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3. The geosphere
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2.1.
Physical properties of minerals
In nature there are more than 4000 known minerals. We can identify some minerals
by studying their physical properties, without analysing their chemical composition.
Physical properties are classified into optical, mechanical and magnetic.
2.1.1.
Optical properties
Understand
11.
Explain the difference between the colour and the streak of a mineral.
A mineral’s
optical properties
relate to how a mineral reacts to light.
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Habit
: some minerals have very characteristic shapes
that reflect their crystalline structure. For example,
pyrite has cubic shapes and aragonite has hexagonal
shapes.
❚
Streak
: is the colour of the powder produced when a
mineral is scratched. The colour of the streak does not
have to be the same as the mineral. For example, solid
quartz can be many different colours, but its streak is
always white.
Pyrite
Aragonite
Galena and its streak
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Colour
: some minerals have a very characteristic
colour. Galena is rusty grey and sulphur is bright
yellow. Other minerals, like quartz, can be found in
different colours, as seen in the images below.
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Lustre
: describes how the mineral reflects light. It can
be metallic if it shines like metals, glassy if it is like
glass, pearly, if it shines like a pearl, diamond-like, or
dull if it doesn’t shine.
Blue quartz
Transparent quartz
Metalic: magnetite
Glassy: fluorite
Pearly: gypsum
Pink quartz
Sulphur
Diamond-like: rutile
Dull: limonite
