What I've found out is that Ammonia has (in the relevant temperature/pressure regime) cubic crystal structure. But what would that mean for ammonia snowflakes? Would they have fourfold symmetry? But then, you can also find threefold symmetries in a cube (just look at it from the corner), and cutting a cube in the right way you even get a hexagon.
Also, I don't know what causes water snowflakes to be planar rather than three-dimensional structures. That mechanism might well be special to water, so maybe ammonia snowflakes wouldn't be planar at all?
Or maybe ammonia wouldn't form snowflakes, but only solid crystals? That would mean that on an ammonia world, it would hail, but never snow.
So does anyone know what ammonia snowflakes would look like, or if they could exist at all?