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

2. Colour 1 Determined by the chemical composition of the mineral
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

3. Colour 2 Determined by the atomic structure of the mineral
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

4. 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

5. Examples of colour variation in Fluorite

6. Colour 5 All these minerals are grey or white in colour
Plagioclase feldspar
Quartz
Calcite
Barytes
Fluorite
Gypsum
All these minerals are grey or white in colour

7. 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

8. 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

9. Lustre Quartz – Vitreous 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

10. Vitreous Lustre Dog-Tooth Calcite Fluorite
The mineral reflects light like glass
Sometimes glassy lustre is used instead of vitreous

11. Metallic Lustre Minerals reflect light like metals.
Malachite
Galena
Minerals reflect light like metals.
Metallic lustre often tarnishes to a dull lustre

12. Pearly Lustre The lustre of a pearl or mother of pearl
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

13. Silky Lustre The lustre of silk
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)

14. Resinous Lustre The lustre of resin
1cm
The lustre of resin
The mineral has a grainy appearance
Sphalerite, opal and amber show resinous lustre
Sphalerite (Zinc Blende)

15. Adamantine Lustre The lustre of a diamond
5mm
The lustre of a diamond

16. Limonite has a 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

17. 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
Any minerals harder than the tile (6) will scratch it
Haematite gives a cherry red streak

18. Streak 2 Malachite – pale green Haematite – cherry red
Iron Pyrite – greenish black
Galena – lead grey
Sphalerite – pale brown
Limonite – yellowish brown

19. Metallic Ore Minerals – Characteristic Streaks

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

21. Relative Density- Some Examples
Kyanite
Gold
Fluorite 3.2
Iron Pyrite
Haematite
Gypsum 2.3

22. Measured on Moh’s scale from 1.0 (softest) to 10 (hardest)
Hardness
Measured 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

23. Moh’s Scale of Hardness Note diamond is over 30 x harder than corundum
8 Topaz
7 Quartz
6 Orthoclase Feldspar
Note diamond is over 30 x harder than corundum

24. Moh’s Scale of Hardness
10. Diamond
9. Corundum
8. Topaz
7. Quartz
6. Orthoclase Feldspar

25. 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
From 1 through to 9 on the scale, hardness increases in equal steps

26. Moh’s Scale of Hardness
5. Apatite
4. Fluorite
3. Calcite
2. Gypsum
1. Talc

27. Moh’s Scale of Hardness
Steel nail
Fingernail 2.5
Copper coin 3.0
Window glass 5.0
Everyday objects can be substituted for minerals on Moh’s scale

28. 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

29. 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

30. The way a mineral breaks when struck by a hammer
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

31. 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

32. 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

33. 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, 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

34. 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.

35. 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

36. 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

37. Cleaved edge of cubic crystal
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

38. 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

39. 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

40. 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

41. 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

42. Talc feels very greasy when rubbed between the fingers
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

43. 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
Example labradorite, a common variety of plagioclase feldspar
2cm

44. Amorphous Chalcopyrite Crystallised Iron Pyrite
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

45. Variations in Habit/Form/Appearance of Minerals

46. Variations in Habit/Form/Appearance of Minerals

47. 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

48. Habit – Stalactitic, Fibrous and Radiating
1cm
2cm
Haematite showing stalactitic form with fibrous and radiating internal structure

49. Habit - Acicular Kyanite Chiastolite
The mineral occurs as thin needle-like crystals
Examples chiastolite, tourmaline, andalusite and kyanite
2cm
2cm
Kyanite
Chiastolite

50. Rhombdodecahedral Garnet Crystals
Habit - Crystallised
1 cm
Rhombdodecahedral Garnet Crystals

51. Habit – Nodular, Fibrous and Radiating
1cm
Iron Pyrite showing nodular habit with fibrous and radiating internal structure

52. Habit – Foliate/Lamellar Muscovite Mica showing foliate/lamellar habit
1cm
Muscovite Mica showing foliate/lamellar habit

53. Tabular mass of Barytes crystals
Habit - Tabular
1cm
Tabular mass of Barytes crystals

54. Randomly oriented barytes crystals up to 8cm long
Habit - Bladed
2cm
Randomly oriented barytes crystals up to 8cm long

55. Habit - Reticulate
1cm
Interlocking framework structure resembling a delicate snowflake shown by Cerussite from Tsumeb, Namibia

56. Habit – Dendritic/Arborescent
Manganese oxide dendrites on limestone, Solnhofen, Germany

57. 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

58. The End