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   Lowitz Oriented Crystals




 

Lowitz oriented plates or columns take rotational positions - they don’t necessarily spin - about a near horizontal Lowitz axis ‘AA’. The axis passes through two opposite prism edges and is parallel to the hexagonal end faces.

The orientation and its halo arcs are named after Tobias Lowitz who first reported them in 1790. Their existence was long doubted but the halos are now occasionally observed and digitally photographed although clear examples remain very rare.

Crystals with the classic Lowitz orientation take all positions about the axis LL. In practice, the crystals may have a more restricted range of angular positions.

Rays can pass through two side faces with a wedge angle of 60° in three ways to form upper, middle and lower Lowitz arcs. Rays internally reflected from end faces form reflected Lowitz arcs, long pillar like extensions from the parhelia when the sun is very low.
  

   Lowitz arcs  Middle Lowitz arc
 



Images: Petri Hakkarainen, Gary Saunders