What defines a mineral? A mineral is a naturally-occurring, inorganic solid having an ordered crystal lattice structure and a well-defined chemical composition.
Rocks are composed of one or more minerals consolidated into a solid massive body. For example, a mass of granite rock might be composed of the minerals quartz, Na-feldspar and mica, all of which formed by cooling of an original body of granitic magma in the Earth's crust.
Some minerals are composed of only one element; native element minerals, such as yellow native sulfur (S) or nuggets of native gold (Au), silver (Ag) or copper (Cu). However, most minerals are composed of chemical compounds involving more than one element.
The mineral halite (NaCl) or rock-salt is composed of the chemical compound sodium chloride in a solid, ionically-bonded mineral. Most minerals are built of lattices that are mostly ionically-bonded, although some bonds have some degree of covalent character.
To form an ionic bond, sodium (Na) and chlorine (Cl) could react directly, in which case Na loses the sole electron in the atom's outermost electron orbit and Cl gains an electron to complete its own outermost electron orbit. Na becomes Na+ and Cl becomes Cl-, and the two link together due to opposite-charge attraction, forming an ionic bond.
Ionic bonds tend to be brittle, so ionically bound materials tend to fracture and break with relative ease.
Covalent bonds involve two atoms with unfilled electron orbits that share their electrons to complete the orbit. Hydrogen forms a covalent bond with another H to form a shared bond making H2. Same-element bonds can be perfectly covalent, otherwise bonds are usually somewhat covalent to some degree and ionic to some degree.
Native metal minerals such as gold and silver possess lattices bound by metallic bonds, a different form of bonding that certain transition elements can exhibit in pure form. Each atom shares its outermost electrons to form a 'sea' of freely-migrating electrons throughout the metal. This form of bonding makes metals electrically conductive and malleable.
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