2. Silicates are classified on the basis of arrangement
of sio4 and Si-O polymerism
[SiO4]4- tetrahedron is basic
building of silicates
3. Structure depend upon sharing of oxygen of basic
unit cell [SiO4]4- Independent tetrahedral Nesosilicates
e.g. olivine and garnet
[Si2O7]6- Double tetrahedra Sorosilicates
e.g. epidote
n[SiO3]2- n = 3, 4, 6 Cyclosilicates
e.g. beryl and axinite.
4. Silicates are classified on the basis of Si-O
polymerism
[SiO3]2- single chains Inosilicates [Si4O11]4- Double tetrahedra
pryoxenes pyroxenoids amphiboles
5. Silicates are classified on the basis of Si-O
polymerism
[Si2O5]2- Sheets of tetrahedra Phyllosilicates
micas talc clay minerals serpentine
6. Silicates are classified on the basis of Si-O
polymerism
[SiO2] 3-D frameworks of tetrahedra: fully polymerized Tectosilicates
quartz and the silica minerals feldspars feldspathoids zeolites
low-quartz
9. Olivine (100) view blue = M1 yellow = M2
b
c
perspective
Nesosilicates: independent SiO4 tetrahedra
10. Olivine (001) view blue = M1 yellow = M2
M1 in rows
and share
edges
M2 form
layers in a-c
that share
corners
Some M2
and M1 share
edges
b
a
Nesosilicates: independent SiO4 tetrahedra
12. Nesosilicates: independent SiO4 tetrahedra
Olivine Occurrences:
Principally in mafic and ultramafic igneous and
meta-igneous rocks
Fayalite in meta-ironstones and in some alkalic
granitoids
Forsterite in some siliceous dolomitic marbles
Monticellite CaMgSiO4
Ca M2 (larger ion, larger site)
High grade metamorphic siliceous carbonates
13. Nesosilicates: independent SiO4 tetrahedra
Garnet (001) view blue = Si purple = A turquoise = B
Garnet: A2+
3 B3+
2 [SiO4]3
“Pyralspites” - B = Al
Pyrope: Mg3 Al2 [SiO4]3
Almandine: Fe3 Al2 [SiO4]3
Spessartine: Mn3 Al2 [SiO4]3
“Ugrandites” - A = Ca
Uvarovite: Ca3 Cr2 [SiO4]3
Grossularite: Ca3 Al2 [SiO4]3
Andradite: Ca3 Fe2 [SiO4]3
Occurrence:
Mostly metamorphic
Some high-Al igneous
Also in some mantle peridotites
14. Nesosilicates: independent SiO4 tetrahedra
Garnet (001) view blue = Si purple = A turquoise = B
Garnet: A2+
3 B3+
2 [SiO4]3
“Pyralspites” - B = Al
Pyrope: Mg3 Al2 [SiO4]3
Almandine: Fe3 Al2 [SiO4]3
Spessartine: Mn3 Al2 [SiO4]3
“Ugrandites” - A = Ca
Uvarovite: Ca3 Cr2 [SiO4]3
Grossularite: Ca3 Al2 [SiO4]3
Andradite: Ca3 Fe2 [SiO4]3
Occurrence:
Mostly metamorphic
Pyralspites in meta-shales
Ugrandites in meta-carbonates
Some high-Al igneous
Also in some mantle peridotites
a1
a2
a3
15. Inosilicates: single chains- pyroxenes
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)
Diopside: CaMg [Si2O6]
b
asin
Where are the Si-O-Si-O chains??
21. Inosilicates: single chains- pyroxenes
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)
Perspective view
22. Inosilicates: single chains- pyroxenes
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)
SiO4 as polygons
(and larger area)
IV slab
IV slab
IV slab
IV slab
VI slab
VI slab
VI slab
b
asin
29. The pyroxene
structure is then
composed of
alternating I-beams
Clinopyroxenes have
all I-beams oriented
the same: all are (+)
in this orientation
(+)
(+)(+)
(+)(+)
Inosilicates: single chains- pyroxenes
Note that M1 sites are
smaller than M2 sites, since
they are at the apices of the
tetrahedral chains
30. The pyroxene
structure is then
composed of
alternation I-beams
Clinopyroxenes have
all I-beams oriented
the same: all are (+)
in this orientation
(+)
(+)(+)
Inosilicates: single chains- pyroxenes
(+)(+)
32. 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
Inosilicates: single chains- pyroxenes
c
a
(+) M1
(+) M2
(+) M2
33. 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)
Inosilicates: single chains- pyroxenes
c
a
(+) M1
(-) M1
(-) M2
(+) M2
34. The general pyroxene formula:
W1-P (X,Y)1+P Z2O6
Where
W = Ca Na
X = Mg Fe2+ Mn Ni Li
Y = Al Fe3+ Cr Ti
Z = Si Al
Anhydrous so high-temperature or dry
conditions favor pyroxenes over amphiboles
35. The pyroxene quadrilateral and opx-cpx solvus
Coexisting opx + cpx in many rocks (pigeonite only in volcanics)
Diopside Hedenbergite
Wollastonite
Enstatite Ferrosilite
orthopyroxenes
clinopyroxenes
pigeonite (Mg,Fe)2Si2O6 Ca(Mg,Fe)Si2O6
pigeonite
orthopyroxenes
Solvus
1200oC
1000oC
800oC
37. 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 A
12.5 A
17.4 A
5.2 A
38. 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
asin
39. Inosilicates: double chains- amphiboles
Hornblende:
(Ca, Na)2-3 (Mg, Fe, Al)5
[(Si,Al)8O22] (OH)2
b
asin
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)
little turquoise ball = H
40. Inosilicates: double chains- amphiboles
Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2
light blue = M3 (all Mg, Fe)
Hornblende:
(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)
41. Inosilicates: double chains- amphiboles
b
asin
(+) (+)
(+)
(+)
(+)
Same I-beam
architecture, but
the I-beams are
fatter (double
chains)
All are (+) on
clinoamphiboles
and alternate in
orthoamphiboles
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)
little turquoise ball = H
Hornblende:
(Ca, Na)2-3 (Mg, Fe,
Al)5 [(Si,Al)8O22]
(OH)2
42. Inosilicates: double chains- amphiboles
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)
little turquoise ball = H
Hornblende:
(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
43. Inosilicates: double chains- amphiboles
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)
little turquoise ball = H
Hornblende:
(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)
44. 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
45. 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)2
Fe7Si8O22(OH)2
Actinolite
Cummingtonite-grunerite
Orthoamphiboles
Clinoamphiboles
46. 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
47. Tremolite (Ca-Mg) occurs in meta-carbonates
Actinolite occurs in low-grade metamorphosed basic igneous rocks
Orthoamphiboles and cummingtonite-grunerite (all Ca-free, Mg-Fe-rich
amphiboles) are metamorphic and occur in meta-ultrabasic rocks and some
meta-sediments. The Fe-rich grunerite occurs in meta-ironstones
The complex solid solution called hornblende occurs in a broad variety of both
igenous and metamorphic rocks
Sodic amphiboles are predominantly metamorphic where they are
characteristic of high P/T subduction-zone metamorphism (commonly called
“blueschist” in reference to the predominant blue sodic amphiboles
Riebeckite occurs commonly in sodic granitoid rocks
Amphibole Occurrences
48. Inosilicates
Pyroxenes and amphiboles are very similar:
Both have chains of SiO4 tetrahedra
The chains are connected into stylized I-beams by M octahedra
High-Ca monoclinic forms have all the T-O-T offsets in the same direction
Low-Ca orthorhombic forms have alternating (+) and (-) offsets
+
+ +
+
++
+
++
-
- -
-
-
-
+
++
a
a
+
+ +
+
+ +
+
+ +
+
+ +
-
-
-
-
-
-
Clinopyroxene
Orthopyroxene Orthoamphibole
Clinoamphibole
49. 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
50. SiO4 tetrahedra polymerized into 2-D sheets: [Si2O5]
Apical O’s are unpolymerized and are bonded to other constituents
Phyllosilicates
51. Tetrahedral layers are bonded to octahedral layers
(OH) pairs are located in center of T rings where no apical O
Phyllosilicates
52. Octahedral layers can be understood by analogy with hydroxides
Phyllosilicates
Brucite: Mg(OH)2
Layers of octahedral Mg in
coordination with (OH)
Large spacing along c due
to weak van der waals
bonds
c
53. Phyllosilicates
Gibbsite: Al(OH)3
Layers of octahedral Al in coordination with (OH)
Al3+ means that only 2/3 of the VI sites may be occupied for charge-balance reasons
Brucite-type layers may be called trioctahedral and gibbsite-type dioctahedral
a1
a2
54. Phyllosilicates
Kaolinite: Al2 [Si2O5] (OH)4
T-layers and diocathedral (Al3+) layers
(OH) at center of T-rings and fill base of VI layer
Yellow = (OH)
T
O
-
T
O
-
T
O
vdw
vdw
weak van der Waals bonds between T-O groups
55. Phyllosilicates
Serpentine: Mg3 [Si2O5] (OH)4
T-layers and triocathedral (Mg2+) layers
(OH) at center of T-rings and fill base of VI layer
Yellow = (OH)
T
O
-
T
O
-
T
O
vdw
vdw
weak van der Waals bonds between T-O groups
56. Serpentine
Octahedra are a bit larger than tetrahedral
match, so they cause bending of the T-O
layers (after Klein and Hurlbut, 1999).
Antigorite maintains a
sheet-like form by
alternating segments of
opposite curvature
Chrysotile does not do this
and tends to roll into tubes
57. Serpentine
The rolled tubes in chrysotile resolves the apparent
paradox of asbestosform sheet silicates
S = serpentine T = talc
Nagby and Faust (1956) Am.
Mineralogist 41, 817-836.
Veblen and Busek, 1979,
Science 206, 1398-1400.
58. Phyllosilicates
Pyrophyllite: Al2 [Si4O10] (OH)2
T-layer - diocathedral (Al3+) layer - T-layer
T
O
T
-
T
O
T
-
T
O
T
vdw
vdw
weak van der Waals bonds between T - O - T groups
Yellow = (OH)
59. Phyllosilicates
Talc: Mg3 [Si4O10] (OH)2
T-layer - triocathedral (Mg2+) layer - T-layer
T
O
T
-
T
O
T
-
T
O
T
vdw
vdw
weak van der Waals bonds between T - O - T groups
Yellow = (OH)
60. Phyllosilicates
Muscovite: K Al2 [Si3AlO10] (OH)2 (coupled K - AlIV)
T-layer - diocathedral (Al3+) layer - T-layer - K
T
O
T
K
T
O
T
K
T
O
T
K between T - O - T groups is stronger than vdw
61. Phyllosilicates
Phlogopite: K Mg3 [Si3AlO10] (OH)2
T-layer - triocathedral (Mg2+) layer - T-layer - K
T
O
T
K
T
O
T
K
T
O
T
K between T - O - T groups is stronger than vdw
63. Chlorite: (Mg, Fe)3 [(Si, Al)4O10] (OH)2 (Mg, Fe)3 (OH)6
= T - O - T - (brucite) - T - O - T - (brucite) - T - O - T -
Very hydrated (OH)8, so low-temperature stability (low-T metamorphism
and alteration of mafics as cool)
Phyllosilicates
64. Why are there single-chain-, double-chain-, and sheet-polymer types,
and not triple chains, quadruple chains, etc??
“Biopyriboles”
![Silicates are classified on the basis of arrangement
of sio4 and Si-O polymerism
[SiO4]4- tetrahedron is basic
building of silicates](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)