  Liquid salty micro-channels (brine) between growing ice platelets in sea ice
are an important habitat for $CO_2$ - binding microalgaea with great impact on
polar ecosystems. The structure formation of ice platelets is microscopically
described and a phase field model is developed. The pattern formation during
solidification of the two-dimensional interstitial liquid is considered by two
coupled order parameters, the tetrahedricity as structure of ice and the
salinity. The coupling and time-evolution of these order parameters are
described by a consistent set of three model parameters. They determine the
velocity of the freezing process and the structure formation, the phase
diagram, the super-cooling and super-heating region, and the specific heat. The
model is used to calculate the short-time frozen micro-structures. The obtained
morphological structure is compared with the vertical brine pore space obtained
from Xray computed tomography.
