Minerals

A mineral is...

• A naturally occuring solid inorganic substance
• It has... a definite chemical composition or range of compositions
• a definite, regular atomic structure
• a definite crystal form

Strangely enough though...

Ice in glaciers is a mineral on this definition

Coal and oil aren't! 

There are only 8 elements in the continental crust in significant amounts. These 8 elements make up over 99.8% of the crustal mass.

In decreasing order, the top 8 are...

• Oxygen
• Silicon
• Aluminium
• Iron
• Calcium
• Sodium
• Potassium
• Magnesium

Approximately 4,000 minerals have been identified, but only 30 are commonly encountered.

tell us

1) about the source of a rock

• Igneous: parent magma
• Sedimentary: sedimentary source
• Metamorphic: parents rock 

2) about the environment at which they were formed

• Temperature
• Pressures
• chemical conditions

Minerals differ from one another because each has a specific chemical composition and a unique atomic structure.

These differences result in a variety of physical properties, including;

• Appearance
• Break - how?
• Resistance to scratch
• Smell
• Taste
• Feel

In reality, minerals in rocks are often identified either by the use of a microscope or chemical analysis.

Nevertheless, the identification of basic minerals is an important skill that a geologist needs to learn.

1) Colour 

This is the most obvious property but is of variable use. Some minerals show a range of colours. For example, quartz can be....

• Clear
• Milky
• White
• Purple (amethyst)
• Pink (rose quartz)
• Smoky
• Yellow (due to trace elements)

Streak is the colour of the powder of the mineral. The powder is produced by rubbing the mineral across an unglazed porcelain tile, called a streak plate.

Some minerals are too hard to give a streak and instead scratch the streak plate. Eg: Quartz.

• Black - Graphite, pyrite, 
• Grey - Galena
• Yellow/Brown - Limonite
• Red/brown - Haemotite 

The lustre of a material is the way its surface reflects light. It is a useful property in some cases, particularly metallic minerals.

• Metallic (Pyrites)
• Vitreous (glassy, eg: quartz)
• Pearly (feldspar)
• Resinous (... looks plasticky)
• Silky (some gypsum)
• Dull or earthy (haematite... little light is reflected)

Transparency is the ability of a mineral to allow light to pass through it. Not an especially useful property but it does help with metallic minerals as no light can get through. 

• Transparent - letters can be read through it. Eg: Quartz
• Translucent - only allows light through. Eg: Some quartz
• Opaque - No light. Eg: Haematite

From softest to hardest...

• Talc
• Gypsum - fingernail 
• Calcite - Copper coin
• Fluorite 
• Apatite 
• Orthoclase Feldspar - steel needle
• Quartz - glass
• Topaz
• Corundum
• Diamond

Hardness scale:

• Fingernail (2.5)
• Copper coin (3-5)
• Steel needle (6-6.5)

This is the tendency of a mineral to break in a definite preferred direction to give a smooth flat cleavage plane. These are parallel to plane of weakness in the chemical structure of the mineral. VERY DIAGNOSTIC!

Is referred to as being, perfect, good, fair or poor, depending on how easy it is for a mineral to break along its cleavage planes.

It can occur in 1, 2 or 3 directions. 

For example, GALENA has 3 perfect cleavages at 90 degrees.