Composition and Structure of Minerals

In order to be classified as a mineral a substance has to be

• Naturally occurring

• Solid

• Definite chemical composition

• Orderly pattern of atoms (crystal)

• Inorganic

Mineral Facts

• 4000 known minerals

• Eight minerals make up >98% of the Earth’s crust

• Most are compounds

• Some are native elements

• Rocks are mixtures of minerals

Mineral Formation • From magma

– As magma cools and hardens chemical bonds form

– More than one mineral is produced from the same magma due to types of elements and cooling times

Minerals formed from magma

Gabbro is a volcanic rock that contains quartz,

feldspar and mica (minerals)

Mineral Formation • From dissolved ions

• Water evaporates, leaving minerals behind

Evaporates

Form when water evaporates and leaves mineral

crystals behind like salt

Mineral Formation • Existing minerals are transformed by heat

and pressure

Structure of Minerals

• Crystal structure

– Regular, geometric, smooth faces

– Orderly arrangements with repeating structures

– Each mineral always forms the same crystal shape

– Six basic crystal shapes

– Crystallographic axes are used to determine structure

Mineral crystals

Crystallographic Axis

Silicates

• Most numerous

• Contain Si and O (and a metal)

• Forms a silica tetrahedron

• Covalent bond

Depending on the arrangement of the silica tetrahedron, different

groups of silicates will form.

Crystal Structure

• Determines cleavage

– Split occurs where bonds are weak

– Halite: cubic cleavage

– Quartz: no cleavage

Hardness

• Depends on arrangement of ions, atoms or molecules and strength of bond

– Carbon can form diamonds in a tetrahedron

– Carbon can form graphite in a hexagonal arrangement