measuring the angles between crystalfaces are called goniometers. Hand Goniometers. The hand-goniometer consists essentially of two pivoted armsand a graduated arc. If the crystal faces permit close contact, an instrument with detachable arms,F°- 271- like that shown in Fig. 271, will yield measurements which are ac-curate within one degree. The arms are first lifted from thearc and slid upon the pivot untilthe pointed ends are of convenient,but not necessarily equal, length tobe applied to the crystal. Theseends are then so placed that eachis perpendicular to the edge of in-tersection of the two crystal faces, and in such close contact thatno light passes
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