Silicates

JD Price

Sil

icat

e S

truct

ure

Sili

ca

te S

tructu

re

(SiO2)

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+)

Shortcuts to mineral formulae

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)

NesosilicatesOlivine

Forsterite Mg2SiO4

Fayalite Fe2SiO4

Image from mineral.galleries.com

Image from Klein and Hurlbut, 1985

With continued

crystallization,

what happens to

the composition

of the olivine s.s.

and the melt?

Solid solution

Image from Klein and Hurlbut, 1985

Olivine structure

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

Image from Klein and Hurlbut, 1985

Habits

Image from Klein and Hurlbut, 1985

Garnet Structure

Other nesosilicates and

subsaturates

Zircon Zr(SiO4)

Titanite CaTiSiO5

Topaz Al2SiO4(F,OH)2

Aluminosilicate Al2SiO5 {AlAl(SiO4)O}

Andalusite - Sillimanite - Kyanite

Staurolite (Fe,

Mg,Zn)2Al9[(Si,Al)4O16]O6(OH)2

Image from mineral.galleries.com

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

Oldest Material

Mount Scott Granite SQ-1 123a

Testing titanite stabilityas a function of fluorineconcentration in the melt

Titanite and fluorite stability

CaTiO5 (Ttn) + F2 ! TiO2 (melt or ilm) + CaF2 (Fl) + SiO2 (melt or qtz)

+ ! O2

Titanite and Fluorite

Fm (wt.%)

H2O

m (

wt.

%)

0.0 0.5 1.0 1.5 2.00

1

2

3

4

5

6

7

8

The stability ofthese two phasesin a naturalgranite, ASInear 1

ASI measures theratio of Na, K,and Ca to Al

F may easilycomplex with Al -could control itsinvolvement inreactions

Titanite and Fluorite

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

CyclosilicatesCyclosilicates

Beryl - BeBeryl - Be33AlAl22(Si(Si66OO1818))

Tourmaline -Tourmaline -

WXWX33YY66(BO(BO33))33(Si(Si66OO1818)(OH,F))(OH,F)44

Image from mineral.galleries.com

Image from

Balckburn and

Dennen, 1988

To

urm

ali

ne

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)

Pegmatites

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

Ultramafic (very Mg-Fe rich) rocks

Henri de la Boisse

Obduction

Classification

Univ. North Carolina, Atlas of rocks,

minerals, and textures website

Kimberlite, lamprophyre

Volcanic ultramafic

rock. Very rare, and

sourced straight from

the mantle - eruption

rates must be

impressive. Some are

diamondiferous.

Coarse: Gabbro, Anorthosite Fine: Basalt

Minerals: Largely olivine (isolated), pyroxene (single-

chain), and feldspar (framework).

Origin: Partial melting of the mantle

Oceanic crust, oceanic islands, the moon.

Univ. North Carolina, Atlas of rocks, minerals,

and textures website

AnorthositeAnorthosite

Univ. North Dakota, Plutonic Images

Mafic (Mg-Fe rich) Rocks

BasaltBasalt

GabbroGabbro

Univ. North Carolina, Atlas of rocks, minerals,

and textures website

Image from

Balckburn and

Dennen, 1988

Be

ryl

Mineralogical and structural

response of a rock to

imposed conditions of T & P

markedly different from those

of its origin.

Metamorphism

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.

Conditions: low to moderate P & T

Minerals: quartz (framework)

Origin: sandstone

Quartzite

Univ. of North Carolina Web atlas of metamorphic textures

Conditions: low to moderate P & T

Minerals: calcite or dolomite (carbonates)

Origin: biogenic or chemogenitc (limestone or

dolostone)

Marble

Silica Polymorphs

More ‘morphs

CaCO3AlSiO5

The end product is is a function of

the starting material, the P-T

history, and fluid volume and

composition.

Open system Metasomatism

Quartz-bearing Dolostone

CaMg(CO3)2 + 2SiO2 = CaMgSi2O6 + 2CO2

Limestone + silica in fluid

Ca(CO3)2 + 2SiO2 (aq) = CaSiO3 + 2CO2

Some reactions

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.

Why the transformation?

P k

bar

Spear, 1993

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

Metamorphic Facies

Heat and pressure

Geothermal Gradient Burial Path

Going down?

Path back

Prograde - the

pathway to peak

conditions.

Retrograde - the

path from peak

conditions.

Spear, 1993

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.

Grid

Spear, 1993

Reaction 11

Ms = Crn + Kfs + H2O

Ms = KAl2(Si3Al)O10(OH)2

Reaction 8

Pg + Qtz = As + Ab+

H2O

Pg = NaAl2(Si3)O10(OH)2

Paragonite Amphibole

Diagram from E.B Watson

Univ. of North Carolina Web atlas of

metamorphic textures

Conditions: relatively low P & T

Minerals: very fine grained feldspar (framework) and

mica (sheet silicates)

Origin: fine grained clastic (shale)

Slate

Conditions: moderate P & T

Minerals: fine grained micas (sheet), some feldspar or

quartz (framework) may include garnet, staurolite

(isolated)

Origin: fine grained clastic (shale)

Schist

Univ. of North Carolina Web atlas of

metamorphic textures

Conditions: high P & T

Minerals: feldspar and quartz (framework), mica (sheet)

Origin: clastic (shale and sandstone), felsic (rhyolite,

granite)

Gneiss

GraniteGranite

GneissGneiss

Fabric

Photo by Mike Brown (UMD)

Metamorphic

materials that

exceed the

solidus for

the system

start to melt

Migmatite

Making granites?