The Physical Properties Of Minerals

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The Physical Properties Of Minerals
WJEC AS Geology I.G.Kenyon
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Colour 1
8cm

• 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
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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
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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

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Colour 4
Examples of colour variation in Fluorite
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Colour 5
Plagioclase feldspar
Quartz
Calcite
Barytes
Fluorite
Gypsum
All these minerals are grey or white in colour
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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
• Determined by the atomic structure and chemical
  composition of the mineral

Calcite Iceland Spar
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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
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Lustre
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 Dull/Earthy
2cm
Quartz Vitreous Lustre
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Vitreous Lustre
Dog-Tooth Calcite
Fluorite
The mineral reflects light like glass Sometimes
glassy lustre is used instead of vitreous
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Metallic Lustre
Malachite
Galena
Minerals reflect light like metals. Metallic
lustre often tarnishes to a dull lustre
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Pearly Lustre
Biotite 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)
Muscovite Mica
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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)
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Resinous Lustre
The lustre of resin The mineral has a grainy
appearance Sphalerite, opal and amber show
resinous lustre
1cm
Sphalerite (Zinc Blende)
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Adamantine Lustre
5mm
The lustre of a diamond
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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 atmosphere
1cm
Limonite has a dull or earthy lustre
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Streak
The colour of a minerals 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
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Streak 2
Malachite pale green
Haematite cherry red
Iron Pyrite greenish black
Galena lead grey
Sphalerite pale brown
Limonite yellowish brown
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Metallic Ore Minerals Characteristic Streaks
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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
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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
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Hardness
Measured on Mohs 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
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Mohs Scale of Hardness 10 Diamond 9 Corundum 8
Topaz 7 Quartz 6 Orthoclase Feldspar
Note diamond is over 30 x harder than corundum
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Mohs Scale of Hardness
10. Diamond
9. Corundum
8. Topaz
7. Quartz
6. Orthoclase Feldspar
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Mohs 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
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Mohs Scale of Hardness
5. Apatite
4. Fluorite
3. Calcite
2. Gypsum
1. Talc
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Mohs 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 Mohs scale
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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 2.53.0 If a
mineral cannot be scratched by steel it has a
hardness of over 6.0
Gypsum is scratched by a fingernail, hardness lt2.5
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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 Mohs Scale and Knoop numbers
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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
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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
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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
5mm
Rose quartz showing conchoidal fracture
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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
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Parallel or Basal Cleavage
1cm
1cm
Barytes
Biotite Mica
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.
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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
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Prismatic Cleavage
1cm
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
Halite
1cm
Calcite
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Octahedral Cleavage
Cleaved edge of cubic crystal
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
1cm
Cleavage Surface
Octahedron
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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
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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
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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
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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 earths surface
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Feel
A characteristic sensation experienced when a
mineral is held and
rubbed between the fingers
2cm
2cm
Graphite feels very cold upon the touch as it is
a very good conductor of heat
Talc feels very greasy when rubbed between the
fingers
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Schiller Effect or Iridescence
The mineral shows a play of colours on the
surfacesimilar 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 feldspar
2cm
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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
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Variations in Habit/Form/Appearance of Minerals
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Variations in Habit/Form/Appearance of Minerals
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Habit Botryoidal/Mammilated
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
Mammilated Haematite
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Habit Stalactitic, Fibrous and Radiating
1cm
2cm
Haematite showing stalactitic form with
fibrous and radiating internal structure
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Habit - Acicular
The mineral occurs as thin needle-like
crystals Examples chiastolite, tourmaline,
andalusite and kyanite
2cm
2cm
Kyanite
Chiastolite
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Habit - Crystallised
1 cm
Rhombdodecahedral Garnet Crystals
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Habit Nodular, Fibrous and Radiating
1cm
Iron Pyrite showing nodular habit with
fibrous and radiating internal structure
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Habit Foliate/Lamellar
1cm
Muscovite Mica showing foliate/lamellar habit
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Habit - Tabular
1cm
Tabular mass of Barytes crystals
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Habit - Bladed
2cm
Randomly oriented barytes crystals up to 8cm long
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Habit - Reticulate
1cm
Interlocking framework structure resembling a
delicate snowflake shown by
Cerussite from Tsumeb, Namibia
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Habit Dendritic/Arborescent
Manganese oxide dendrites on limestone,
Solnhofen, Germany
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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
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The End