Inosilicates (chain)


Common Fe/Mg – bearing silicates
Two common groups




Pyroxenes: single chains
Amphiboles: double chains
Pyroxenes are common in MORB
Amphiboles more common on continents
because of weathering
Pyroxene group





General formula: XYZ2O6
Z/O ratio = 1/3
Z cations usually Si, occasionally Al
Single chain extend along c axis
Chains are stacked along a axis,
alternating:


Base faces base
Apex faces apex
View down
a axis
View down
c axis
Two distinct
sites, depending
on location
relative to chains
M1 and M2
Base
facing
base
Apex
facing
Apex
Fig. 14-1

X cations in M2 sites




Between bases of tetrahedrons
Distorted 6- and 8- fold coordination
Depends on stacking and the size of the
cations
Y cations in M1 sites

6-fold coordination between apical oxygen

“I-beams”

Consist of two chains connected by Y cations


Located in M1 sites
Closeness of apical oxygen and 6-fold
coordination make bonds strong
Apex
pointed at
apex
I-beam

I-beams held together by X cations in M2
site



Coordination number depends on how chains
line up
6-fold coordination gives orthorhombic
symmetry - OPX
8-fold coordination gives monoclinic symmetry
- CPX
Crystallographic
and optical axes
align
C crystallographic
axis at 32 to 42º
angle to the Z
optical axis
OPX - Orthorhombic
Pigeonite – CPX Monoclinic

Crystal shapes



Blocky prisms, nearly square
Elongate along c axis
Cleavage controlled by I-beams



Cleavage typically between 87º and 93º
Only when viewed down the c axis
Mineral grain must be cut parallel to (001)
I beams – tightly
bonded
Weak zones
between faces of I
beams
Cleavage angles
are 87º and 93º
Weak planes
between “I beams”
= cleavage
Fig. 14-1
Classification

Based on two linked things



Which cations occurs in M2 sites (facing bases
of tetrahedron)
Cation determines symmetry
Most plot on ternary diagram with apices:



Wollastonite, Wo
Enstatite, En
Ferrosilite, Fe

Three major groups




Orthopyroxenes (opx) – orthorhombic
Low-Ca clinopyroxenes (cpx) – monoclinic
Ca-rich clinopyroxenes (cpx) – monoclinic
The amount of Ca in the mineral controls
the extinction angle

Orthopyroxenes: Fe and Mg, but little Ca


Both M1 and M2 are octahedral
Larger Fe ion more concentrated in M2 site
(larger)

Low-Ca clinopyroxene: more Ca, but no
solid solution with Hi-Ca clinopyroxene



Mineral species is Pigeonite
Ca restricted to M2 sites, these still mostly Fe
and Mg
M1 sites all Mg and Fe

Ca- clinopyroxene




Diopside Mg(+Ca) to Hedenbergite Fe (+Ca)
M2 site contains mostly Ca
M1 site contains mostly Fe and Mg
Most common specie is augite


Al substitutes in M1 site, and for Si in
tetrahedral site
Na, Fe or Mg substitutes for Ca in M2 site

Other common pyroxenes


Jadeite NaAlSi2O6
Spodumene LiAlSi2O6
Possible ranges
solid solutions
of
“Augite”
Clinopyroxene
Orthopyroxenes
Na – bearing pyroxenes
Fig. 14-2

Identification in hand-sample difficult




Mostly based on occurrence
Also color can be indicative
Optical properties distinguish clino- from
ortho-pyroxenes
If composition is important, need chemical
analysis
Geology of pyroxenes

Igneous





Common igneous pyroxenes: augite,
pigeonite, and opx
Augite most common
Usually in mafic and intermediate volcanics
Both intrusive and extrusive
Zoning common: magma becomes enriched in
Fe because of partition of Mg into crystals


Requires 3 component phase diagram
Exsolution common – cooling allows
rearrangement of Ca
Exsolution mechanisms

Augite original crystallization



Ca substitution in M2 sites
restricted
As cools, Ca reorganizes
Generally find exsolution lamellae
of pigeonite (low Ca cpx) within
host augite parallel to (001) or
opx parallel to (100)
Augite Matrix

Opx crystallize at high T with
excess Ca – up to 10%



Slow cooling allows Ca expelled
to form exsolution of augite (hiCa cpx)
Single lamellae of augite parallel
to (100)
Bushveld variety – S. Africa type
location
Opx Matrix

Pigeonite grows in mafic
magma



Up to 10% Ca in M2 site
Cooling causes Ca to expel
and form augite (hi-Ca cpx)
lamellae
Single lamellae parallel to
(001)
Pigeonite Matrix

If slow enough pigeonite
converts to opx



Pigeonite only preserved where
cooling fast (volcanic)
Slow cooling creates second set
augite (hi-Ca cpx) parallel to
(100)
“Stillwater type”
Opx Matrix
Metamorphic

Carbonate rocks, typically diopside
because of Ca and Mg from calcite and
dolomite


Amphibolite common association (water)
Na and Ca clinopyroxenes


Typically restricted to high T and low P
conditions
Found at subduction zones (blue schist facies)

Opx also in granulite facies rocks


Hot enough to remove water
Derived from amphiboles
Sedimentary


Not stable (anhydrous)
Converts to clay minerals
Amphibole Group



Structure, composition, and classification
similar to pyroxenes
Primary difference is they are double
chains
Z/O ratio is 4/11
Structure



Chains extend parallel to c
axis
Stacked in alternating
fashion like pyroxenes
Points face points and
bases face bases


Chains are linked by
sheets of octahedral sites
Three unique sites: M1,
M2, and M3
 Depend on location
relative to Si
tetrahedron
Not
shared
O
Shared
O
OH
Fig. 14-12

TOT layers



Two T layers (tetrahedral layers with Z ions)
Intervening O layer (octahedron) with M1,
M2, and M3 sites
Form “I-beams” similar to pyroxenes

Geometry produces
five different
structure sites


M1, M2, and M3
between points of
chains
M4 and A sites
between bases of
chains




Bonds at M4 and A sites
weaker than bonds
within “I-beams”
Cleavage forms along
the weak bonds
“I-beams” wider than
pyroxenes
Cleavage angles around
56º and 124º
Weak planes
between “I beams”
= cleavage
Composition
W0-1X2Y5Z8O22(OH)2


Each cation fits a particular site
W cation



Occurs in A site
Has ~10 fold coordination
Generally large, usually Na+
W0-1X2Y5Z8O22(OH)2

X cations





Located in M4 sites
Analogous to M2 sites in pyroxenes
Have 6 or 8 fold coordination depending on
arrangement of chains
If 8-fold, X usually Ca
If 6-fold, X usually Fe or Mg
W0-1X2Y5Z8O22(OH)2

Y cations



Located in M1, M2, and M3 sites; Octahedral
cations in TOT strips
Usually Mg, Fe2+, Fe3+, Al
Z cations

Usually Si and Al

Composition





Most common amphiboles shown on ternary
diagram
Wide variety of substitution, simple and
coupled
Divided into ortho and clino amphiboles
Depends on X cations in M4 site (largely
amount of Ca), distorts structure
Reduces symmetry from orthorhombic to
monoclinic
W0-1X2Y5Z8O22(OH)2
Tremolite
Anthophylite
Orthorhomic
Ferroactinolite
~30% Ca
exactly
2/7 of
sites
available
for Ca
Grunerite
Monoclinic
Fig. 14-13
Pyroxenes
and
Amphiboles

Identification




Hand sample and thin section difficult
Best method is association
Ca and Na amphiboles commonly dark green
to black, pleochroic: usually Hornblende
White or pale green amphiboles usually called
tremolite
Geology of amphiboles

Several important aspects




Hydrous – water part of their structure
Not stable in anhydrous environments
Dehydrate at high temperature
High Z/O ratio (4/11) mean they should occur
in Si-rich rocks

Generalization
1.
Not common in mafic and ultramafic rocks
1.
2.
2.
Crystallize late in magmatic history; melt rich in
Si and H2O
Overgrowths of amphibole on pyroxenes
common
Common in felsic to intermediate rocks
Fe and Mg minerals either amphibole or biotite
2. Depends on abundance of K (biotite) and Ca/Na
(amphiboles)
 Generally amphibole tends toward intermediate
rocks; biotite toward felsic
1.
1)
Amphiboles common in regional
metamorphism of intermediate to mafic
rocks
Usually water rich from breakdown of clay
and micas
2) Metamorphic rock with abundant amphiboles
called amphibolite facies
3) At high T, amphiboles break down to
pyroxenes
Note – these generalities are likely to be wrong
1)
Pyroxenoid Group

Similar to pyroxenes



Differ in repeat distance along c axis




Single chains
Z/O ratio 1/3
Pyroxene – 2 tetrahedron repeat (5.2 Å)
Pyroxenoid – 3 or more repeat (more than 7.3
Å)
Difference is the pyroxenes are straight
pyroxenoids are kinked
Cased by larger linking cations
Pyroxenes
Rhodenite - Mn
Wollastonite - Ca

Only a few minerals



Most common Wollastonite – Ca
Others are Rhodonite – Mn
Pectolite – Ca and Na

Wollastonite



Composition: Ca with some Mn and Fe
substitution
Common in altered carbonate rocks,
particularly with reaction with qtz
Useful industrial mineral, replacing asbestose,
also used in paints and plastics