Oblique extinction

Fig. 3 Schematic representations of extinction character (a) isotropic, (b) parallel, (c) symmetrical, and (d) oblique extinction.

Some crystals will show parallel or symmetrical extinction, and others oblique extinction. 

R (calcd) =19.7 (imidine value for N s). R (obsd) = 15.6 Fusion Data Tetrazole melts without decompn at 155.5° (equilibrium micro melting point) and crystallizes without supercooling to form flat rods with oblique extinction (ca 15°) and showing centered BXafigures. Negative, v>r, 2E = 90°... 

Chromium Pyroarsenate, Cr4(As20-)3, is produced when a small quantity of chromium sesquioxide is fused at a low temperature with sodium or potassium meta-arsenate. It jdelds green transparent prisms, sometimes elongated, sometimes flattened, wnth oblique extinctions insoluble in dilute acids. 

Figure 2. Circular domains with curved column axes showing oblique extinction brushes in pure benzene hexa-n-hexanoate (a b) and in its mixture with a few per cent by weight of benzene (c). The mesophase of the mixture is slightly more mobile than that of the pure material, but has the same structure. Polarizer- analyser directions are vertical and horizontal ((a) Courtesy of K. A. Suresh (b and c), from Frank and Chandra-.sekhar [11], reproduced by permission of the Commission des Publications Franfaises de Physique).

For biaxial crystals, similar results are obtained as with uniaxial crystals. The exception to this rule is that in monoclinic and triclinic systems, the polarization directions need not be parallel to faces or to the bisectors of face angles. If the prominent faces or edges of an extinguished crystal are not parallel to the axes of the initial polarizer, the extinction is said to be oblique (Fig. 3d). 

Extinction calculations for obliquely incident light, also taken from Asano (1979), are shown in Fig. 11.16. The symmetry axis is parallel to the z axis and the direction of the incident beam, which makes an angle with the symmetry axis, lies in the xz plane, the plane of incidence. The incident light is polarized cither with its electric field or its magnetic field perpendicular to the plane of incidence these two polarization states are denoted by TE (transverse electric) and TM (transverse magnetic). 

Figure 11.16 Calculated extinction of obliquely incident light by spheroids. From Asano (1979).

Figure 11.17 Calculated extinction by infinite cylinders for obliquely incident light = 90° corresponds to normally incident light. TE and TM denote light with the electric and magnetic vectors, respectively, perpendicular to the xz plane. From Lind and Greenberg (1966).

There is an imperfect cleavage parallel to a(100), through which an optic axis emerges very obliquely. Another optic axis is -visible through c(001), so that the axial plane is A 35° extinction on m (with the vertical edge) is proof that the system is not orthorhombic. 

Birefringent crystals will turn dark every 90° as the stage is rotated. The angle at which they turn dark is called the extinction angle and may be paralled (Fig. 15), oblique (Fig. 16), or symmetrical (Fig. 17) and is typical of the substance. 

The extinction position is oblique. This may be hard to see in many rock specimens, but in selinite it is apparent (Fig. 22). 

Since transition through an optically isotropic phase, cubic, is excluded by x-ray data (12), the 57°C. extinction of rotation indicates the formation of a phase with disorder normally associated with a melt. This indicates that the chain separation and dislocation during the transition from the perpendicularly oriented q l hexagonal structure to the obliquely oriented 13l triclinic structure, Figure 2, are larger than has previously been described or implied. 

Extinction straight or oblique if oblique angle Interference colours and order With transmitted cros.sed-polarized light and Bertrand lens ... 

Extinction, oblique n. Extinction is oblique if the vibration directions are oblique to the long direction of the crystal. 

Here, a = )k, where k is the dimensionless extinction coefficient, which is the imaginary part ofthe complex refi active index, n = n — ik (n being the real part ofthe refractive index). IR absorption spectroscopy can also be performed with IR radiation at oblique... 

---