Science Chemistry: Matter and Change

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 Problem of the Week
Chapter 8
Minerals
Minerals are naturally occurring, inorganic crystalline solids that have a specific chemical composition. Minerals are the building blocks of rocks and many minerals are essential to plant and animal life. The science of mineralogy studies minerals that make up rocks, clays, soils, and sands of Earth, as well as, meteorites and other extraterrestrial rocks.

Minerals have highly-ordered three dimensional internal arrangements of atoms or ions. Thus, minerals are crystalline solids. Crystals are built up by the repetition of a unit structure such as a cube or a hexagon. See Table 1 below for three minerals whose structural unit is the cube.

Table 1
Mineral Comparison

Mineral Cation Anion Melting point (ºC) Hardness Anion-cation distance (nm)
Halite Na+ Cl- 801 2.5 281
Sylvite K Cl- 771 2.0 315
Fluorite Ca2+ F- 1418 4.5 236

Note the anions and cations in each mineral in Table 1. These oppositely charged ions are held by ionic bonds to form crystals. The strength of the bonds within a crystal is responsible for the crystal’s physical properties: solubility, hardness, melting point, and lack of conductivity of heat and electricity. Bond strength is determined by the spacing between the ions and the amount of charge on the ions.

The chemical formula of an ionic crystal is determined by the ratio of the cations and anions that make up the crystal. In addition, the radii of the ions determine the arrangement of ions in the crystal. Ions of different elements can substitute for one another in a crystal if their sizes are similar. Therefore, most minerals are not pure substances. Often, mineral colors result from minute traces of transition elements (Ti, V, Cr, Mn, Fe, Co, Ni, Cu) called chromophores.

Rubies and sapphires have the same chemical composition, Al2O3, as the mineral corundum. The difference in color is due to the chromophore chromium in rubies and the chromophores titanium and iron in sapphires.
Problem 1

Determine the correct chemical formula and chemical name for each crystal.

a. galena, composed of lead(II) ions and sulfide ions
b. hematite, composed of iron (III) ions and oxide ions
c. aragonite, composed of calcium ions and carbonate ions
d. barite, composed of barium ions and sulfate ions
Problem 2 Examine the properties of halite, sylvite, and fluorite listed in Table 1. Explain the trends you observe.
Useful Web Sites:
Colgate Mineralogy
Covalent Crystals
Halite
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Matter and Change