The Physical Properties Of Minerals

WJEC AS Geology

I.G.Kenyon

Colour 1

• Determined by the chemical composition of the mineral

• Minerals rich in Al, Ca, Na, Mg, Ba and K are often light coloured

• Minerals rich in Fe, Ti, Ni, Cr, Co, Cu and Mn are often dark in colour Haematite, Kidney Ore

8cm

Colour 2

• Determined by the atomic structure of the mineral

• Atomic structure controls which components of white light are absorbed or reflected

• White minerals reflect all components of white light

• Black minerals absorb all components of white light

• Green minerals reflect green light and absorb the others

Pyrite Cubes with Striated Faces

5cm

Colour 3

•Colour is not particularly useful as a diagnostic property

•Some minerals show a wide variety of colours

•Quartz can be transparent, white, pink, brown, purple, yellow, orange and even black

•Many minerals show very similar colours

•Calcite, gypsum, barytes, fluorite, plagioclase feldspar and halite are

commonly grey or white in colour

Colour 4

Examples of colour variation in Fluorite

Colour 5Plagioclase feldspar

All these minerals are grey or white in colour

Quartz Calcite

Barytes Fluorite Gypsum

Transparency

Calcite – Iceland Spar

• When outlines of objects seen through it appear

sharp and distinct

•A good examples is Iceland Spar, a variety of calcite that

is used for optical lenses

•Iceland Spar also shows the remarkable property of

double refraction

• Determined by the atomic structure and chemical

composition of the mineral

2cm

Translucency

Fluorite

1 cm

•The ability for a mineral to let light pass through it

•Many minerals if cut thin enough will show some degree of translucency

•Controlled by atomic structure and chemical composition

•All transparent minerals are also translucent

LustreThe way in which a mineral reflects light

Controlled by the atomic structure of the mineral

Main types of lustre are

Vitreous

Metallic

Pearly

Resinous

Adamantine

Dull/EarthyQuartz – Vitreous Lustre

2cm

Vitreous LustreDog-Tooth CalciteFluorite

The mineral reflects light like glass

Sometimes glassy lustre is used instead of vitreous

Metallic Lustre

Minerals reflect light like metals.

Metallic lustre often tarnishes to a dull lustre

Malachite Galena

Pearly LustreBiotite Mica

Muscovite Mica

The lustre of a pearl or mother of pearl

Shows clearly on the cleavage surfaces

of biotite and muscovite mica

Also shown by Talc and selenite (a variety

of gypsum)

Silky Lustre

The lustre of silk

Occurs in minerals with a fibrous structure

Satin spar (a fibrous form of gypsum) shows

this to good effect

1cm

Gypsum (Satin Spar)

Resinous Lustre

The lustre of resin

The mineral has a grainy appearance

Sphalerite, opal and amber show

resinous lustreSphalerite (Zinc Blende)

1cm

Adamantine Lustre

The lustre of a diamond

5mm

Dull or Earthy Lustre

The mineral does not reflect light and has the

same appearance as soil.

Minerals such as galena have metallic lustres on freshly broken surfaces but they tarnish to dull

with prolonged exposure to the atmosphere1cm

Limonite has a dull or earthy lustre

StreakThe colour of a mineral’s powder

Obtained by rubbing a mineral specimen on an unglazed white

porcelain tile

Useful for identifying metallic ore minerals

Silicates generally do not mark the tile and have no

streak

White minerals streaked on a white tile will have a white streak

Any minerals harder than the tile (6) will scratch it

Haematite gives a cherry red streak

Streak 2

Malachite – pale green Haematite – cherry red Iron Pyrite – greenish black

Galena – lead grey Sphalerite – pale brown Limonite – yellowish brown

Metallic Ore Minerals – Characteristic Streaks

Relative Density

Measured relative to an equal volume of distilled water at 4 degrees centigrade.

1 litre = 1000g (1kg) 1 cubic centimetre = 1g

Controlled by the atomic weight of the constituent atoms (chemical composition) and the packing (atomic structure)

A useful property for identifying metallic ore minerals, these usually have relative densities over 5.0.

The only non-metallic mineral which is quite dense is barytes (4.5)

Most of the silicate minerals have densities between 2.5 and 3.2

Relative Density- Some Examples

Kyanite 3.5-3.7 Gold 12.0-20.0 Fluorite 3.2

Iron Pyrite 4.9-5.2 Haematite 4.9-5.3 Gypsum 2.3

HardnessMeasured on Moh’s scale from 1.0 (softest) to 10 (hardest)

Scale was devised by measuring the amount of noise and powder produced from rubbing a mineral on a metal file

Talc 1.0 Diamond 10.0

Moh’s Scale of Hardness

10 Diamond

9 Corundum

8 Topaz

7 Quartz

6 Orthoclase Feldspar

Note diamond is over 30 x harder than corundum

Moh’s Scale of Hardness

10. Diamond 9. Corundum 8. Topaz

7. Quartz 6. Orthoclase Feldspar

5 Apatite

4 Fluorite

3 Calcite

2 Gypsum

1 Talc

Moh’s Scale of Hardness

From 1 through to 9 on the scale, hardness increases in equal steps

Moh’s Scale of Hardness

5. Apatite 4. Fluorite 3. Calcite

2. Gypsum 1. Talc

Moh’s Scale of Hardness

Everyday objects can be substituted for minerals on Moh’s scale

Steel nail 5.5-6.0

Fingernail 2.5

Copper coin 3.0

Window glass 5.0

Testing For Hardness

Try to scratch mineral specimens with substances

of known hardness

If a mineral is not scratched by your fingernail, but is

scratched by a copper coin then it will have a hardness

of 2.5–3.0

If a mineral cannot be scratched by steel it has a

hardness of over 6.0

Gypsum is scratched by a fingernail, hardness <2.5

Mineral HardnessSmaller atoms/ions promote greater

hardness in minerals generally

Minerals with large ions such as carbonates and sulphates are soft

Atomic structure and bond type also control hardness. Covalent bonds are

generally stronger than ionic ones

Hardness should not be confused with difficulty of breaking-a hard mineral

may be very brittle

Graph to illustrate difference between Moh’s Scale and Knoop numbers

FractureThe way a mineral breaks when struck by a hammer

The type of fracture is not controlled by any weaknesses in the atomic structure of the mineral

Types of Fracture

Conchoidal – Like Glass

Even – Flat fracture surface

Uneven – Irregular fracture surface

Hackly – Very jagged like cast iron

Fracture is only described when the mineral has no cleavage

Conchoidal Fracture

This type of fracture is the same as that shown

by window glass

A series of concentric curved lines can be

seen on the fractured surface

A diagnostic property of the mineral quartz

Rose quartz showing conchoidal fracture

5mm

CleavageThe way a mineral breaks when struck by a hammer

Cleavage is controlled by lines of weakness in the atomic

structure of the mineral

Minerals can have 1, 2, 3 or 4 planes of cleavage

1 plane, parallel or basal cleavage

2 planes of cleavage that intersect at a characteristic angle

3 planes (cubic, rhombohedral)

4 planes, octahedral cleavage

Parallel or Basal Cleavage

One plane of cleavage enables the mineral to part along parallel lines. It is analogous to a ream of paper that can

be separated into individual sheets.

Biotite Mica Barytes

1cm

1cm

Minerals Showing 2 Sets of Cleavage Planes

Feldspars – intersect at 90 degrees

Augite (Pyroxene) – intersect at 90 degrees

Hornblende (Amphibole) – Intersect at 60/120 degrees

Augite Plagioclase Feldspar

1cm

1cm

Prismatic Cleavage

Produced by the intersection of three

cleavage planes

Cubic cleavage 3 planes intersect at 90 degrees

e.g. halite

Rhombohedral cleavage 3 planes intersect at

60/120 degrees e.g. calcite

Calcite

Halite

1cm

1cm

Octahedral Cleavage

Fluorite shows well developed octahedral

cleavage

The cubic crystals are truncated across their corners at 45° by four

cleavage planes

This can eventually lead to the formation of

octahedrons from the original cubic crystals

Cleaved edge of cubic crystal

1cm

Octahedron

Cleavage Surface

Acid Reaction

Use dilute hydrochloric acid to test for carbonates

Calcite effervesces (fizzes) and gives off carbon

dioxide gas

2cm

Calcite reacting and giving off carbon dioxide

Taste

If a mineral can be tasted in the mouth, then it is soluble in

fresh water

Halite (rock salt) tastes salty and is a

diagnostic property of the mineral

Striking Fire With Steel

Iron Pyrite (Fools Gold) sparks when struck with a steel

hammer and releases a sulphurous odour

Iron Pyrite was used as flints in flintlock pistols to ignite the

gunpowder

Pyritohedrons

Pyrite cubes

Magnetism

The ability of a mineral to attract iron filings and pick up steel pins

Magnets stick to magnetite quite readily and is the only strongly magnetic mineral found at the earth’s surface

Octahedral crystals of MagnetiteSteel pins and magnet attracted to magnetite

1cm

FeelA characteristic sensation experienced when a

mineral is held and rubbed between the fingers

Graphite feels very cold upon the touch as it is a

very good conductor of heat

2cm 2cm

Talc feels very greasy when rubbed between the fingers

Schiller Effect or IridescenceThe mineral shows a

‘play of colours’ on the surface–similar to the

effect of oil/petrol spills in water

Produced by the scattering of light by fine planar zones

of compositional variation called

exsolution lamellae

Example labradorite, a common variety of plagioclase feldspar2cm

Form or Habit

This refers to the common appearance of the mineral and varies from crystallised to amorphous or massive

Amorphous Chalcopyrite

Crystallised Iron Pyrite

Variations in Habit/Form/Appearance of Minerals

Variations in Habit/Form/Appearance of Minerals

Habit – Botryoidal/Mammilated

Mammilated Haematite

The specimen has spherical; lumps or

mounds encrusting the surface

Botryoidal – the lumps or mounds are less

than 2mm in diameter

Mammilated – the lumps or mounds are over

2mm in diameter (‘breast-like’)

1cm

Haematite showing stalactitic form with fibrous and radiating internal structure

2cm

1cm

Habit – Stalactitic, Fibrous and Radiating

Habit - Acicular

Chiastolite

2cmThe mineral occurs as thin

needle-like crystals

Examples chiastolite, tourmaline, andalusite

and kyanite

Kyanite

2cm

Habit - Crystallised

1 cm

Rhombdodecahedral Garnet Crystals

Iron Pyrite showing nodular habit with

fibrous and radiating internal structure

1cm

Habit – Nodular, Fibrous and Radiating

Muscovite Mica showing foliate/lamellar habit

1cm

Habit – Foliate/Lamellar

Habit - Tabular

Tabular mass of Barytes crystals

1cm

Habit - Bladed

Randomly oriented barytes crystals up to 8cm long

2cm

Habit - Reticulate

Interlocking framework structure resembling a delicate snowflake shown by Cerussite from Tsumeb,

Namibia

1cm

Habit – Dendritic/Arborescent

Manganese oxide dendrites on limestone, Solnhofen, Germany

Diagnostic Properties

Those properties that allow any mineral to be identified

Most minerals have two to four diagnostic properties

Hardness, cleavage, streak and habit are most useful

Colour, lustre, transparency and density are less useful

Special properties such as acid reaction, taste, magnetism, striking fire with steel and feel are

often used to identify a mineral

The End