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
8cm
Haematite, Kidney Ore
Colour 2
5cm
• 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
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 5
Plagioclase feldspar
Barytes
Quartz
Fluorite
Calcite
Gypsum
All these minerals are grey or white in colour
Transparency
2cm
• 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
Calcite – Iceland Spar
• Determined by the atomic
structure and chemical
composition of the mineral
Translucency
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
Fluorite
Lustre
2cm
The 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
Quartz – Vitreous Lustre
Dull/Earthy
Vitreous Lustre
Fluorite
Dog-Tooth Calcite
The mineral reflects light like glass
Sometimes glassy lustre is used instead of vitreous
Metallic Lustre
Malachite
Galena
Minerals reflect light like metals.
Metallic lustre often tarnishes to a dull lustre
Pearly Lustre
Biotite 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
1cm
The lustre of silk
Occurs in minerals with
a fibrous structure
Satin spar (a fibrous
form of gypsum) shows
this to good effect
Gypsum (Satin Spar)
Resinous Lustre
1cm
The lustre of resin
The mineral has a
grainy appearance
Sphalerite, opal
and amber show
resinous lustre
Sphalerite (Zinc Blende)
Adamantine Lustre
5mm
The lustre of a diamond
Dull or Earthy Lustre
The mineral does not
reflect light and has the
same appearance as soil.
1cm
Minerals such as galena
have metallic lustres on
freshly broken surfaces
but they tarnish to dull
with prolonged exposure
to the atmosphere
Limonite has a dull or earthy lustre
Streak
The 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
Haematite gives a
cherry red streak
Any minerals harder than the tile
(6) will scratch it
Streak 2
Malachite – pale green
Galena – lead grey
Haematite – cherry red
Iron Pyrite – greenish black
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
Hardness
Measured on Moh’s scale from 1.0 (softest) to 10 (hardest)
Talc 1.0
Diamond 10.0
Scale was devised by measuring the amount of noise and
powder produced from rubbing a mineral on a metal file
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
7. Quartz
9. Corundum
8. Topaz
6. Orthoclase Feldspar
Moh’s Scale of Hardness
5 Apatite
4 Fluorite
3 Calcite
2 Gypsum
1 Talc
From 1 through to 9 on the scale, hardness increases in equal steps
Moh’s Scale of Hardness
5. Apatite
2. Gypsum
4. Fluorite
3. Calcite
1. Talc
Moh’s Scale of Hardness
Steel nail 5.5-6.0
Fingernail 2.5
Copper coin 3.0
Window glass 5.0
Everyday objects can be substituted for minerals on Moh’s scale
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 Hardness
Smaller 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
Fracture
The 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
5mm
Rose quartz showing conchoidal fracture
A diagnostic property
of the mineral quartz
Cleavage
The 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
1cm
1cm
Biotite Mica
Barytes
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.
Minerals Showing 2 Sets of Cleavage Planes
1cm
1cm
Augite
Plagioclase Feldspar
Feldspars – intersect at 90 degrees
Augite (Pyroxene) – intersect at 90 degrees
Hornblende (Amphibole) – Intersect at 60/120 degrees
Prismatic Cleavage
1cm
Produced by the
intersection of three
cleavage planes
Halite
1cm
Cubic cleavage 3 planes
intersect at 90 degrees
e.g. halite
Rhombohedral cleavage
3 planes intersect at
60/120 degrees
e.g. calcite
Calcite
Octahedral Cleavage
Cleaved edge of
cubic crystal
Fluorite shows well
developed octahedral
cleavage
1cm
Cleavage
Surface
Octahedron
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
Acid Reaction
Use dilute hydrochloric
acid to test for carbonates
Calcite effervesces (fizzes)
and gives off carbon
dioxide gas
Calcite reacting and
giving off carbon dioxide
2cm
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
Pyritohedrons
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
Pyrite cubes
Magnetism
1cm
Steel pins and magnet
attracted to magnetite
Octahedral crystals of Magnetite
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
Feel
A characteristic sensation experienced when a
mineral is held and rubbed between the fingers
2cm
Graphite feels very cold
upon the touch as it is a
very good conductor of heat
2cm
Talc feels very greasy when
rubbed between the fingers
Schiller Effect or Iridescence
The 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
2cm
Example labradorite,
a common variety of
plagioclase feldspar
Form or Habit
Amorphous Chalcopyrite
Crystallised Iron Pyrite
This refers to the common appearance of the mineral and
varies from crystallised to amorphous or massive
Variations in Habit/Form/Appearance of Minerals
Variations in Habit/Form/Appearance of Minerals
Habit – Botryoidal/Mammilated
The specimen has
spherical; lumps or
mounds encrusting
the surface
Botryoidal – the lumps
or mounds are less than
2mm in diameter
1cm
Mammilated Haematite
Mammilated – the lumps
or mounds are over
2mm in diameter
(‘breast-like’)
Habit – Stalactitic, Fibrous and Radiating
1cm
2cm
Haematite showing stalactitic form with
fibrous and radiating internal structure
Habit - Acicular
2cm
The mineral occurs as thin
needle-like crystals
Examples chiastolite,
tourmaline, andalusite
and kyanite
2cm
Chiastolite
Kyanite
Habit - Crystallised
1 cm
Rhombdodecahedral Garnet Crystals
Habit – Nodular, Fibrous and Radiating
1cm
Iron Pyrite showing nodular habit with
fibrous and radiating internal structure
Habit – Foliate/Lamellar
1cm
Muscovite Mica showing foliate/lamellar habit
Habit - Tabular
1cm
Tabular mass of Barytes crystals
Habit - Bladed
2cm
Randomly oriented barytes crystals up to 8cm long
Habit - Reticulate
1cm
Interlocking framework structure resembling a delicate
snowflake shown by Cerussite from Tsumeb, Namibia
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