Summer School Geosciences 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 Copyright MII Photos

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 Copyright MII Photos

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 Malachite Copyright 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 Minerals 6 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 Copyright MII Photos Gold Satin Spar Gypsum

Studying Minerals 7 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 Copyright MII Photos

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 Mohs 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 4 o C 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. Reading Press and Siever Ch.3 Rocks and Minerals Thomson and Turk Ch. 1 and 4