Hot Black Ice

December 13, 2021

Cold winter weather is arriving at Tikalon's Northeast New Jersey home; so, it's time to tune-up the snow blower. Illustrators depict cold in various ways, such as icicles on an old-style glass thermometer and stylized snowflakes. Nineteen different phases of ice are known, each existing in its own temperature and pressure range. The common ice that we fight in cold weather is, not surprisingly, known as Ice-I.

While Ice-I specifies the hexagonally symmetric crystal cell from which all snowflakes are constructed, the random nature of the actual crystallization ensures that none of the septillion (1024) annually produced snowflakes are identical.[1] Robert Hooke's 1665 book, Micrographia, included many drawings of snowflakes. Hooke's drawings were the first to show the complex shapes within shapes in snowflakes; and, after the invention of photography, Vermont farmer, Wilson Alwyn Bentley (1865-1931), took numerous photomicrographs of snowflakes that showed how unlikely that two could be identical (see figure).

Snowflakes photographed in 1902 at Jericho, Vermont, by Wilson Alwyn Bentley (1865-1931)). Bentley took numerous photomicrographs of snowflakes starting in 1885, and these gave anecdotal evidence to the conjecture that no two could be identical. (Reformatted Wikimedia Commons image.)

A snowflake's uniqueness and fragility are the origin of the slang term, snowflake, to describe a person with an inflated sense of uniqueness who is easily offended. A mathematical object known as the Koch snowflake is a fractal curve invented in 1904 by the Swedish mathematician, Helge von Koch (1870-1924) and shown in the following figure.

A Koch snowflake in its fourth iteration.

The snowflake, as shown, is simply constructed. (1) Draw an equilateral triangle. (2) Taking each line segment in turn, divide it into three equal segments. (3) Draw an equilateral triangle pointing outwards using the middle segment as the base. (4) Remove the base. (5) Continue this operation for the next two line segments of the original triangle; and then recursively for all line segments.

In the limit of an infinite number of such iterations, the Koch snowflake has a fractal dimension, ln(4)/ln(3), or about 1.26.

(Wikimedia Commons image. Click for larger image.)

As I wrote in a previous article (Snowflakes, January 18, 2011), it's even possible to have ice under room temperature and atmospheric pressure conditions when it's confined to a small dimension.[2] As the phase diagram of water reveals, application of higher pressure allows some stable forms of ice to exist at higher temperatures. A new extreme at the high pressure and high temperature corner of the water phase diagram was achieved in research published in Nature Physics.[3-5]. Team members were from the University of Chicago (Chicago, Illinois), the Carnegie Institution of Washington (Washington, DC), and the GFZ German Research Center for Geosciences (Potsdam, Germany). This ice, a superionic ice is designated as ice XVIII.[6]

Superionic ices, ices with with highly mobile protons contained in a stable oxygen crystal sublattice have been predicted at high pressures, but it's been difficult for theory to predict the exact pressure and temperature conditions at which they would occur, and prior experiments gave contradictory results depending on the techniques used.[3] This ice XVIII superionic phase was created for an instant in shockwave experiments, but this new study created the phase for a much longer time using the traditional diamond anvil cell technique and laser heating.[5] X-ray analysis was used to determine the crystal structure of the ice.[5]

The experiments revealed that the superionic ice phase was stable at about 20 gigapascals, much lower than the 50 gigapascals predicted by theory.[5-6] Says Vitali Prakapenka, a study co-author and professor at the University of Chicago,
"It was a surprise - everyone thought this phase wouldn't appear until you are at much higher pressures than where we first find it... But we were able to very accurately map the properties of this new ice, which constitutes a new phase of matter, thanks to several powerful tools."[5]

Apparatus used for the creation and analysis of the superionic phase of ice, ice XVIII.

The cube at the left is a photon-counting X-ray detector used for determination of the crystal structure by X-ray diffraction. The device converts X-ray photons to electrical charges in an imaging array.

(Argonne National Laboratory image by Vitali Prakapenka. Click for larger image.)

While all other ice crystals are built from intact water molecules, the superionic Ice XVIII crystal has oxygen atoms that form a cubic lattice with freely moving hydrogen atoms existing as a liquid in the rigid oxygen lattice.[6] The hydrogen atoms are responsible for the electrical conductivity of the ice, and this has consequences for the magnetic fields of water-rich gas giant planets, such as Neptune and Uranus.[3,5-6] This ice is a mushy solid, so it would flow, but not truly churn, and this would restrict the magnetic field dynamo action to shallower depths.[6] The physical properties of Ice XVIII have yet to be explored.[4-5] Says Prakapenka, ​"It's a new state of matter, so it basically acts as a new material, and it may be different from what we thought."[5]

References:

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