Summer SchoolGeosciences

Geology

Lecture 3 Minerals

A Mineral• A mineral is a naturally occurring

inorganic substance with a chemical composition and physical properties. Under favourable circumstances a mineral assumes a characteristic crystalline form.

e.g. Feldspar Biotite

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Studying Minerals

• Macroscopic characteristics – seen with the naked eye

• Microscopic characteristics – seen with a microscope

Studying Minerals

Most Helpful Equipment for studying Macroscopic Properties

• Penknife

• 10-power magnifying lens

• Piece of broken white porcelain with a rough unglazed surface

• Small hand magnet

Studying Minerals

• Macroscopic Characteristics

• 1 Mode of occurrence – where it is in the field and what it is associated with

• 2 Structure- does the mineral show cleavage, occur as fibres or is it granular….?

Studying Minerals

Halite or Rock Salt Cubic fracture

Asbestos - fibrous

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Studying Minerals

• 3. Form and Habit

• Individual crystals

• Tabular or platy – one pair of parallel faces much larger than the others

• Pyramidal - main faces meet in an apex

Studying Minerals

• Form and Habit cont..• Prismatic – crystals elongated in one

direction

• Acicular – crystals long and thin (needle-like)

• Fibrous – crystals like fine threads/fibres

Studying Minerals

• Individual Crystals – surface characteristics

• Smooth

• Striated

• Curved

Studying Minerals

• Crystal Aggregates – external appearance

• Nodular – irregular lumps/nuggets

• Botryoidal - like a bunch of grapes

• Reniform - kidney shaped

MalachiteCopyright MII Photos

Studying Minerals

• Crystal Aggregates – internal

• Laminar – sheets

• Columnar – in columns

• Granular – composed of grains or small irregular-shaped crystals

• Massive – outline of crystals cannot be seen by eye

Studying Minerals

Amorphous

• Vitreous/glassy – uniform masses without form or structure

• Earthy – loosely coherent particles, which are not crystalline

• Some amorphous minerals may be like gels

Studying Minerals

4 Colour

Minerals can be identified by colour but some minerals have more than one colour e.g. Quartz

Studying Minerals

5 Transparency

• Transparent – an object can be seen clearly through it

• Translucent – light is transmitted, but an object cannot be seen through it

• Opaque – no light passes through

Studying Minerals6 Lustre – intensity of the light reflected

from the surface of a mineral• Metallic – like polished metal• Resinous – like the surface of broken

resin• Vitreous – like broken glass• Greasy – as if covered by a film of oil• Pearly – like Mother of Pearl• Silky – generally characteristic of • a fibrous surface

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Gold

Satin Spar Gypsum

Studying Minerals7 Streak - colour of the powder of a

mineral, seen by rubbing the mineral on an unglazed porcelain plate

8 Cleavage – some crystals show a tendency to split along flat surfaces parallel to a certain plane in the crystal e.g. Mica or like a cube e.g. Halite

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Studying Minerals

9 Fracture – some minerals do not cleave but fracture instead. The appearance of the fractured surface may be distinctive.

• Terms used – even, uneven, conchoidal (curved like a shell), hackly (jagged and rough)

Studying Minerals

10 Hardness – this is estimated using Moh’s Scale of ten standard minerals

• A mineral will scratch other minerals, which are softer and will be itself be scratched by those, which are harder.

• Hardness is expressed by the serial no. which is closest to the mineral being tested

Studying Minerals

11 Specific Gravity• Weight of a mineral/Weight of equal volume

of water at 4oC• 12 Reaction with acid – does effervescence

take place with acid e.g. Chalk• 13 Touch – soapy, silky etc• 14 Taste – Salty, chalky etc• 15 Magnetism – attraction with mineral and

magnet• 16 Characteristics related to cohesion –

elastic, malleable, ductile

Studying Minerals• Not all minerals will have every property,

you may be able to identify a mineral by just a few diagnostic properties.

• http://www.mii.org/commonminerals.html

Reading

• Press and Siever Ch.3 Rocks and Minerals

• Thomson and Turk Ch. 1 and 4