Silicate mineral structure

What are cations?

ions that have a positive charge

What are anions?

ions that have a negative charge

what is a covalent bond?

A chemical bond that is formed by two atoms sharing a pair of electrons.

What is an ionic bond?

An electrostatic force of attraction between oppositely charged ions (cation & anion)

What mineral structure occurs when silicon and oxygen combine?

Silicate tetrahedra - SiO₄^-4

Can silicate tetrahedra exist by itself?

It cannot, it must join with other atoms to form a stable molecule.

Give an example of an island silicate

Olivine

How are island silicates formed?

Either two Fe⁺² ions or two Mg⁺² ions join with the spare bonds of the SiO₄^-4 tetrahedron, balancing the 4- charge, forming strong covalent bonds. The tetrahedra are packed closely together with the metal ions inbetween.

Give an example of a single chain silicate

Pyroxene (Augite)

How are single chain silicates formed?

These are formed when each tetrahedron share two of its oxygen atoms with adjacent tetrahedra. These long single chains of molecules are joined together by weaker ionic bonds involving the metallic ions.

In what ways do the single chain structure influence the mineral’s properties?

Lower density (than olivine) - tetrahedra can’t pack so closely & there is less iron.

Lines of weaknesses - this is due to the weaker bonds between the chains, lines of cleavage may develop along these lines of weaknesses.

Lower hardness (than olivine)

Give an example of a double chain silicate

Amphiboles (hornblende)

How are double chain silicates formed?

Single chains link together to form double chains, this leaves fewer bonds available for metal elements to join. Double chain structures are less compact then single chains, reducing their density. Mg²⁺, Fe²⁺, Al²⁺ often link the chains together.

Give an example of a sheet silicate

Mica (muscovite or biotite)

Describe the process of how sheet silicates are formed?

• continuous sheets of tetrahedra are joined in pairs by aluminium ions.

• The pairs of sheets are held together by potassium ions.

• this creates a perfect cleavage in one plane, meaning that the sheets of mica are as thin as the two sheets of tetrahedra.

Where are the planes of cleavage found in Mica?

Along the lines of potassium ions because their bonds are the weakest.

Give an example of framework silicates

Quartz and feldspar

How are framework silicates formed?

When the oxygens bond together at each of the four corners of each tetrahedron. This forms a continuous 3-dimensional framework (sheet upon sheet) of hexagons joined together.

What properties are associated with framework silicates?

High hardness due to covalent bonds.

No cleavage due to strong covalent bonds in the 3D framework.

Low density as there are less/no metal ions & rigid framework.