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

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Presentation on theme: "The Physical Properties Of Minerals WJEC AS Geology I.G.Kenyon."— Presentation transcript:

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

2 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

3 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

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 Colour 4 Examples of colour variation in Fluorite

6 Colour 5 Plagioclase feldspar All these minerals are grey or white in colour QuartzCalcite BarytesFluoriteGypsum

7 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

8 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

9 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 Quartz – Vitreous Lustre 2cm

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. Metallic lustre often tarnishes to a dull lustre MalachiteGalena

12 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)

13 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)

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

15 Adamantine Lustre The lustre of a diamond 5mm

16 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 greenHaematite – cherry redIron Pyrite – greenish black Galena – lead greySphalerite – pale brownLimonite – yellowish brown

19 Metallic Ore Minerals – Characteristic Streaks

20 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

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

22 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 1.0Diamond 10.0

23 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

24 Moh’s Scale of Hardness 10. Diamond9. Corundum8. Topaz 7. Quartz 6. Orthoclase Feldspar

25 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

26 Moh’s Scale of Hardness 5. Apatite4. Fluorite3. Calcite 2. Gypsum1. Talc

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

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 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 Rose quartz showing conchoidal fracture 5mm

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, 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

34 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

35 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 AugitePlagioclase Feldspar 1cm

36 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

37 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

38 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

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 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 Magnetite Steel pins and magnet attracted to magnetite 1cm

42 Feel A 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 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 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

45 Variations in Habit/Form/Appearance of Minerals


47 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

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

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

50 Habit - Crystallised 1 cm Rhombdodecahedral Garnet Crystals

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

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

53 Habit - Tabular Tabular mass of Barytes crystals 1cm

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

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

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

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