Ice crystal

Formation

At ambient temperature and pressure, water molecules have a V shape. The two hydrogen atoms bond to the oxygen atom at a 105° angle. Ice crystals have a hexagonal crystal lattice, meaning the water molecules arrange themselves into layered hexagons upon freezing.

Slower crystal growth from colder and drier atmospheres produces more hexagonal symmetry. Depending on environmental temperature and humidity, ice crystals can develop from the initial hexagonal prism into many symmetric shapes. Possible shapes for ice crystals are columns, needles, plates and dendrites. Mixed patterns are also possible. The symmetric shapes are due to depositional growth, which is when ice forms directly from water vapor in the atmosphere. Small spaces in atmospheric particles can also collect water, freeze, and form ice crystals. This is known as nucleation. Snowflakes form when additional vapor freezes onto an existing ice crystal.

Trigonal and cubic crystals

Supercooled water refers to water below its freezing point that is still liquid. Ice crystals formed from supercooled water have stacking defects in their layered hexagons. This causes ice crystals to display trigonal or cubic symmetry depending on the temperature. Trigonal or cubic crystals form in the upper atmosphere where supercooling occurs.

Square crystals

Water can pass through laminated sheets of graphene oxide unlike smaller molecules such as helium. When squeezed between two layers of graphene, water forms square ice crystals at room temperature. Researchers believe high pressure and the van der Waals force, an attractive force present between all molecules, drives the formation. The material is a new crystalline phase of ice.