What is a crystal made out of and what holds the pieces together?

Crystals are built out of atoms. The fundamental building blocks for atoms are protons, neutrons and electrons. Atoms are listed and arranged in the periodic table according to their number of protons (increasing left to right across the rows).

In general, the elements found in gemstones are those that are relatively abundant in the crust (however, this is not always true).

What are ions and how do they form?

Ions form when atoms lose (cations) or gain (anions) electrons.

Cations are positively charged ions.

Anions (negatively charged ions) charged ions.

Atoms are held together in crystals by atomic bonding.

The most important types of bonds are via electron exchange (ionic) or electron sharing (covalent).

How are minerals grouped?

Minerals are grouped firstly based on what they contain:

• silicates – silica (Si)-rich minerals
• oxides – anion is oxygen (O)
• carbonates – contain carbon and oxygen (C)
• phosphates – contain phosphorus (P)
• borates – contain boron (B)
• sulfides and sulfates – contain sulfur (S)
• halides – contain chlorine (Cl) or other elements from group VIIa in the periodic table

What is a crystal?

Something is crystalline if the atoms or ions that compose it are arranged in a regular way (i.e., a crystal has internal order due to the periodic arrangement of atoms in three dimensions). Gems are described as amorphous if they are non-crystalline.

Crystals characterized by well developed crystal faces (external surfaces) are described as euhedral. Crystals do not always show well developed crystal faces seen on euhedral examples.

A crystal is built up by arranging atoms and groups of atoms in regular patterns, for example at the corners of a cube or rectangular prism.

The basic arrangement of atoms that describes the crystal structure is identified. This is termed the unit cell.

Crystals must be charge balanced. This means that the amount of negative charge must be compensated by the same amount of positive charge.

Example:

Al 3+ and O 2-: These are combined as Al2O3 (two aluminum atoms per three oxygen atoms). They would not combine as AlO due to the above rule.

How are cations and anions arranged?

The atomic cluster consisting of a regular arrangement of anions around a central cation (or vice versa) is described as a coordination polyhedron. Coordination polyhedra are larger building blocks than individual atoms.

Coordination arrangements are determined by relative sizes of cations (+) and anions (-).

How are coordination polyhedra arranged?

Crystals are formed by linking of coordination polyhedra:

Imagine building a crystal by linking the building blocks (tetrahedra or octahedra) into chains or frameworks.

Example: silicate polyhedra may be linked by sharing of oxygen atoms (corners) between two adjacent silicon tetrahedra.

Other polyhedra (e.g., octahedra) may be linked by sharing edges (e.g., two adjacent oxygens are shared between two octahedra) and faces (e.g., three adjacent oxygens are shared between two adjacent octahedra).

Is more than one arrangement of coordination polyhedra possible?

Yes. At different temperatures and pressures, different arrangements of the same elements are stabilized. A polymorph is a chemical composition that can crystallize into more than one type of structure. This results in different minerals with identical compositions and distinguished by their crystallography.

A well-known example Diamond and Graphite, two different minerals composed of the same exact substance, though they crystallize distinctly..

Symmetry and Crystals

The symmetry involves the pattern of arrangement of atoms

“Symmetry” refers to sameness. All minerals are assigned to one of six (if we group rhombohedral and hexagonal together) crystal systems. Crystal systems are determined based on the symmetry of the mineral. The crystal’s external form is the direct result of addition of growth by addition of groups of atoms (unit cells) in a fixed arrangement.

Crystal Systems:

• Cubic

• Hexagonal

• Tetragonal

• Orthorhombic

• Monoclinic

• Triclinic

Examples of crystals: silicate and oxide minerals

Silicate minerals: Silicates have structures containing abundant silica tetrahedra, i.e., a tetrahedron with a Si at the center, surrounded by four oxygen anions.

Many silicates contain linkages of silica tetrahedra. This forms the backbone of the structure. Silicate minerals are classified based upon the way in which the tetrahedra are linked together.

Gem examples of the extremes of this classification system are:

• amethyst (quartz, in which each tetrahedron is linked to four other tetrahedra, making a completely cross-linked framework)

• peridot (olivine, in which the silicate tetrahedra are not linked to any other tetrahedra

Oxide minerals: Simple oxide minerals consist of metals and oxygen. Examples include corundum (Al2O3) and hematite (Fe2O3). These contain metals (aluminum or iron) in a six coordinated site (octahedral site) formed by oxygen anions.

There are thousands of naturally occurring compounds (minerals). Most are not suitable for use as gems.

Credit: https://nature.berkeley.edu/