HALITE: PASS THE SALT

We've all said "pass the salt" at the dinner table.

- STEVE VOYNICK

Because it's so commonplace and often taken for granted, it is easy to sometimes forget that halite is a mineral. With its perfectly formed, sharp-edged cubic crystals, it's a collectible one at that. Halite's solubility, hygroscopicity, and distinctive taste set it apart from all other minerals and account for its great economic importance and prominent role in history.

IONIC BONDING

Halite's unique properties are because of ionic bonding, the attraction between ions of metallic elements (sodium) and nonmetallic elements (chlorine).

Atoms are considered stable when their outer shells contain eight electrons. Atoms with fewer than eight outer electrons achieve stability by gaining, losing, or sharing electrons. Both sodium and chlorine atoms have three shells. Sodium, with one outer-shell electron, achieves stability by losing its electron to create the positively charged sodium cation Nal+. Chlorine, with seven electrons in its outer shell, achieves stability by acquiring an electron from another atom to create the negatively charged chlorine anion Cl¹.

When chlorine atoms contact sodium atoms, chlorine aggressively grabs sodium's single outer electron, transforming both atoms into oppositely charged ions. This opposite-charge attraction is the ionic bond that holds sodium and chlorine ions together in the stable halite molecule.

STRUCTURE AND SOLUBILITY

Halite's crystal lattice consists of alternating rows of sodium and chlorine ions arranged in three axial directions at right angles in a cubic structure. Because ionic bonding is relatively weak, moderate mechanical pressure easily separates the rows to account for halite's perfect, three-directional cleavage and its softness of Mohs 2.0.