Content deleted Content added
m I added a picture with conoscopic figures of cm large mineral spheres under crossed polars of a gemmological polariscope, some uniaxial, some biaxial |
|||
Line 6:
In [[optical mineralogy]] a [[petrographic microscope]] and cross-[[polarized light]] are often used to view the interference pattern. This is done by placing a [[Bertrand lens]] (Emile Bertrand, 1878) between a high-power microscope objective and the eyepiece. The microscope's condenser is brought up close underneath the specimen to produce a wide divergence of polarized rays through a small point. There are many other techniques used to observe the interference pattern.
A uniaxial mineral will show a typical 'Maltese' cross shape and its isogyres, which will revolve/orbit around a projection of the optical axis as the stage is rotated.
A biaxial mineral 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 "2V" angle. 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>
| |||