Geology

Minerals are the 'building blocks' of geology - all rocks and the fossils are composed of them.

Minerals: Inorganic substances composed of one or more than one element, with a fixed internal atomic structure that produces fixed external features by which they can be recognised.

1) Crystal form

Crystals form when fluids in which the material is dissolved either cool or evaporate and the mineral then precipitates as a solid. if there is enough room and time for them to develop properly they will show regular shapes ( shapes depend on the mineral )

If there isn't enough room to grow then the crystals become poorly formed and are described as crystalline.

If the mineral cools too quickly the crystals wont form properly and are invisible. This is referred to as microcrystallinc.

Types

1) Cube/ triangular prism - equidimensional prism

Halite (sodium chloride), Galena (lead sulphide), Pyrite (iron sulphide = fools gold)

2) Prismatic- one dimension is elongated compared to the other two.

3) Tabular - one dimension is shortened to the other two.

Lines of weakness in the mineral along which they break, forming cleavage planes.

Cleavage is a very useful way of describing and identifying minerals.

1) Minerals with no cleavage

When broken will show an irregular fracture plane. Typically quartz (and glass shows a ''conchoidal'' fracture.

Typical minerals

Quartz, haematite, pyrite,chalcopyrite.

2) Minerals with cleavage(one direction)

White (silvery) = muscovite

Mica

Black = biotite

3) Two directions

Feldspar (white or pink) has two cleavage meeting at 90

4) Three Directions

a) meeting at 90 degrees

b) Meeting at 60/120 degrees

How easy it is to scratch a mineral (how easy it is to break the atomic bonding). Minerals are arranged in a scale of hardness.

Refer to book

Testing for hardness

See if you can make a surface scratch with one of the familiar objects on a fresh surface of the mineral.

• Start with the softest and work up.

Eg. If the mineral doesn't scratch with your finger nail it must be >2.5. If it scratches with a copper coin it must be <3.5, the hardness must be roughly = to 3.

How easily or not it is to scratch will give you an idea. eg. If a mineral doesn't scratch with a copper coin (3.5) but scratches easily with a steel nail (5.5) you know it is closer to 3.5 than 5.5.

4) Density

Mass per unit of volume. how heavy a specimen is compared to its size.

• Rocks formed by magma (molten rock)

They can be distinguished from sedimentary and metamorphic rock quite easily.

The following way:

Igneous rocks: randomly arranged crystals

Metamorphic rocks: Often have crystals growing in a preferred direction due to pressure.

Sedimentary rocks: made of fragments (clasts) of pre-existing igneous/metamorphic rock.

Formed by magma - molten rock, produced by rock, produced by mealting, especially at plate boundries.

Magma is less dense than the overlying rock so tries to rise.

It can rise in:

• large balloon shapes = diapirs
• or become igneous intrusions
• or become extrusive if it makes it to the surface.

Composition

If visable, the minerals in igneous rocks can be identified using the following key:

rounded irregular lumps no cleavage = Olive green/clear grey = olivine/Quartz

short rectangular crystals with cleavage = Pink,white/Black, green = feldspar/augite

long rectangular crystals with cleavage = White,clear,grey/Black,dark green = feldspar/hornblend

shimmering plates = Silvery/Black = Muscovite mica/biotite mica