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  Pyramidal Crystals
 
The internal hexagonal structure of ice produces pyramidal crystals in addition to ordinary plates and columns.

Pyramidal crystals have the same central hexagonal shape as plate or column crystals but six sided pyramids cap the middle prism section. The pyramids can be truncated and 18 different habits or variations are possible having from 12 to 20 faces.

All the crystals have the same pyramidal angle where the faces slope at 28° to the crystal ‘c’ axis.

  These are the faces with Miller indices like
{10-11}. Higher order faces of greater slope are, in principle, possible but unlikely. However, elliptical halos can currently only be explained by hypothesising very flat pyramids with improbably large Miller indices.

Pyramidal crystals tend to be aerodynamically rather rounded and are therefore less well aligned than plates and columns. Many different light paths are possible through them and produce multiple circular halos called the 'Odd radius halos'.

More rarely, the crystals are better aligned and form pyramidal plate and column arcs.

Pyramidal crystals are not so rare as previously thought. Search carefully for their halos whenever the skies look favourable