insolicates, mineralogy

1. 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??

2. Inosilicates: single chains- pyroxenes
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)
b

3. Inosilicates: single chains- pyroxenes
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)
b

4. Inosilicates: single chains- pyroxenes
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)
b

5. Inosilicates: single chains- pyroxenes
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)
b

6. Inosilicates: single chains- pyroxenes
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)
b

7. Inosilicates: single chains- pyroxenes
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)
Perspective view

8. Inosilicates: single chains- pyroxenes
M1 octahedron

9. Inosilicates: single chains- pyroxenes
M1 octahedron

10. Inosilicates: single chains- pyroxenes
M1 octahedron
(+) type by convention
(+)

11. Inosilicates: single chains- pyroxenes
M1 octahedron
This is a (-) type
(-)

12. Inosilicates: single chains- pyroxenes
T
M1
T
Creates an “I-beam”
like unit in the
structure.

13. Inosilicates: single chains- pyroxenes
T
M1
T
Creates an “I-beam”
like unit in the
structure
(+)

14. 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

15. 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
(+)(+)

16. 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

17. 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

18. Pyroxene Chemistry
The pyroxene quadrilateral and opx-cpx solvus
Coexisting 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

19. Pyroxene Chemistry
“Non-quad” pyroxenes
Jadeite
NaAlSi2O6
Ca(Mg,Fe)Si2O6
Aegirine
NaFe3+Si2O6
Diopside-Hedenbergite
Ca-Tschermack’s
molecule CaAl2SiO6
Omphacite
aegirine-
augite
Augite
Spodumene:
LiAlSi2O6

20. 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

21. 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

22. Inosilicates: double chains- amphiboles
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

23. Inosilicates: double chains- amphiboles
Hornblende (001) view dark blue = Si, Al purple = M1 rose = M2
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)

24. Inosilicates: double chains- amphiboles
b
(+) (+)
(+)
(+)
(+)
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,K)
little turquoise ball = H
Hornblende:
K,<1(Ca, Na)2-3 (Mg, Fe,
Al)5 [(Si,Al)8O22] (OH)2

25. 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, K)
little turquoise ball = H
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

26. 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:
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)

27. 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

28. 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

29. 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

30. 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