III.3 Elongation
When a section through a mineral is clearly elongated, one can speak of the elongation of that section. The elongation is called positive when the longest axis of the elliptical section makes the smallest angle with the longitudinal direction of the crystal section, in the other situations the elongation is called negative. This is only useful when the longitudinal shape of the crystal is not caused by coincidence, and when the length or width represent a clearly recognisable crystallographic direction. The determination of these two axes of the elliptical section is accomplished by observing the position of extinction in crossed polarised light. A gypsum plate is then used to tell which of the two directions is fast and which is slow. If the slow ray of the gypsum plate coincides with the slow ray of the mineral an increase in retardation occurs and the order of the interference colour increases. The mineral is said to be length slow. If the slow ray of the gypsum plate coincides with the fast ray of the mineral a decrease in interference colour occurs. The minerals is now said to be length fast.
Fig 3.6
For uniaxial crystals with a platy habit parallel to the basis the elongation is in longitudinal sections opposite to the optic sign.
Fig. 3.7
Similarly, longitudinal sections of biaxial platy minerals always have a negative elongation when γ, and a positive elongation when α, coincides with the smallest angle with the normal to the plate. In some biaxial minerals β will almost coincide with the prisma-axis or the normal to the plate. In such cases the prismatic sections can show both negative and positive elongation.
Fig. 3.8
III.4 Dispersion of extinction
This effect is only observed in biaxial crystals, and only when the orientation of one of the optical axes or indicatrix axes is changes with changes in wavelength. When a crystal is slowly rotated through extinction the crystals show the following abnormal colours: abnormal blue, dark, abnormal brown. The interference colour is normally not effected. Expecially in sections roughly perpendicular to one of the optical axes is the effect visible. Examples can be found in Ti-augite and labradorite.
Twins that are easily recognisable through their crystal shape with inward angles are rather rare. Known are cross and X-twins of staurolite. The arrowhead or knee forms of rutile and the swallow tail of gypsum and titanite. More often the twins are only visible with the polarising microscope, because the individual crystals in the twin have different extinction angles or different interference colours. Common are the (100) twins in amphiboles and pyroxenes, which can be single or lamellar in nature. (100) is here the common plane between the two crystals, or the symmetry plane of the twin (see Fig 3.9 for a few examples).
Fig. 3.9 Twins
Cordierite can among other things, although rare, be recognised by its trilings. Lamellar types of twinning are very commonly observed in feldspars. The extinction angles for the different lamellae can be used to determine the composition of the feldspar (see II.2). Lamellar twins often point towards a triclinic nature of the crystal. But they can also be formed by oriented pressure in uniaxial minerals (pressure twinning in calcite for example).