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Minerals
Naturally occurring inorganic substances
that have definite chemical composition and
definite atomic structure
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Physical properties1. Colour
2. Lustre
3. Streak
4. Hardness
5. Cleavage
6. Fracture
7. Tenacity
8. Structure
9. Specific gravity
10. form
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Colour Classification of
mineral based on
colour
characteristics example
Idiochromatic Fairly constant colour related
primarily to the composition
of mineral
Metallic minerals
(copper group)
Allochromatic •Variable colour due to minute
quantities of colouring
impurities
Non metallic minerals
(quartz, calcite, fluorite
and tourmaline)
Pseudochromatic •Shows false colour
•Exhibit play of colours
•Simultaneous refraction and
reflection from the mineral
surface due to impuritiesDownloaded from Ktunotes.in
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Colour
Iredescence – minerals show rainbow colours either in
exterior or interior surface. Eg: limonite, hematite.
Tarnish – phenomenon of change of original colours of
a mineral to some secondary colours at the surface due
to its oxidation. Eg: bornite
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Lustre
Shine of mineral; intensity of reflection of light
from the mineral surface.
Lustre depends on three factors: refractive index of
mineral, absorption capacity of the mineral, nature
of reflecting surface.
Lustre classified into metallic and non-metallic
lustre
Metallic lustres are characteristics of high density,
high refractive index and opaque minerals.
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Lustre
Metallic Shine resembles to metals galena
No
n-
metallic
Adamantine Lustre of diamonds diomond
vitreous Shine typical of glass and ice quartz
pearly Resembles shines of pearls muscovite
silky Like the shine of pure silk gypsum
resinous Shine is oily, waxy or greasy talc
Dull or earthy Shine is almost absent, no light reflected
due to high porous nature
Chalk,
bauxite, clay
Lustre
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Streak
Diagnostic property of coloured minerals.
The colour of a powdered mineral obtained by
scratching or rubbing the mineral over a unglazed
white porcelain plate (streak plate).
Brown – chromite
Black – magnetite
Colour of the mineral may or may not be the same as
its streak.
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Hardness
Resistance, which a mineral offers to an external
deformation action such as scratching, abrasion,
rubbing or indentation.
Hardness – qualitative property
Generally determined by scratching a given mineral
with a mineral of known hardness.
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Mohs’ scale of hardness
Mohs’ Scale of hardness comprises ten minerals
arranged in order of ascending hardness; softest is
assigned a value of 1 and the hardest a value of 10.
Hardness of any mineral can be determined with the
help of minerals from the above scale.
The harder mineral will scratch the softer mineral.
Minerals of equal hardness scratch each
other.(diamond cut diamond)
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hardness Standard mineral
1 talc
2 gypsum
3 calcite
4 fluorite
5 apatite
6 orthoclase
7 quartz
8 topaz
9 corundum
10 diamond
Hardness- Mohs’ scale of hardness
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Mohs’ scale of hardness
Hardness is only a relative property. If talc has H=1 and
quartz H=7, it does not indicate quartz is seven times
harder than talc.
Hardness is anisotropic property – different properties in
different directions.
Hardness decreases on decomposition of a mineral.
Hence, it must checked on unweathered and unaltered
surface only.Downloaded from Ktunotes.in
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Cleavage
Tendency of crystallized mineral to break along a certain
definite directions yielding more or less smooth, plane
surfaces.
Cleavage plane – plane of easiest fractures.
Indicative of directions/ plane of least cohesion in the
atomic constitution of a mineral.
In terms of perfection, cleavage is termed as eminent,
perfect, good, distinct and indistinct.
Eminent cleavage – mineral can be split very easily
yielding extremely smooth surfaces.
Based on direction of cleavage – cubic, rhombohedral,
prismatic, basal and octahedral cleavage.Downloaded from Ktunotes.in
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Fracture
The appearance of broken surface of a mineral in adirection other than that of a cleavage
Common type of fractures are :
1. Even: broken surface is smooth and flat (chert)
2. Uneven: broken surface is irregular, full of ridges anddepressions (fluorite)
3. Conchoidal: broken surface shows broadly concentricrings (quartz)
4. Splintry: mineral breaks with rough woody fractureresulting in rough projection at the surface (kyanite)
5. Hackly: the broken surface is highly irregular withnumerous sharp, fine, pinching projections (nativecopper)
6. Earthy: smooth, soft and porous surface (chalk)
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earthy
Chonchoidal
hackly
splintry
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Tenacity
Behaviour of a mineral towards the forces that tend to break,
bend or cut or crush.
Sectile: mineral can cut with a knife (talc)
Malleable: minerals flatens to sheet when hammered(silica)
Brittle: minerals crumbles to grains when hammered
Flexible: can bent: chlorite
Plastic
elastic
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Structure Physical make-up of a mineral
1. Tubular
2. Elongated
3. Bladed
4. Lamellar
5. Foliated
6. Fibrous
7. Radiating
8. Granular
9. Globular
10. Reniform
11. Mammillary
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Fibrous - Asbestos
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Specific gravity
The ratio between the density of a mineral and that of
water at 4oC.
Also termed as relative density
Composition: non-metallic – low sp. Gravity
Atomic constitution: minerals with atoms of greater
atomic radii – low sp. gravity
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Form
Crystallized: well defined crystals
Crystalline: well defined crystals are absent. But, shows
tendency towards crystallization
Amorphous: neither a crystal face nor a cleavage is seen
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Miscellaneous
Magnetism
Electricity – electric charge may develop by heating –
pyroelectric minerals , electric charge develop by
pressure – piezo electric
Fluorescence – property of glowing or emitting light
when they are exposed to radiation
Phosphorescence - light is emitted by mineral not
during the act of exposure to radiation but after the
substance is transferred rapidly to a dark place.
Fusibility – behaviour on heating
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