Why do ice crystals always* form hexagonal structures?
Water, containing two hydrogen atoms and an oxygen atom is bent. The angle of the hydrogen atoms from the oxygen atom is 105 degrees:
and the oxygen atom has a partial negative charge making it very attractive to the partially positive hydrogen atoms …
So the hydrogen atoms orient them selves to be facing the oxygen and we get hexagons.
Different conditions change the type of ice crystal formed, for instance higher moisture levels are required for snow flakes (dendrites), and relatively higher temperatures for ice needles. (See diagram below, moisture content is the vertical axis, temperature is horizontal).
But that’s not all:
*While the hexagonal ice form discussed above is the primary form of ice and is the dominant form from the freezing point at 273K down to about 72 K, 13 different crystalline forms of ice have been identified according to Debenedetti and Stanley