10 inosilicates

Inosilicates: single chains- pyroxenes

Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)

Diopside: CaMg [Si2O6]

b

Where are the Si-O-Si-O chains??



b



Perspective view


M1 octahedron


M1 octahedron

(+) type by convention

(+)


M1 octahedron

This is a (-) type

(-)


T

M1

T

Creates an “I-beam”

like unit in the

structure.


T

M1

T

Creates an “I-beam”

like unit in the

structure

(+)

The pyroxene

structure is then

composed of

alternating I-beams

Clinopyroxenes have

all I-beams oriented

the same: all are (+)

in this orientation

(+)

(+)(+)

(+)(+)


Note that M1 sites are

smaller than M2 sites, since

they are at the apices of the

tetrahedral chains

The pyroxene

structure is then

composed of

alternation I-beams

Clinopyroxenes have

all I-beams oriented

the same: all are (+)

in this orientation

(+)

(+)(+)


(+)(+)

The tetrahedral chain

above the M1s is thus

offset from that below

The M2 slabs have a

similar effect

The result is a

monoclinic unit cell,

hence clinopyroxenes


c

a

(+) M1

(+) M2

(+) M2

Orthopyroxenes have

alternating (+) and (-)

I-beams

the offsets thus

compensate and result

in an orthorhombic

unit cell

This also explains the

double a cell dimension

and why orthopyroxenes

have {210} cleavages

instead of {110) as in

clinopyroxenes (although

both are at 90o)


c

a

(+) M1

(-) M1

(-) M2

(+) M2

Pyroxene Chemistry

The pyroxene quadrilateral and opx-cpx solvusCoexisting opx + cpx in many rocks (pigeonite only in volcanics)

Diopside Hedenbergite

Wollastonite (pyroxenoid)

Enstatite Ferrosilite

orthopyroxenes

clinopyroxenes

pigeonite (Mg,Fe)2Si2O6 Ca(Mg,Fe)Si2O6

pigeonite

orthopyroxenes

Solvus

1200oC

1000oC

800oC

Augite

Miscibility Gap

Pyroxene Chemistry

“Non-quad” pyroxenesJadeite

NaAlSi2O6

Ca(Mg,Fe)Si2O6

Aegirine

NaFe3+Si2O6

Diopside-Hedenbergite

Ca-Tschermack’s

molecule CaAl2SiO6

Omphaciteaegirine-

augite

Augite

Spodumene:

LiAlSi2O6

Pyroxenoids“Ideal” pyroxene chains

with 5.2 A repeat (2

tetrahedra) become

distorted as other cations

occupy VI sites

Wollastonite

(Ca M1)

3-tet repeat

Rhodonite

MnSiO3

5-tet repeat

Pyroxmangite

(Mn, Fe)SiO3

7-tet repeat

Pyroxene

2-tet repeat

7.1 A12.5 A

17.4 A

5.2 A

Inosilicates: double chains- amphiboles

Tremolite (001) view blue = Si purple = M1 rose = M2 gray = M3 (all Mg)

yellow = M4 (Ca)

Tremolite:

Ca2Mg5 [Si8O22] (OH)2

b


Hornblende:

K,<1(Ca, Na)2-3 (Mg, Fe,

Al)5 [(Si,Al)8O22] (OH)2

b

Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2

light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na+K)

little turquoise ball = H



light blue = M3 (all Mg, Fe)

Hornblende:

K,<1(Ca, Na)2-3 (Mg, Fe,

Al)5 [(Si,Al)8O22] (OH)2

Same I-beam

architecture, but

the I-beams are

fatter (double

chains)


b

(+) (+)

(+)

(+)

(+)

Same I-beam

architecture, but

the I-beams are

fatter (double

chains)

All are (+) on

clinoamphiboles

and alternate in

orthoamphiboles


light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na,K)


Hornblende:

K,<1(Ca, Na)2-3 (Mg, Fe,

Al)5 [(Si,Al)8O22] (OH)2



light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na, K)


Hornblende:

K,<1(Ca, Na)2-3 (Mg, Fe,

Al)5 [(Si,Al)8O22] (OH)2

M1-M3 are small sites

M4 is larger (Ca)

A-site is really big

Variety of sites

great chemical range



light blue = M3 (all Mg, Fe) yellow ball = M4 (Ca) purple ball = A (Na)


Hornblende:

K,<1(Ca, Na)2-3 (Mg, Fe,

Al)5 [(Si,Al)8O22] (OH)2

(OH) is in center of

tetrahedral ring where O

is a part of M1 and M3

octahedra

(OH)

See handout for more information

General formula:

W0-1 X2 Y5 [Z8O22] (OH, F, Cl)2

W = Na K

X = Ca Na Mg Fe2+ (Mn Li)

Y = Mg Fe2+ Mn Al Fe3+ Ti

Z = Si Al

Again, the great variety of sites and sizes a great chemical range,

and hence a broad stability range

The hydrous nature implies an upper temperature stability limit

Amphibole Chemistry

Ca-Mg-Fe Amphibole “quadrilateral” (good analogy with

pyroxenes)

Amphibole Chemistry

Al and Na tend to stabilize the orthorhombic form in low-Ca amphiboles, so

anthophyllite gedrite orthorhombic series extends to Fe-rich gedrite in more Na-

Al-rich compositions

Tremolite

Ca2Mg5Si8O22(OH)2

Ferroactinolite

Ca2Fe5Si8O22(OH)2

Anthophyllite

Mg7Si8O22(OH)2Fe7Si8O22(OH)2

Actinolite

Cummingtonite-grunerite

Orthoamphiboles

Clinoamphiboles

Hornblende has Al in the tetrahedral site

Geologists traditionally use the term “hornblende” as a catch-all term for

practically any dark amphibole. Now the common use of the microprobe

has petrologists casting “hornblende” into end-member compositions

and naming amphiboles after a well-represented end-member.

Sodic amphiboles

Glaucophane: Na2 Mg3 Al2 [Si8O22] (OH)2

Riebeckite: Na2 Fe2+3 Fe3+

2 [Si8O22] (OH)2

Sodic amphiboles are commonly blue, and often called “blue

amphiboles”

Amphibole Chemistry

Inosilicates

Cleavage angles can be interpreted in terms of weak bonds in M2

sites (around I-beams instead of through them)

Narrow single-chain I-beams 90o cleavages in pyroxenes while wider

double-chain I-beams 60-120o cleavages in amphiboles

pyroxene amphibole

a

b

10 inosilicates

Engineering

Transcript of 10 inosilicates