Re: Explain why salt water forms cubic crystals and ice is hexagonal?

Purification by selective crystallization is not well supported by theory.
 Thermodynamics can explain after observation but often not usefully
predict before. 

 http://cst-www.nrl.navy.mil/lattice/struk/b1.html

 http://webmineral.com/data/Halite.shtml
 sodium chloride (halite); cubic 

 http://www.lsbu.ac.uk/water/ice1h.html

 http://en.wikipedia.org/wiki/Ice_Ih

 http://www.tms.org/pubs/journals/JOM/9902/Schulson-9902.html
 Ice, hexagonal

The drug industry desperately funds research to predict crystal structure
from molecular structure.  After more than a decade of efforts not much has
obtained.  Crystal structure of simple ionic salts depends on radius ratios
of the ions,

J. Chem. Ed. 62(3) 215 (1985) 
 http://www.chem.ox.ac.uk/icl/heyes/structure_of_solids/Lecture3/Lec3.html

 It's complicated!
 http://www.lsbu.ac.uk/water/phase.html
Water is tremendously studied.  Water is not well understood. 
 http://www.chemguide.co.uk/physical/phaseeqia/saltsoln.html
 Even if we don't understand it we can observe it. 

 http://www.ucalgary.ca/~kmuldrew/cryo_course/cryo_chap6_1.html

 http://mysite.du.edu/~jcalvert/phys/phase.htm#Salt
 Wow.

All that... and it's only water.