Framework Silicates

& Building the Crust

JD Price

Sil

icat

e S

truct

ure

Sili

ca

te S

tructu

re

(SiO2)

Zeolites

Natrolite - mineralien-verkauf.de

Zeolites have cage-like

structures that store water (or

other large molecules).

Typically found in amygdales

within basaltic rocks.

Natrolite Na2[Al2Si3O10] 2(H2O)

Thomsonite NaCa2Al5Si5O20

6(H2O)

Chabazite CaAl2Si4O12 6H2O

Natrolite

Water is located

in open

structures

within the

lattice. But note

the framework

of Si-O

tetrahedra.

SiO2

Quartz | Coesite | Stishovite

mineral.galleries.com

De minuscules cristaux de quartz ont une

disposition radiale autour de la coesite : ceci

montre que le quartz se forme au d師riment de la

coesite (LPA)

High birefringence Stishovite in coesite, synthetically grown

by J. Mosenfelder, CalTech.

Cristobalite nodules

within a vitrophere -

Snowflake obsidianRockhoundblog.com

gemandmineral.com

Tridymite crystal, from

deposit associated with

Mono Lakes volcano.

SiO2

Image from Blackburn and Dennen, 1988

Silica Polymorphs

Beta quartz form

from Bishop Tuff,

CA. Note that

crystal is actually

alpha quartz

CalTech

SiO2Agate | Jasper | Chert |

Flint | Chalcedony

Agate is name applied

banded rocks made of

microcrystalline quartz,

typically made of fibrous

quartz, called

chalcedony. Colors result

from impurities within the

crystals

Chert and flint are

homogenous chalcedony

- often related to

fossilizationLace agate / www.lhconklin.com

Silica saturation in water is quite low at

ambient conditions, pH=7, but increases

rapidly with T. Note that amorphous silica

has a higher solubility than quartz, Rimstidt

& Cole 1983.

• In the absence of organic ligands or fluoride,quartz solubility is relatively low in natural waters.

• Below pH 9, the dissolution reaction is:

SiO2(quartz) + 2H2O(l) ! H4SiO40

• Quartz is frequently supersaturated in naturalwaters because quartz precipitation kinetics areslow.

• Quartz saturation does not usually control theconcentration of silica in low-temperature naturalwaters.

opal

Image from Klein and Hurlbut, 1985

Opals are made of

spheres of silica and

water - not exactly a

mineral.

Feldspars

Plagioclase Feldspar

Anorthite (CaAl2Si2O8)

Alkali Feldspar

Albite (NaAlSi3O8)

Orthoclase (KAlSi3O8)

Images from mineral.galleries.com

Pla

gio

cla

se S

olid

So

luti

on Liquid

Liquid +

Plag

Plag

M

T

T

Co

mm

on

Feld

sp

ar

Tern

ary

Feldspar Twinning

Image from Blackburn and Dennen, 1988

Feldspathoids

Leucite (KAlSi2O6)

Nepheline (NaAlSiO4)

Sodalite (Na4Al3Si3O12Cl)

Image from mineral.galleries.com

dkimages

Foids

Nephaline vs. Albite

NaAlSiO4 NaAlSi3O8

1 Si per NaAl 3 Si per NaAl

Leucite vs. Orthoclase

KAlSi2O6 KAlSi3O8

2 Si per NaAl 3 Si per NaAl

Lower Si than feldspar counterparts.

Arise from melts that are low in SiO2

undersaturated.

Igneous nomenclature“archaic” texture discrimination

boundary at 1mm.

An alkali feldspar and quartzdominant rock with an averagegrainsize >1 mm is a granite, <1mm is a rhyolite.

> 1mm Phaneritic

< 1 mm Aphanitic

Glassy - vitrophyric

Rhyolite

Dacite

Andesite

Basalt

Phonolite

Granite

Diorite

Tonalite

Gabbro

Anorthosite

Foid Syenite

After Strekeisen, 1978

Rock names

Mean grain size < 1mmMean grain size > 1mm

Igneous rocks are the end products of partial

melting in the earth, so their compositions are

determined by the chemical processes

involved in melting earth materials

Source, conditions, and degree of melting

Partial melting in S.W. Maine

Tectonic thinning of the crust

Tectonic increase of

crustal thickness

(after Burnham and Davis,

1974)

H2O

and

melting

(after Burnham and Davis,

1974)

H2O

and

melting

IUGS Phaneritic

Phaneritic rock

classification -

note that the

scheme

precludes the

coexistence of

foids and SiO2.

Aphanitic rock

classification -

compare with

the previous

slide.

This is a nice

diagram, but

the fine-grained

nature of these

rocks can

impede its use.

IUGS Aphanitic

IUGS TAS

If you really want to classify a volcanic rock - powder

it and analyze its composition. How does this relate

to the previous slide? What of plutonic rocks?

Again – our understanding

of the earth falls off with

depth.

Mafic (Mg-Fe rich) Rocks

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

Anorthosite

Univ. North Dakota, Plutonic Images

Basalt

Gabbro

Univ. North Carolina, Atlas of rocks, minerals,

and textures website

Plume theory

Earth is hottest at its

core, the release of heat

may be convective -

mass transferred. Some

masses may move as

buoyant ductile bodies.

Diagram by J. Tarney

End of the ride… if such plumes exist, they would

become more or less stalled at the transition to

brittle mantle.

Diagram by J. Tarney

If most of the heat needed

for melting is deep within

the earth, why are there

volcanoes?

Magma is driven upwards.

Parameters - density, viscosity

Pd = Ph + Po - Pvis - Sh

Hogan et al., 1998

Ascent of Magmas

Kilauea, Hawaii

March, 1996

20-406Figure 20.19

A cross section of a shield

volcano

Generally low Si

Low volatile

High rate

Mauna Kea, Hawaii

Pu’u Hulu

Mauna Loa, Hawaii

Pahoehoe

Oceanic rifting

Continental Rift

Example: the East African Rift

http://jsc.nasa.gov

STS-32

Rifting - Birth of an ocean

Diagram by J. Tarney

Diagram by J. Tarney

Cartoon to left shows

some general attributes

of rift formation

Presumably, the tension

that drives rifting is not

well understood.

Intermediate rocks (less Fe-Mg, more Na-K-Si-Al)

Coarse: Granodiorite, Tonalite Fine: Andesite, Dacite

Minerals: Feldspar (framework), pyroxene (single

chain), amphibole (double chain), and quartz

(framework)

Origin: liquid fraction of crystallizing mafic liquids.

Location: Convergent tectonic settings

Tonalite

Subduction: Island Arcs

Example: the Aleutians

Subduction: Continental Arcs

Example: the Cascades

Mount Saint Helens:

May 18, 1980 0832

Q: What is a

volcanic dome?

View from South Rim, June 1991

Crater Lake

Oregon

Felsic (K, Na, and Si rich) rocks

Coarse: Granite

Fine: Rhyolite

Minerals: Feldspar and quartz (framework), with small

amounts of amphibole (double chain) and mica (sheet).

Origins: Partial melting of crustal rocks and liquid

fraction of crystallizing intermediate liquids.

Location: continental magmatism

Granite

Univ. North Carolina, Atlas of rocks,

minerals, and textures website

Rhyolite

Vitrophyre

©Pam Gore, GPC

Felsic rocks

Barker, 1983 after

Bowen, 1928

Fractional Crystallization

Pegmatite Pegmatites are

typically coarsely

crystalline - may

contain rare

minerals. Most

result from

extreme

fractionation.

Slow cooling

Low nucleation

Lots of H2O