Lecture 17
Systematic Description of Minerals
 Part 4:
Silicates II:
 Cyclosilicates, Inosilicates,
Phyllosilicates and Tectosilicates
Silicate Mineral Classification
(based on arrangement of SiO4 tetrahedra)
Cyclosilicates (Rings)


x(SiO3) Unit Composition
Hexagonal and Orthorhombic (pseudohexagonal) symmetries

Forms silicate minerals with:
Moderate density(2.6-3.2) and hardness (7-8)
Prismatic habits
Poor cleavage
Beryl
Beryl
Common Cyclosilicates
Beryl
Be3Al2(Si6O18)
Common accessory mineral in granite pegmatite
Gem varieties – Aquamarine, Emerald, Rose Beryl,
Golden Beryl
Cordierite
(Mg,Fe)2Al4Si5O18·nH20
Common mineral in contact metamorphosed
argillaceous rocks
Resembles quartz in appearance and hardness (77.5)
Tourmaline
Hexagonal, pleochroic, parallel extinction.
Na,Ca)(Li,Mg,Al)3(Al,Fe,Mn)6(BO3)3(Si6O18)(OH)4
(
Common accessory mineral in granite pegmatite
Characteristic striated prisms with trigonal outline
watermelon
tourmaline
Inosilicates (Chain)
 XY(Si2O6) in Pyroxenes, WX2Y5Si8O22(OH,F)2 in Amphiboles
 Single and double silicon tetrahedra chains respectively
 Typically monoclinic and orthorhombic symmetry
 Single chains (Pyroxenes) develop ~90° cleavage
 Double chains (Amphiboles) develop 120 ° cleavage
Pyroxene
Structure
Amphibole
Structure
O-coordination and Bond Strength of Other
Common Cations in Silicate Minerals
Electostatic
Valence w/ O-2
big
medium
small
1/8 - 1/12 Weak
1/6 - 1/8
1/3 – 1/4
2/6 = 1/3
2/6 = 1/3
2/6 = 1/3
3/6 = 1/2
4/6 = 2/3
3/6 = 1/2
3/4
4/4 = 1 Strong
Pyroxenes (XYZ2O6 )
X (M2) – Na+, Ca++, Mn++, Fe+2, Mg++, Li+ [8] Cubic
Y (M1) – Mn++, Fe+2, Mg++, Fe+3, Cr+3 , Ti+4 [6] Octahedral
,
Z (Tetrahedral site) - Al+3
Si+4 [4]
Single Si2O6 chains (the
tetrahedral sites) that run
parallel
to the c-axis
Monoclinic Pyroxenes (Clinopyroxenes - Cpx)
The Diopside- Hedenbergite series - Diopside (CaMgSi2O6) - Ferrohedenbergite (CaFeSi2O6)
There is complete Mg-Fe solid solution between Diopside and (Ferro)Hedenbergite
Augite - (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6 is closely related to the Diopside Hedenbergite series with addition of Al and minor Na substitution
There is also a complete Mg-Fe
substitution and small amounts of Ca
substitution into the Orthopyroxene
solid solution series. Old name
Hypersthene
Pigeonite is a high Temperature clinopyroxene
Orthorhombic Pyroxenes (Orthopyroxenes - Opx)
These consist of a range of compositions between Enstatite - MgSiO3 and Ferrosilite - FeSiO3
Pigeonite (Ca,Mg,Fe)(Mg,Fe)Si2O6
Augite
(Ca,Na)(Mg,Fe,Al)(Si,Al)2O6
Solid immiscibility
Pigeonite crystallizes in the
monoclinic system, as does
Augite, and a miscibility gap
exists between the two
minerals.
Cpx
Opx
At lower temperatures,
Pigeonite is unstable relative to
Augite plus Orthopyroxene.
Pigeonite => Augite + Opx
Pigeonite is only found in hot volcanic
and shallow intrusive igneous rocks,
or as exsolution lamellae
Exsolution in Pyroxene
Common Pyroxene Species
Diopside (Cpx)
Augite (Cpx)
Enstatite (Opx)
Look at these two drawings.
Is M1 = larger or smaller than M2?
Recall
X (M2) – Na+, Ca++, Mn++, Fe+2, Mg++, Li+ [8] Cubic
Y (M1) – Mn++, Fe+2, Mg++, Fe+3, Cr+3 , Ti+4 [6] Octahedral
Cleavage in Pyroxenes
Look at this drawing.
What axis are we
looking down?
Cleavage in Pyroxenes
Amphiboles (A0-1X2Y5Z8O22 (OH,F))
A-site – Na+, K+ loose coordination 10-12 Oxygens
X (M4) – Na+, Ca++, Mn++, Fe+2, Mg++, Li+ 8-fold
Y (M1-3) – Mn++, Fe+2, Mg++, Fe+3, Cr+3 , Ti+4 6-fold octohedral
Z (Tetrahedral T-site) - Al+3, Si+4
double-chain
backbone
Common Types of Amphiboles
Double chains in Amphiboles
TOT stacks separated by big Na+
and K+ “A” cations
TOT – A assemblies staggered
Cleavage ~120 – 60o
Distinguishing
Amphiboles with
the Petrographic
Microscope
Inosilicates - Pyroxenoids
• Wollastonite CaSiO3 is a common pyroxenoid
pyroxenoid.
Note pattern:
Up up down
up up down
Wollastonite CaSiO3: Connection of silicate
chains through [CaO6] octahedra in
direction of [100] and [001], repeats
every third tetr.
Rhodonite MnSiO3 repeats structure every
fifth tetrahedron
Is Wollastonite stable at the surface?
• CaSiO3(s) + CO2 (g) => CaCO3(s) + SiO2 (s)
DGf
-370.313 - 94.257 => -269.908 -204.65 Kcal/gfw
Reactants
Products
DGrxn = S DGproducts – S DGreactants
Kcal/gfw
DGrxn = -474.554 + 464.57
DGrxn = -9.984 Kcal/gfw negative, so the reaction
will go to the right, as written. Wollastonite will
break down if exposed to CO2 at STP.
Data source: Robie and Waldbaum (1968) Thermodynamic Properties of Minerals….
Pyroxenoids: Wollastonite and
Rhodonite
Wollastonite
Rhodonite
Phyllosilicate
Structures
Alternating Tetrahedral
and Octahedral layers
bound by large cations
or weak electrostatic
bonds
Common Phyllosilicates
Kaolinite
Chrysotile
Antigorite
Talc
Pyrophyllite
Muscovite
Biotite
Lepidolite
Chlorite
Prehnite
Alternating Tetrahedral and Octahedral layers bound
by large cations or weak Van Der Waals bonds
 Infinite sheets of silicon tetrahedra
 Charge balancing metals in [6] (octahedral)
 Strong single cleavage parallel to silicon sheets
Pyrophyllite (clay)
Muscovite (mica)
Mica Structures
KAl2(AlSi3O10)(F,OH)2
Alternating Si Tetrahedral and Octahedral layers (TOTs)bound by large
cations
Phlogopite is the Magnesium end-member of the Biotite solid solution
series, with the chemical formula KMg3AlSi3O10(F,OH)2.
Tectosilicates (Framework)
 3-D framework of linked
silicon tetrahedra
 Variable physical properties
and symmetries depending
on linkage of framework
groupings
SiO2 Group
Quartz SiO2
Feldspar Group
Microcline
XAl(Al,Si)3O8
Most abundant minerals, by
mass or volume, in the crust
Notice that Albite is
an end member of
both the Plagioclase
and K-Spar (Alkali
Feldspar) groups
Compositions
for Feldspars are
commonly
described in terms
of mole percents
of the end member
components (e.g.
Or85Ab15,
An54Ab39)
Anorthite
Feldspars are Tectosilicates
with every oxygen atom shared
by adjacent silicon or aluminum
tetrahedra. The tetrahedra are
arranged in four-member rings
that are stacked to form
“crankshafts” parallel to the aaxis of the monoclinic or
triclinic structure. The
crankshafts are joined together
in an open structure with large
voids to hold the alkali metals
K+ or Na+, or the alkaline earth
ion Ca++ .
Alkali Feldspars
At low temperatures solid solution
(ss) is unstable, ss exsolves to
Albite + Microcline. We say the two
phases are immiscible
Triclinic K-spar “Microcline”
Sanidine
Albite
Perthite (albite exsolution in microcline)
High Sanidine is fully disordered
with a statistically random Al-Si
distribution: each tetrahedron has,
on averaging over a reasonable
volume, 0.25 Al atoms and 0.75 Si
atoms. The Al+3 can be anywhere.
Low Sanidine and Orthoclase
are more ordered. For these
minerals to be monoclinic, the
center of symmetry in each ring
must be preserved.
Looking down the a-axis
At still lower temperatures, the Al+3
will be completely ordered:
always on the two t1 tetrahedra.
This ordering will destroy the
center of symmetry and the
mineral will become triclinic
Microcline.
“TOT”
Close O=
Attracted to
K+
K-spars
diagonal b -c
Looking down c-axis
Plagioclase Feldspars
Albite Twinning
Compositional
Zoning
(Oscillatory)
Plagioclase Composition from Albite Twins
Albite twins in
Plagioclase reveal
solid solution
composition.
Feldspathoids (Si-poor)
We will see Alkaline plugs on the Petrology field trips next semester
Common in Alkaline (Si-undersaturated) igneous rocks
Leucite – KAlSiO4
Nepheline – (Na,K)AlSiO4
Sodalite – Na8(AlSiO4)6Cl2
Scapolite
Hydrous Tectosilicates
 Analcime (Scapolite Gp)
NaAlSi2O6·H2O
 Natrolite (Zeolite Gp)
Na2Al2Si3O10·2H2O
 Heulandite (Zeolite Gp)
CaAl2Si7O18·6H2O
 Stilbite (Zeolite Gp)
NaCa2Al5Si13O36·14H2O