EPSC210 Introductory Mineralogy

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EPSC210 Introductory Mineralogy
Inosilicates
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Bowens reaction series

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Bowens reaction series describe the typical
order of crystallization from a basaltic magma.
The nesosilicate olivine and the inosilicates
(chain silicates pyroxenes and amphiboles)
precipitate before phyllosilicates and quartz
because they contain a lower proportion of
SiO2. They are also less polymerized (less
corner sharing among SiO4) and generally denser
than phyllosilicates and quartz. Cations must
occupy a larger share of the space among
tetrahedra to satisfy the valence of oxygen.
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3 important groups among inosilicates A)
pyroxenes (single chain) b) pyroxenoids (single
chain, twisted) c) amphiboles (double
chain) Being chain silicates gives all
inosilicates a prismatic cleavage, prismatic
habit and a moderate hardness (5.5). The
prismatic cleavage and growth habit are most
pronounced in the amphiboles.
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Nearly each pyroxene has a corresponding
amphibole, i.e. one where the same type of
metallic cations joins the chains together.
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Amphiboles, with (OH)- groups, are stable at
lower temperatures than the equivalent pyroxenes.
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• Common metamorphic reactions include
• - the alteration of pyroxenes to amphiboles, if a
  rock is reheated (without melting it) and is
  brought in contact with hot water. H2O supplies
  the OH- groups to build an amphibole.
• - amphiboles breaking down to pyroxene in rocks
  that are re-heated enough that the OH- groups
  break away from the structure.

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The growth habit of inosilicates generally obeys
the law of Bravais (lecture notes 13) i.e. their
largest faces are found along lattice planes of
highest node density. Since the chains of SiO4
tetrahedra (the shortest and strongest bonds in
the structure) are along the c axis, growth is
expected to be fastest along that direction.
The slow-growing faces are therefore parallel
to the c axis, and often end up being the largest
ones on the crystals.
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Pyroxenes have the general formula X Y Z2
O6 Where X are cations in 6 to 8-fold
coordination, Y are cations in 6-fold
coordination, Z are cations in 4-fold
coordination. These sites are also given labels
M2, M1, T
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The oxygen ions are further apart along the base
of the tetrahedra than around the apex (tips) of
the chains. This defines the two types of
octahedral sites for cations, called M2 (larger,
blue) and M1 (smaller, red). M2 corresponds to X,
and M1 to Y in the general formula.
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The M1 cations are bonded mostly to apical
oxygens (at tips of tetrahedra ). They have
exactly 6 oxygens around the cation arranged into
a regular octahedron. The M2 sites, at the base
of tetrahedra, can house slightly larger divalent
cations than the M1 sites. They are coordinated
to the 6-8 nearest oxygen ions.
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The shortest and strongest bonds are Si-O
followed by M1-O bonds. In 3-D, they form strong
chains sometimes described as I-beams because
they give strength to the crystalline structure.
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The M2-O bonds are the easiest ones to break. The
cleavage runs between the base of tetrahedra, in
a jagged pattern, making it rough, less than
perfect.
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Clinopyroxenes are monoclinic when X and Y
cations have very different sizes. The beta
angle, between the a and c axes, is gt 90 degrees.
The parallelohedron 001 is inclined (but not
perpendicular) at the end of the prism.
Orthopyroxenes have an ortho-rhombic symmetry,
and their M2 and M1 site are filled by cations
fairly similar in size...
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(left orthopyroxene) See how the M1 octahedra
alternate in orientation? Below clinopyroxenes
are monoclinic (note shorter a axis and beta
angle.)
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Seen down their c axis (chains perpendicular to
the screen), the orthopyroxenes and
clino-pyroxenes show the same cleavage
angles. But go back to the previous slide and see
how the unit length along the a axis extends
twice as far in an orthopyroxene than in a
clinopyroxene
sketch the axes and the cleavage for yourself
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Seen down their c axis (chains perpendicular to
the screen), the orthopyroxenes and
clino-pyroxenes show the same cleavage
angles. But go back to the previous slide and see
how the unit length along the a axis extends
twice as far in an orthopyroxene than in a
clinopyroxene
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In opx, cleavage is indexed as 210 because the
unit length along the a axis is about twice as
long. Cleavage intercepts it at 1/2 its length.
cell of MgSiO3 a 18.2 b 8.8 c 5.2
In cpx, cleavage is 110. The cell of diopside
a 9.73, b 8.9, c 5.3.
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Pyroxenes which ones are opx, cpx... (opx)
enstatite MgSiO3 (opx) enstatite-ferrosilite
series (Mg, Fe)SiO3 (cpx) diopside
CaMgSi2O6 (cpx) augite Ca(Mg, Fe)(Al,
Si)2O6 (cpx) spodumene LiAlSi2O6 (cpx) jadeite
NaAlSi2O6
(Compare the ionic radii of the first two ions)
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There is a fair amount of flexibility in the
structure of single chain silicates. The
tetrahedra can twist in order to share corners
with the octahedra surrounding M1 and M2 cations
of various sizes.
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view down (100) see how chains are slightly
twisted
clinopyroxenes
view down (010)-gtgt
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Rhodonite and wollastonite are pyroxenoids.
Their divalent cations, Mn2 or Ca2, are all in
8-fold coordination, and the SiO4 chains are
strongly twisted.
Above MnSiO3 Right CaSiO3
Both minerals show some compositional variation
Fe2, Mn2, Ca2, Zn2, and (to a lesser degree)
Mg 2 ...
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Pyroxenoids have a twisted chain structure to
accommodate all MeO8 polyhedra. This further
lowers their symmetry to triclinic.
CaSiO3, wollastonite is increasingly used in
industry as a filler in rubber, asphalt tiles,
etc... Its fibrous texture can be a good
substitute for asbestos.
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Being triclinic, the pyroxenoids MnSiO3 or
CaSiO3 do not form a solid solution with
pyroxenes because their structure cannot mix
largish (Ca2, Mn2) and smallish ions (Mg2or
Fe2) in M1 sites.
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There are no pyroxenes of intermediate
compositions in this area of the diagram...
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Some versions of this diagram show tie-lines
connecting the pairs of minerals that would form
from a melt of intermediate composition. Many
igneous rocks contain both orthopyroxenes and
clinopyroxenes as separate crystals because of
this limited solid solution.
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What is pigeonite?
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At high temperature, orthopyroxenes and
clinopyroxenes can tolerate mixing Ca 2 and
smaller Mg2 and Fe2 ions in the M2 sites.
However, these ions tend to unmix during cooling.
This chemical umixing of one mineral into two
different species is called exsolution. It occurs
in several types of magmatic minerals, but it is
not necessarily visible to the naked eye.
The same process is responsible for the perthite
in a feldspar paler veins of NaAlSi3O8 unmixed
from KAlSi3O8.
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From this diagram, you see that an opx
(Mg,Fe)SiO3 can contain more Ca at high
temperature, but would unmix during slow cooling
to exsolve lamellae of augite.
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Pigeonite is rare, but it is a sensitive
indicator of cooling history. See how it should
not exist at lower temperatures according to the
phase diagram?
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It is only at high temperatures that M2 sites can
hold appreciable amounts of cations of different
sizes such as Ca2 and Mg2 . If cooling is
slow, pigeonite unmixes to a Ca-free
orthopyroxene, (Mg,Fe)SiO3 containing small
lamellae of augite, a more stable Ca-rich
clinopyroxene with a M2 site filled mostly by
Ca2 (and possibly other large ions such as
Na). The clinopyroxene pigeonite is only
preserved if quick magmatic crystallization
prevents diffusion and exsolution. This happens
more often in in volcanic rocks than in intrusive
rocks.
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Dar al Gani 476
This meteorite (a shergottite, if you must know)
is a piece of Martian basaltic lava that landed
in the Libyan Sahara desert. Larger crystals are
magnesian olivine (forsterite), and some of the
smaller ones are pigeonite. Pigeonite is common
enough in basaltic flows that spilled out to form
plateaus on the ocean floor and on continents.
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Pigeonite grew in this basaltic rock. Being 2/m,
it is prone to twinning along the plane (010)
shown as a dotted line. During slow cooling,
this pigeonite unmixed to an orthopyroxene
(Ca-poor) and thin lamellae of clinopyroxene
(Ca-rich augite).
The cpx lamellae form a herringbone pattern
within the yellow opx crystal. But they first
exsolved from pigeonite crystals (cpx) related
by twinning. Pigeonite lost enough Ca to the
lamellae and became an opx crystal.
Thin section of basalt under crossed polarizers.
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In the crystal traced in orange, the lamellae are
clearly mirrored in each part of the twinned
crystal.
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What substitutions relate a diopside to an
augite? Composition substitutions? CaMgSi2O6
viiiCa2 viMg2 viAl3 viiiNa
viMg2 ivSi4 ivAl3 viAl3 (Na, Ca)
(Mg,Fe,Al)(Al,Si)2O6 ...
two coupled substitutions (Note Al3 occurs in
two different types of sites.)
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Another rare inosilicate Mt Saint Hilaire is
world famous for the occurrence of large,
euhedral serandite crystals, first found in 1963.
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serandite viiiNaviMn2ivSi3O8(OH) is a
quasi-pyroxenoid which shows a twisted chain.
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What substitutions relate wollastonite CaSiO3 to
serandite viiiNaviMn2ivSi3O8(OH) ? (Hint...
start from 3CaSiO3 Ca3Si3O9) Check the ionic
radii to find largest ion... ... vi2Mn2 for
viii2Ca2 -gt CaMn2Si3O9 ... viiiNa for
viiiCa2 (charge balance?) ... O2-H or (OH)-
for O2- (charge balance?) ... these last two
substitutions must be combined in a single
equation as the charge balance is solved by
coupling them Na OH- Ca2 O2-
-gt viiiNaviMn2ivSi3O8)(OH)
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Amphiboles are double-chained silicates.
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Many more types of sites between the oxygen ions
M4, M3, M2, M1 but also a larger A site between
the chains. OH groups line up with tetrahedral
tips.
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General formula of amphiboles W X2
Y5 Z8 O22 (OH)2 A 0-1 (M4 )2
(M1,2,3)5 T8 O22 (OH)2 where A is a large
cation (can be totally absent) M4 is a cation
equivalent to M2 in a pyroxene M1,2,3 are cations
equivalent to M1 in a pyroxene T is a small
cation in tetrahedral coordination The size
difference between X, Y (M4 vs M1,2,3) cations
also controls the overall symmetry. Those radii
are close in orthoamphiboles Radius in M4 gtgt
than in M1, 2, 3 for clinoamphiboles.
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The difference in cleavage angles among pyroxenes
and amphiboles are obvious in thin section, under
the microscope.
Pyroxene (left) angles of 87 93 degrees.
Cleavage is coarser, less regular, parallel to
smaller faces.
Amphibole (right) angles of 120 and 60 degrees.
Cleavage is better developed, parallel to larger
faces, more evenly spaced.
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The cleavage breaks the weakest bonds, M4-O and
A-O, along the bases of tetrahedra. It is better
(nearly planar) than in pyroxenes.
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Whats wrong with this picture? Angles are OK,
but what bonds are being broken?
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The shape of some of the fields (solid solution)
expands at higher temperature.
Tie-lines
Tie-lines connect pairs of amphiboles that would
form from a melt of intermediate composition.
If, at high temperature, cummingtonite can take
more Ca than is shown here, it will tend to
exsolve (unmix) actinolite lamellae when it cools
down...
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• T (tetrahedra) Si, Al. The limit of Al
  substitution in these sites is about 2 out of 8.
• M2 (small octahedron) Al3, Cr3,Fe3,Ti4,Fe2,
  Mg2
• M1, M3 (medium octahedra) Fe2, Mg2, Mn2.
• M4 (larger cation site) Ca2, Na, Mn2, Fe2,
  Mg2.
• A Na, K, or vacancies (i.e. can be left
  empty).

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Common substitutions in amphiboles, written in a
more compact notation Al2Mg-1Si-1 is the same
as writing the following equation 2 Al3 Mg2
Si4 isomorphous Fe2Mg-1 , MnMg-1, MgCa-1
coupled Al2Mg-1Si-1 Fe3AlMg-1Si-1
TiAl2Mg -1Si-2 These coupled substitutions
fill the A site. The V stands for a vacant
(empty) site. NaAlV-1Si-1 (equivalent to NaAl?-1
Si-1) KAlV-1Si-1 (equivalent to KAl
?-1Si-1) NaAlCa-1Mg-1
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Which ones of these amphiboles are ortho- or
clino?
tremolite Ca2Mg5Si8O22(OH)2 actinolite
Ca2(Mg,Fe)5Si8O22(OH)2 glaucophane
Na2Mg3Al2Si8O22(OH)2 anthophyllite Mg7Si8O22(OH)2
hornblende (see why its called a garbage
can?) (Na,K)0-1Ca2(Mg,Fe,Al,Ti)5(Si6-8Al0-2)8O22(
OH)2
W X2 Y5 Z8 O22 (OH)2 A
0-1 (M4 )2 (M1, 2, 3)5 T8 O22 (OH)2
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The difference in radii of X vs. Y cations
determines which amphiboles are ortho- or
clino...
tremolite Ca2Mg5Si8O22(OH)2
ltltclinogtgt actinolite Ca2(Mg,Fe)5Si8O22(OH)2
ltltclinogtgt glaucophane Na2Mg3Al2Si8O22(OH)2
ltltclinogtgt anthophyllite Mg7Si8O22(OH)2
ltltorthogtgt
hornblende (its a clino garbage
can...) (Na,K)0-1Ca2(Mg,Fe,Al,Ti)5(Si6-8Al0-2)8O2
2(OH)2
Is the A site filled in any of them?
W X2 Y5 Z8 O22 (OH)2 A
0-1 (M4 )2 (M1, 2, 3)5 T8 O22 (OH)2
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The bad name of asbestos comes from amphiboles!
Some amphiboles, including crocidolite, an
iron-rich variety of glaucophane,
Na2Mg3Al2Si8O22(OH)2, grow with a fibrous habit.
Crocidolite has been used as blue asbestos.
Over long periods of exposure, its fibers are far
more damaging to lung tissues than chrysotile.
Ironically, the popular gemstone tigereye or
hawkeye is a pseudomorphic replacement of
crocidolite by quartz