Download Mineral Identification

The Silicates
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These are common rock forming minerals.
The minerals we discuss today comprise over
90% of the igneous rocks minerals you will
encounter
The properties they share are hardness (all
above 5), colourful and often good cleavage.
Though they have little economic importance,
they must be studied because they are so
common.
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Despite the fact that I will give you the properties
for each identification test – you should only
focus on the 2 or 3 properties that really give
away this mineral!
I will outline the most important properties by
highlighting them in red!
Identification in the field is more difficult because
the geologist does not have the benefit of perfect
specimens HOWEVER the geologist uses
knowledge of the geological environment and lab
mineral analysis to aid identification
PROPERTY
Colour
Usually clear or white BUT CAN BE ANY COLOUR
Ex. Purple (amethyst), pink (rose Qtz), black
(smoky qtz)
Crystal Form
Hexagonal crystals are common.
Cleavage/Fracture
Conchoidal fracture (curved edges like broken
glass)
Lustre
Non-metallic – usually transparent
Hardness
Very Hard (7) – scratches glass
Specific Gravity
Light-medium
Streak
None
Other
none
PROPERTY
Colour
Dioside-augite (green most common)
But other varieties are black and even white
Crystal Form
Usually massive (no crystals) but sometimes
blocky crystals with a square cross section
Cleavage/Fracture
90˚ cleavage
Lustre
Non-metallic – usually opaque
Hardness
Hard (5-6) – wont scratch glass
Specific Gravity
Light-medium
Streak
None
Other
Most often found in mafic igneous rocks
Pyroxenes are a family of minerals:
Diopside/Augite – CaMgSi2O6 – most common – green in colour
Enstatite – MgSiO3 –
Acmite - NaFe
PROPERTY
Colour
Most often black, dark green is also seen
Crystal Form
Common, long bladed crystals
Cleavage/Fracture
120˚ crystals
Lustre
Non-metallic
Hardness
5-6 does not scratch glass
Specific Gravity
Light-medium
Streak
none
Other
Easily confused with pyroxene, but the difference
in cleavage is useful.
Amphiboles are a family of minerals:
Hornblende – most common – black in colour – found in granite
Tremolite- found as white or green wheat sheaves (sprays of long
bladed crystals)
PROPERTY
Colour
White often with a pale green cast
Crystal Form
Common flaky masses
Cleavage/Fracture
Perfect – finger easily scratches off small flakes
Lustre
Non-metallic - pearly
Hardness
1 – softest of all minerals – easily scratched with
the fingernail
Specific Gravity
Very light
Streak
none
Other
Greasy feel
Talc is commonly a metamorphic mineral
PROPERTY
Colour
black
Crystal Form
Hexagonal “books”
Cleavage/Fracture
Perfect – layers can be peeled off easily
Lustre
Non-metallic – often very sparkly due to light
reflecting off of many flat crystal surfaces.
Hardness
3 – soft – easily scratched with a knife/nail
Specific Gravity
Very light
Streak
none
Other
Found commonly as fine black flakes in
metamorphic rocks and granites
PROPERTY
Colour
Clear, grey
Crystal Form
Hexagonal “books”
Cleavage/Fracture
Perfect – layers can be peeled off easily
Lustre
Non-metallic – often very sparkly due to light
reflecting off of many flat crystal surfaces.
Hardness
3 – soft – easily scratched with a knife/nail
Specific Gravity
Very light
Streak
none
Other
Found commonly as fine white flakes in
metamorphic rocks. A common constituent of
schist
PROPERTY
Colour
Golden brown
Crystal Form
Hexagonal “books”
Cleavage/Fracture
Perfect – layers can be peeled off easily
Lustre
Non-metallic – often very sparkly due to light
reflecting off of many flat crystal surfaces.
Hardness
3 – soft – easily scratched with a knife/nail
Specific Gravity
Very light
Streak
none
Other
Found commonly as fine gold flakes in
metamorphic or igneous rocks. A common
constituent of schist
PROPERTY
Colour
Typically pinkish, red, orange (but can be white or
green)
Crystal Form
Rare – usually massive
Cleavage/Fracture
Perfect – 2 directions 90˚ apart
Forms blocky cleavage fragments
Lustre
Non-metallic – can be shiny due to flat cleavage
surfaces
Hardness
6 – hard – cannot be scratched with a knife/nail
Specific Gravity
medium
Streak
none
Other
Feldspars are the most common mineral in the
Earth’s crust. Often contains wormy “Perthite”
intergrowths. Common in Felsic rocks
Feldspars are a mineral family:
Orthoclase (KAlSi3O8); Albite (NaAlSi3O8)
and Plagioclase (CaAl2Si2O8)
PROPERTY
Colour
Typically black, dark grey. Albite is white or
cream coloured.
Crystal Form
Rare – usually massive
Cleavage/Fracture
Perfect – 2 directions 90˚ apart
Forms blocky cleavage fragments
Lustre
Non-metallic – can be shiny due to flat cleavage
surfaces
Hardness
6 – hard – cannot be scratched with a knife/nail
Specific Gravity
medium
Streak
none
Other
Most common mineral in the Earth’s crust.
Common twinning causes a record groove effect
to be seen on crystal faces – look carefully!
Feldspars are a mineral family:
Orthoclase (KAlSi3O8); Albite (NaAlSi3O8)
and Plagioclase (CaAl2Si2O8)
PROPERTY
Colour
Olive green – sometimes brownish if Fe
substitutes for Mg.
Crystal Form
Rare – usually massive – forms sugary vitreous
(glassy) grains
Cleavage/Fracture
Hard to see (perfect in the rare good crystal)
Lustre
Non-metallic – vitreous
Hardness
6 – hard – cannot be scratched with a knife/nail
Specific Gravity
medium
Streak
none
Other
Most commonly found in basalts and other mafic
rocks. Common mineral in Hawaiian volcanoes.
*Often written as (Mg,Fe)SiO4
Mr. Snyder did his M.Sc thesis on this
mineral: (NiMgSiO4)
PROPERTY
Colour
Most commonly deep red to black – but can be
many colours.
Crystal Form
Common almost spherical irregular crystals.
Perfect crystals are dodecahedrons (12-sides) and
trapezohedrons (24-sides)
Cleavage/Fracture
Hard to see (perfect in the rare good crystal)
Lustre
Non-metallic – vitreous
Hardness
7.5 – very hard – scratches glass
Specific Gravity
medium
Streak
none
Other
Commonly seen as red spherical grains in
metamorphic rocks. They grow at high pressures
and temperatures. Can be used as gems due to
hardness. Used to make sandpaper
Garnets are a large family of minerals with a wide variety
of chemical compositions – ex. Pyrope (Mg3Al2Si3O12)
PROPERTY
Colour
Bright blue
Crystal Form
Almost always massive
Cleavage/Fracture
none
Lustre
Non-metallic – opaque
Hardness
6 –hard – cannot scratch glass
Specific Gravity
medium
Streak
none
Other
Often found in a matrix of a white mineral.
Found in rare sodium rich rocks
Sodalite is rare, but is included here because it is
Ontario’s provincial gemstone. Found near Bancroft.