Acute bisectrix

Fig. 52. Convergent light figures, a. Uniaxial crystal with optic axis parallel (left) and slightly inclined (right) to line of vision. 6. Biaxial crystal with acute bisectrix parallel (left) and inclined (right) to fine of vision.

Biaxial crystals under similar optical conditions produce convergent light figures like that shown in Fig. 52 6, when the acute bisectrix of the optic axes lies along the line of vision and the vibration directions... 

The observation of convergent light figures may often provide confirmation of the orientation of the indicatrix. The plane of the optic axes is the ay plane, while the normal to this plane is the j8 vibration direction. For a positive crystal the acute bisectrix is the y vibration direction, while for a negative crystal it is the a vibration direction. 

The optic axial plane is (010) and the negative acute bisectrix is inclined at about 10° to the vertical in the acute angle j8. An optic axis is visible through c(001). The dispersion is abnormally strong, p > v. 

The Fedorov complex symbol is 4a(45° 36 ) 0. Optically the crystals are biaxial the acute bisectrix is negative. 

Dispersion v>r very slight Sign of Double Refraction Negative Acute Bisectrix Almost 001 Extinction yAa = 13° on 001 Molecular Refraction (R) (5893 A 25°C). 

The assignment to the monoclinic system was based not so much on the uncertain measures on the inclined facets of the laminae but on the accurate corrosion-figures (Baumhauer, 1874, 1875) measured on both light and dark micas and, above all, on the final ascertainment by Hintze in 1875 of minimal deviation of the perpendicularity of the acute bisectrix with respect to the lamina, that had been suspected previously (Rose 1844, Marignac 1847, Kenngott 1848, Miller 1852). After the studies of Baumhauer and of Hintze, the last word was written by Bauer in 1877 and by Tschermak in 1875-1877 who performed a series of measures on numerous specimens from various locations. 

Acute bisectrix (BXg) The bisector of the acute optic axial angle for biaxial crystals. Rhodes G (1999) Crystallography made crystal clear a guide for users of macromo-lecular models. Elsevier Science and Technology Books, New York. 

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