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Silicates
JD Price
Silicate Structure
Silicate Structure
(SiO2)
Shortcuts to mineral formulae
W cations with 8- (Ca 2+, Fe 2+, Mn2+,
Na+) to 12-fold coordination (K+, Ba 2+)
X divalent cations in 6-fold coordination
(Ca 2+, Mg 2+, Mn 2+, Fe2+)
Y tri- or quadrivalent in 6-fold
coordination (Al3+, Fe3+, Ti4+)
Z tri- or quadrivalent in 4-fold
coordination (Al3+, Fe3+, Si4+, Ti4+)
Components of the earth
• Continental crust - felsic minerals (e.g.
quartz and feldspar)
• Oceanic crust - felsic + mafic (e.g.
feldspar, pyroxene, hornblende)
• Mantle - mafic minerals (e.g. olivine and
pyroxene)
Nesosilicates
Olivine
Forsterite Mg2SiO4
Fayalite Fe2SiO4
Image from mineral.galleries.com
Solid solution
With continued
crystallization,
what happens to
the composition
of the olivine s.s.
and the melt?
Image from Klein and Hurlbut, 1985
Olivine structure
Image from Klein and Hurlbut, 1985
Garnet
X3Y2(ZO4)3
Spessartine Mn3Al2(SiO4)3
Almandine Fe3Al2(SiO4)3
Pyrope Mg3Al2(SiO4)3
Grossular Ca3Al2(SiO4)3
Uvarolite Ca3Cr2(SiO4)3
Andradite Ca3Fe2(SiO4)3
Image from mineral.galleries.com
Habits
Image from Klein and Hurlbut, 1985
Garnet Structure
Image from Klein and Hurlbut, 1985
Other nesosilicates and
subsaturates
Aluminosilicate Al2SiO5 {AlAl(SiO4)O}
Andalusite - Sillimanite - Kyanite
Staurolite (Fe,
Mg,Zn)2Al9[(Si,Al)4O16]O6(OH)2
Zircon Zr(SiO4)
Titanite CaTiSiO5
Topaz Al2SiO4(F,OH)2
Image from mineral.galleries.com
Oldest Material
Recent evidence shows that
the oldest materials on earth,
Jack Hills Zircons, are 4.4
biliion years old.
These record crystallization
temperatures in the 600-750 ºC
range - implying wet
magmatic conditions and the
possible establishment of the
hydrosphere.
NASA’s Earth Observatory
Titanite and fluorite stability
Testing titanite stability as
a function of fluorine
Mount Scott Granite SQ-1 123a concentration in the melt
Titanite and Fluorite
CaTiO5 (Ttn) + F2 TiO2
+ ½ O2
(melt or ilm) +
CaF2 (Fl) + SiO2 (melt or qtz)
Titanite and Fluorite
8
The stability of
these two phases
in a natural
granite, ASI near
1
ASI measures the
ratio of Na, K, and
Ca to Al
F may easily
complex with Al could control its
involvement in
reactions
7
6
5
4
3
2
1
0
0.0
0.5
1.0
Fm (wt.%)
1.5
2.0
Sorosilicates
Hemimorphite Zn4(Si2O7)(OH)2 H2O
Epidote Ca2(Al,Fe)3(SiO4)(Si2O7)(O,OH)
Image from mineral.galleries.com
Image from Klein and Hurlbut, 1985
Cyclosilicates
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Beryl - Be3Al2(Si6O18)
Tourmaline WX3Y6(BO3)3(Si6O18)(OH,F)4
Image from mineral.galleries.com
Tourmaline
Image from
Balckburn and
Dennen, 1988
Pegmatites
We’ve covered
pegmatites before.
But it’s worth
reviewing because of
it’s import to many
ring silicates. A
movie that illustrates
the timing and
processes of
pegmatite (from Jill
Banfield’s website,
Cal-Berkeley)
QuickTime™ and a
Graphics decompressor
are needed to see this picture.
Ultramafic (very Mg-Fe rich) rocks
Coarse: Dunite, Lherzolite, Harzburgite, Periodotite
Fine: Kimberlite
Minerals: these are rocks largely comprised of olivine
(isolated silicate) and pyroxene (single-chain silicate)
Origin: solidification of early Earth
Location: the mantle
Univ. North Carolina, Atlas of rocks,
minerals, and textures website
Obduction
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Henri de la Boisse
Classification
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Kimberlite, lamprophyre
Volcanic ultramafic
rock. Very rare, and
sourced straight from
the mantle - eruption
rates must be
impressive. Some are
diamondiferous.
Univ. North Carolina, Atlas of rocks,
minerals, and textures website
Mafic (Mg-Fe rich) Rocks
Coarse: Gabbro, Anorthosite
Fine: Basalt
Minerals: Largely olivine (isolated), pyroxene (singlechain), and feldspar (framework).
Origin: Partial melting of the mantle
Oceanic crust, oceanic islands, the moon.
Anorthosite
Univ. North Dakota, Plutonic Images
Univ. North Carolina, Atlas of rocks, minerals,
and textures website
Basalt
Gabbro
Univ. North Carolina, Atlas of rocks, minerals,
and textures website
Beryl
Image from
Balckburn and
Dennen, 1988
Metamorphism
Mineralogical and structural
response of a rock to
imposed conditions of T & P
markedly different from those
of its origin.
Metamorphism
•Could happen to any rock
•Occurs in the solid state
•Fluids may be present
•A continuous process
Closed system Isochemical
The end product is a function of
the starting material and the P-T
history.
Quartzite
Conditions: low to moderate P & T
Minerals: quartz (framework)
Origin: sandstone
Marble
Conditions: low to moderate P & T
Minerals: calcite or dolomite (carbonates)
Origin: biogenic or chemogenitc (limestone or
dolostone)
Univ. of North Carolina Web atlas of metamorphic textures
Silica Polymorphs
More ‘morphs
CaCO3
AlSiO5
Open system Metasomatism
The end product is is a function of
the starting material, the P-T
history, and fluid volume and
composition.
Some reactions
Quartz-bearing Dolostone
CaMg(CO3)2 + 2SiO2 = CaMgSi2O6 + 2CO2
Limestone + silica in fluid
Ca(CO3)2 + 2SiO2 (aq) = CaSiO3 + 2CO2
Why the transformation?
If you have a rock made of CaO,
MgO, SiO2, CO2, the group of
minerals that form at a given P&T is
the lowest overall G of all
possibilities.
Each mineral is a phase.
Each group of minerals is an
assemblage.
Metamorphic Facies
P kbar
All of these
conditions are
relevant to
metamorphism on
Earth. Note that
some rocks will
melt at lower T
than others
Grade - the degree
of advancing
metamorphic
conditions
Spear, 1993
Heat and pressure
Going down?
Geothermal Gradient
Burial Path
Path back
Prograde - the
pathway to peak
conditions.
Retrograde - the
path from peak
conditions.
Grid
This can be mapped
out using the
thermodynamic
boundaries for
reactions.
The presence or
absence of minerals
in rocks can
illuminate the P-T
pathway.
Spear, 1993
Spear, 1993
Reaction 8
Pg + Qtz = As + Ab+
H2O
Pg = NaAl2(Si3)O10(OH)2
Paragonite Amphibole
Reaction 11
Ms = Crn + Kfs + H2O
Ms = KAl2(Si3Al)O10(OH)2
Diagram from E.B Watson
Slate
Conditions: relatively low P & T
Minerals: very fine grained feldspar (framework) and
mica (sheet silicates)
Origin: fine grained clastic (shale)
Univ. of North Carolina Web atlas of
metamorphic textures
Schist
Conditions: moderate P & T
Minerals: fine grained micas (sheet), some feldspar or
quartz (framework) may include garnet, staurolite
(isolated)
Origin: fine grained clastic (shale)
Gneiss
Conditions: high P & T
Minerals: feldspar and quartz (framework), mica (sheet)
Origin: clastic (shale and sandstone), felsic (rhyolite,
granite)
Univ. of North Carolina Web atlas of
metamorphic textures
Fabric
Gneiss
Granite
Making granites?
Metamorphic
materials that
exceed the
solidus for
the system
start to melt
Migmatite
Photo by Mike Brown (UMD)