Mineral IdentificationThe Silicates

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.

The Silicates

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

Identification notes

Identifying minerals in thin section

PROPERTY

Colour Usually clear or white BUT CAN BE ANY COLOUREx. 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

1) Quartz (SiO2)

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

2) Pyroxene (XYSi2O6)

Pyroxenes are a family of minerals:Diopside/Augite – CaMgSi2O6 – most common – green in colourEnstatite – 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.

3) Amphiboles (A2Z5Si8O22(OH)2)more simply a complex mafic silicate

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 feelTalc is commonly a metamorphic mineral

4) Talc (Mg3Si4O10(OH)2)Mg sheet silicate

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

5) Biotite (mafic mica)

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

6) Muscovite (felsic mica)

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

7) Phlogopite (intermediate mica)

PROPERTY

Colour Typically pinkish, red, orange (but can be white or green)

Crystal Form Rare – usually massive

Cleavage/Fracture Perfect – 2 directions 90˚ apartForms 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

8) Orthoclase (KAlSi3O8) (K-spar)

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˚ apartForms 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!

9) Plagioclase (CaAl2Si2O8)

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.

10) Olivine (Mg2SiO4)*

*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

15) Garnet (A3B2Si3O12)

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

15) Sodalite (Na8Al6Si6O24Cl2)

Sodalite is rare, but is included here because it is Ontario’s provincial gemstone. Found near Bancroft.