physical properties minerals a ian kenyon

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The Physical Properties Of Minerals WJEC AS Geology I.G.Kenyon Colour • 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 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 •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 Examples of colour variation in Fluorite Colour 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 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 Fluorite •All transparent minerals are also translucent 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 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 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, 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 [...]... 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... 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... 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... 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 (Zinc Blende) Sphalerite, opal and amber show resinous lustre 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... 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... 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... 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... 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...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... 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
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