The shape of all snowflakes depends on the temperature and supersaturation of the air mass in which they form. Under Mars atmospheric pressure, carbon dioxide ice forms at 140°K, or -130°C or -200°F. Water on Mars will boil at 0°C until it freezes.
Carbon dioxide ice crystallizes with cubic symmetry, this means that it can form cubes, octahedrons, or cuboctahedrons, i.e. cubes with the corners cut off. Under Martian conditions carbon dioxide ice forms cuboctahedrons, just like iron pyrite crystals.
Extrapolating to cold, low water vapor densities found on Mars we expect to find water ice snowflakes shaped like hexagonal plates and prisms. These shapes will produce halos around the sun with a radius of 22.5 degrees. The angle of ice crystal halos around the Sun when seen on Mars made by carbon dioxide ice cuboctahedrons, form at 28 degrees from the sun.
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