Conoscopic interference pattern: Difference between revisions

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An interference figure produced looking straight down or close to the optic axis of a uniaxial mineral will show a characteristic 'Maltese' cross shape to its isogyres. If you are looking perfectly down the optic axis, the pattern will remain completely unchanging as the stage is rotated. However, if the viewing angle is slightly away from the optic axis, the centre of the cross will revolve/orbit around the central point as the stage is rotated. However, the form of the cross will stay constant as it moves.

[[File:Biaxial interference figures.jpg|thumb|center|upright=2.5|InterferencePossible interference figures for a biaxial mineral with a fairly large 2V, viewed along one of its two optic axes. The curved shape of the dark arc (the "isogyre") is characteristic of biaxial minerals - though the degree of curvature will change as the microscope stage is rotated, and at some orientations the pattern will resemble the "maltese cross" pattern of a uniaxial mineral. The left hand image illustrates the figure alone; the grey patch at the centre indicates the low first order (grey) birefringence colours seen here (the order of the colours seen would in reality increase away from the center, but these colours are not shown). The two right hand figures show the effect of adding a sensitive tint plate to the setup, replacing the grey at the centre with second order blue and first yellow birefringence colours. The polarity of the yellow and blue reveals whether the mineral being viewed is optically "biaxial positive" (top) or "biaxial negative" (bottom), which can be a key property in identifying the mineral (or investigating its composition).]]

The optic axis figure of a biaxial mineral is more complex. It will typically show a saddle-shaped figure (with one isogyre thicker than the other, typically) that will often morph into two curved isogyres (called brushes) with rotation of the stage. The difference in these curved isogyres is known as the ''optic angle'', or"2V". In minerals that have far-off-center optic axes, only one part of the above sequence may be seen. On either side of the saddle the interferences rings surround two eye like shapes called melanotopes. The closest bands are circles, but further out they become pear shaped with the narrow part pointing to the saddle. The larger bands surrounding the saddle and both melanotopes are figure 8 shaped.<ref name="hartshorne">{{cite book|last1=Hartshorne|first1=N. H.|last2=Stuart|first2=A.|title=Practical Optical Crystallography|year=1964|publisher=Edward Arnold|___location=London|pages=210–211}}</ref> By combining interference pattern microscopy with use of a [[sensitive tint plate]], the optic sign and optic angle can be determined together. This information can help both with mineral identification, and with interpreting the chemical composition of some minerals (for example, feldspars).