Lecture 6 F 2014 - Silicates
Lecture 6 F 2014 - Silicates
Lecture 6 F 2014 - Silicates
Si and Al are about the same size, but have different charges
(SiO4) -4 and (AlO4) -5
Feldspar
Plagioclase Orthoclase
(NaAlSi3O8 (KAlSi3O8)
CaAl2Si2O8)
Rock-Forming Minerals
About 20 common minerals make up
most rocks
Silicates dominate
Quartz, Feldspars, Mica, Amphiboles,
Pyroxenes
Silicates can also be subdivided by
composition
Ferromagnesium High in Iron and Magnesium
Sialic High in Silica and Aluminum
Ferromag Minerals
MAFIC
Silicate minerals rich in iron and
magnesium
Relatively high density and higher
crystallization temperatures
Olivine Isolated SiO4 Tetrahedra
Pyroxenes Single Chains
Amphiboles Double Chains
Mica biotite Sheet silicates
Sialic Minerals
FELSIC
Silicate minerals rich in silicon and
aluminum
Relatively low densities and low crystallization
temperatures
Quartz Framework Silicates
Feldspars - Framework Silicates
Potassium feldspar
Plagioclase feldspar
Mica muscovite Sheet silicates
Sialic Minerals
Minerals
Naturally occurring, inorganic solid with a
specific chemical formula and definite
crystal structure
The crystal structure and the elements
involved indicate much about the
conditions in which a mineral formed
Silicate minerals (SiO4), comprise about
95 % of earth material
Minerals and Temperature
Non-silicate minerals, in general, form at lower
temperature and pressure conditions than do
silicate minerals
Oxides, Halides, Hydroxides, Carbonates, Sulfates,
etc
Ferromag silicate minerals form at higher
temperatures than do Sialic minerals
Ferromags high in Fe and Mg (as well as Ca)
Sialic high in Si, Al and O, little or no Fe, Mg, Ca
Rocks = Aggregates of minerals
Therefore, minerals provide clues as to how the rocks
may have formed
What parts of earth do you expect lower Pressures and
Temperature? Where would you expect to find Higher P and T?
Further, the presence or absence of certain minerals
may also indicate the processes that have acted upon
our rocks, over time
How minerals are arranged in rocks (their relationship to
one another) also provides insight towards the rock-
forming processes
Rocks are classified by: 1) Texture the size, shape and
arrangement of crystals, and 2) Composition
(mineralogy)
Example of a rock and the mineral composition + texture
3 Main Categories of Rocks
Each category will be further subdivided
Igneous new rocks being made
Form from crystallizing magma/lava
High temperature minerals
Sedimentary weathering of pre-existing rocks
Form from weathering, erosion and deposition of
sediments and minerals
Chemical precipitates (Abiotic and biotic)
Lower temperature minerals
Metamorphic alteration of pre-existing rocks
Form from combinations of Heat, Pressure, + Fluids
High T and/or P minerals
Recrystallize, but DO NOT melt!
The role of Plate Tectonics
to Rock formation
Rock Cycle
Igneous Rocks
Temperature
And Pressure
Igneous Rocks
From Fire
Crystallization of Magma/Lava
2 subcategories (with further subdivision)
Extrusive (Volcanic)
Lava at or very near the earths surface
Intrusive (Plutonic)
Magma at depth in earths crust
Silicate minerals dominate; some oxides
Igneous Rocks Texture
Texture size, shape and arrangement of
minerals
Igneous rocks > Interlocking crystals
Size of the crystals help us make the first
division of Igneous Rocks
Crystal size determined by cooling rates of
magma
Extrusive (Volcanic)
Intrusive (Plutonic)
The Nature of Igneous Rocks
Form from Magma
Hot, partially molten mixture of solid
liquid and gas
Mineral crystals form in the magma
making a crystal slush
Gases - H2O, CO2, etc. - are
dissolved in the magma
Magma is less dense than solid rock
The Nature of Igneous Rocks
Magma vs. Lava
Magma is molten rock beneath the
surface
Lava is molten rock that has reached
the surface
Magma solidifies to form intrusive
igneous rocks
Lava solidifies to form extrusive
igneous rocks
Igneous Rocks Composition
Composition = the mineralogy of the
magma and hence, rocks
While texture allows us to separate
Plutonic from Volcanic rocks, composition
allows us to identify and name the rocks in
each category
Composition based on Silica content and
the relative abundance of Ferromag and
Sialic minerals
Name change between Mineral composition
and Magma/Rock composition
Slow cooling
and
crystallization
Igneous Rocks Texture
Texture size, shape and arrangement of
minerals
Igneous rocks > Interlocking crystals
Size of the crystals help us make the first
division of Igneous Rocks
Crystal size determined by cooling rates of
magma
Rapid cooling
and
crystallization
Slow cooling
and
crystallization
Slow cooling Intrusive (plutonic)
Large, visible interlocking crystals
Phaneros = to see
PHANERITIC texture
Rapid cooling Extrusive (volcanic)