the physical properties of minerals wjec as geology i.g.kenyon
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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 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
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
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
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
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
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
Iron Pyrite showing nodular habit with
fibrous and radiating internal structure
1cm
Habit – Nodular, Fibrous and Radiating
Habit - Reticulate
Interlocking framework structure resembling a delicate snowflake shown by Cerussite from Tsumeb,
Namibia
1cm
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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