inosilicates: single chains- pyroxenes diopside (001) view blue = si purple = m1 (mg) yellow = m2...
TRANSCRIPT
Inosilicates: single chains- Inosilicates: single chains- pyroxenespyroxenes
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)
Diopside: CaMg [SiDiopside: CaMg [Si22OO66]]
bb
a si
na
sin
Where are the Si-O-Si-O chains??Where are the Si-O-Si-O chains??
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)
bb
a si
na
sin
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)
bb
a si
na
sin
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)
bb
a si
na
sin
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)
bb
a si
na
sin
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)
bb
a si
na
sin
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)
Perspective viewPerspective view
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)Diopside (001) view blue = Si purple = M1 (Mg) yellow = M2 (Ca)
SiOSiO44 as polygons as polygons
(and larger area)(and larger area)IV slabIV slab
IV slabIV slab
IV slabIV slab
IV slabIV slab
VI slabVI slab
VI slabVI slab
VI slabVI slab
bb
a si
na
sin
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
M1 octahedronM1 octahedron
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
M1 octahedronM1 octahedron
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
M1 octahedronM1 octahedron
(+) type by convention(+) type by convention
(+)
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
M1 octahedronM1 octahedron
This is a (-) typeThis is a (-) type
(-)
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
TT
M1M1
TT
Creates an “I-beam” Creates an “I-beam” like unit in the like unit in the
structure.structure.
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
TT
M1M1
TT
Creates an “I-beam” Creates an “I-beam” like unit in the like unit in the
structurestructure
(+)(+)
The pyroxene The pyroxene structure is then structure is then
composed of composed of alternating I-beamsalternating I-beams
Clinopyroxenes have Clinopyroxenes have all I-beams oriented all I-beams oriented the same: all are (+) the same: all are (+) in this orientation in this orientation
(+)(+)
(+)(+)(+)(+)
(+)(+)(+)(+)
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
Note that M1 sites are Note that M1 sites are smaller than M2 sites, since smaller than M2 sites, since they are at the apices of the they are at the apices of the
tetrahedral chainstetrahedral chains
The pyroxene The pyroxene structure is then structure is then
composed of composed of alternation I-beamsalternation I-beams
Clinopyroxenes have Clinopyroxenes have all I-beams oriented all I-beams oriented the same: all are (+) the same: all are (+) in this orientation in this orientation
(+)(+)
(+)(+)(+)(+)
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
(+)(+)(+)(+)
Tetrehedra and M1 Tetrehedra and M1 octahedra share octahedra share
tetrahedral apical tetrahedral apical oxygen atoms oxygen atoms
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
The tetrahedral chain The tetrahedral chain above the M1s is thus above the M1s is thus offset from that below offset from that below
The M2 slabs have a The M2 slabs have a similar effectsimilar effect
The result is a The result is a monoclinicmonoclinic unit cell, unit cell, hence hence clinopyroxenesclinopyroxenes
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
cc
aa
(+) M1(+) M1
(+) M2(+) M2
(+) M2(+) M2
OrthopyroxenesOrthopyroxenes have have alternating (+) and (-) alternating (+) and (-)
I-beams I-beams
the offsets thus the offsets thus compensate and result compensate and result in an in an orthorhombicorthorhombic
unit cellunit cell
This also explains the This also explains the double double aa cell dimension cell dimension and why orthopyroxenes and why orthopyroxenes
have have {210}{210} cleavages cleavages instead of {110) as in instead of {110) as in
clinopyroxenes (although clinopyroxenes (although both are at 90both are at 90oo))
Inosilicates: single chains- pyroxenes Inosilicates: single chains- pyroxenes
cc
aa
(+) M1(+) M1
(-) M1(-) M1
(-) M2(-) M2
(+) M2(+) M2
Alternative clinopyroxene structure (P21/c)
174° 149° 170°
The C2/c and P21/c structures differ in the way the tetrahedral chains are kinked.
In the C2/c structure the chains are relatively straight and are all symmetry-related to each other.
In the P21/c structure, chains in the same (100) layer are kinked in opposite senses, so they are no longer symmetry-related.
Table 1 C2/c Pbca P21/c
Size andcoordination
Small[6]
Small[6]
Small[6]
Shape Regular Regular RegularM1
CationsSmall cations such
asMg, Fe2+, Al
Small cations suchas
Mg, Fe2+, Al
Small cations suchas
Mg, Fe2+, Al
SizeLarge
[8]Small[6]-[7]
Small[7]
Shape Irregular Irregular IrregularM2
CationsLarge cations such
asNa, Ca
Only small cationssuch as Mg and Fe2+
Very little Ca!
Small amount ofCa is tolerated.Mainly Mg and
Fe2+
Crystal chemistry of the pyroxene polymorphs
Non-convergent cation ordering in orthopyroxenes
Fe2+ is slightly larger than Mg and prefers to sit on the larger M2 site (i.e. the crystal has a lower enthalpy when Fe2+ is sitting on M2).
Example: For a composition (Mg0.5Fe2+0.5)SiO3
M1 M2Low temperature Mg Fe2+
Intermediate temperature
“Infinite” temperature
Mg1-xFe2+x MgxFe2+
1-x
Mg0.5Fe2+0.5 Mg0.5Fe2+
0.5
We can measure “x” experimentally and use it to determine what the cooling rate and effective equilibration rate of the mineral was
(geospeedometry).
Pigeonite and augite solid solutions
Hypersthene transforms to the pigeonite (C2/c) structure at high
temperatures. Pigeonite has an expanded M2 site, and can accept larger amounts of Ca substituting
for (Mg, Fe2+).
The endmembers diopside (CaMgSi2O6) and hedenbergite
(CaFeSi2O6) are both clinopyroxenes (C2/c). Ca occupies M2 and there is
complete solid solution between Mg and Fe2+ on M1. The term
augite is used to describe the Ca-rich clinopyroxene solid solution.
Pigeonite and augite are separated by a large
miscibility gap because of the large difference between the radius of (Mg, Fe) and
Ca
Phase diagram for the pigeonite-diopside “binary”
At high temperature, both pigeonite and augite are monoclinic with the same C2/c structure. Miscibility between these two endmembers is limited due to the large difference in the ionic radii of (Mg, Fe) and Ca.
The eutectic melting loops are typical features of the solidification of a solid solution with limited miscibility.
Mg
0.86 Å
Ca
1.14 Å
32%
Reconstructive phase transition and low-T eutectoid point
In low-Ca pigeonite, the M2 site is too large for the small Mg and Fe cations.
The mismatch is tolerated at high temperature because thermal vibration of the Mg and Fe atoms prevents the structure from collapsing.
At low temperature there is a reconstructive phase transition to the orthopyroxene (hypersthene structure, which has a much smaller M2 site. The reconstructuve phase transition leads
to the development of a eutectoid point.
OPX - Orthorhombic Pigeonite – CPX - Monoclinic
Crystallographic and optical axes align
C crystallographic axis at 32 to 42º angle to the Z optical axis
Reconstructive phase transition and low-T eutectoid point
The reconstructuve phase transition from monoclinic pigeonite to orthorhombic hypersthene is very slow, and will only occur in very slowly-cooled rocks.
If the transition doesn’t take place, the structure needs another way of coping with the small Mg and Fe cations in M2.
Displacive phase transition to the low pigeonite structure (P21/c) occurs instead.
Transition temperature decreases with increasing Ca content, as the larger Ca atoms hold the structure apart.
Exsolution phenomena in pyroxenes
No time for (Mg, Fe)-Ca diffusion, therefore no exsolution.
No time for reconstructive phase transition.
Displacive phase transition from high to low pigeonite occurs below Tc
(Mg, Fe)-Ca diffusion can occur, therefore augite exsolution lamellae develop on entering the miscibility gap. Lamellae are parallel to (001) of monoclinic host.
No time for reconstructive phase transition. Displacive phase transition occurs below Tc in the pigeonite component of the intergrowth
(Mg, Fe)-Ca diffusion can occur. Augite lamellae develop parallel to (001) of monoclinic host.
Reconstructive phase transition occurs in pigeonite component of the intergrowth.
Further exsolution of augite occurs // (100) of orthorhombic host.
Microstructures of exsolved pyroxenes
The exsolving phase forms as lamellae (thin slabs). The orientation of the lamellae is determined by the plane of best fit between the two phases.
Augite in pigeonite (and vice versa) has best fit close to (001)
Augite in hypersthene (and vice versa) has best fit close to (100)
35
Coherence at interfaces• Coherent/semi-coherent/incoherent interfaces: these terms
are based on the degree of atomic matching across the interface.
• Coherent interface means an interface in which the atoms match up on a 1-to-1 basis (even if some elastic strain is present).
• Incoherent interface means an interface in which the atomic structure is disordered.
• Semi-coherent interface means an interface in which the atoms match up, but only on a local basis, with defects (dislocations) in between.
37
Homophase vs. Heterophase• There is a useful comparison that can be made between
grain boundaries (homophase) and interphase boundaries (heterophase).Structure Grain Boundary Interface
atoms no boundary coherentmatch (or, 3 coherent twin in fcc) interface
dislocations low angle g.b. semi-coherent
disordered high angle g.b. incoherent• Remember: for a grain boundary to exist, there must be a difference in
the lattice position (rotationally) between the two grains. An interface can exist even when the lattices are the same structure and in the same (rotational) position because of the chemical difference.
39
LAGB to HAGB Transition
• LAGB: steep risewith angle.HAGB: plateau
Dislocation Structure
Disordered Structure
Phase Transformation 40
Read-Shockley model
• Start with a symmetric tilt boundary composed of a wall of infinitely straight, parallel edge dislocations (e.g. based on a 100, 111 or 110 rotation axis with the planes symmetrically disposed).
• Dislocation density (L-1) given by:
1/D = 2sin(/2)/b /b for small angles.
b
D
Phase Transformation 41
Read-Shockley, contd.
• For an infinite array of edge dislocations the long-range stress field depends on the spacing. Therefore given the dislocation density and the core energy of the dislocations, the energy of the wall (boundary) is estimated (r0 sets the core energy of the dislocation):
gb = E0 ln, where
µb/4π(1-); A0 = 1 + ln(b/2πr0)
Phase Transformation 42
High angle g.b. structure
• High angle boundaries have a disordered structure.
• Bubble rafts provide a useful example.
• Disordered structure results in a high energy.
Low angle boundarywith dislocation structure
Microstructures of exsolved pyroxenes
Exact orientation depends on lattice parameters of the two phases. The orientation varies systmatically with temperature. This can be used to constrain the temperature at which a particular generation of lamellae grew.
Microstructures of exsolved pyroxenes
The thickness and spacing of lamellae depends on temperature and the time available for them to grow.
By performing annealing experiments and measuring the wavelength of exsolution features using transmission electron microscopy, we are able to calibrate the changes in wavelength as a function of isothermal annealing time.
For a process determined by volume diffusion, we observe that the spacing of lamellae is proportional to (time)1/3.
Example: Cooling rate of chondrules in the Allende carbonaceous chondrite
Chondrules are a major component of chondritic meteorites. It is believed that chondrules formed in the solar nebula prior to accretion of the meteorite parent bodies.
Chondrules are thought to have formed by crystallisation of melt droplets.
Pyroxene textures from granular olivine-pyroxene chondrules (GOP’s) provide constraints on their cooling history and therefore provide information about the conditions in the solar nebula.
Wavelengths between 25 and 33 nm are observed, translating to cooling rates between 25 and 0.4 °C/hour over the temperature range 1350-1200 °C.
No orthopyroxene suggests more rapid cooling (>104 °C/hour) below 1000 °C.
Pyroxene ChemistryPyroxene Chemistry
““Non-quad” pyroxenesNon-quad” pyroxenesJadeiteJadeite
NaAlSiNaAlSi22OO66
Ca(Mg,Fe)SiCa(Mg,Fe)Si22OO66
AegirineAegirine
NaFeNaFe3+3+SiSi22OO66
Diopside-HedenbergiteDiopside-Hedenbergite
Ca-Tschermack’s Ca-Tschermack’s moleculemolecule CaAl2SiOCaAl2SiO66
Ca / (Ca + Na)Ca / (Ca + Na)
0.20.2
0.80.8
Omphaciteaegirine- augite
AugiteAugite
Spodumene: Spodumene: LiAlSiLiAlSi22OO66
Sodic Pyroxenes
Jadeite (NaAlSi2O6) is a high-pressure pyroxene, formed by reactions such as:
nepheline (NaAlSiO4) + albite (NaAlSi3O8) -> 2 jadeite (NaAlSi2O6)
and
albite (NaAlSi3O8) -> jadeite (NaAlSi2O6) + quartz (SiO2)
Because it contains a mixture of a monovalent and a trivalent cations, it forms coupled substitution solid solutions with the (Ca, Mg, Fe) pyroxenes.
Jadeite-diposide solid solution
In the solid solution between NaAlSi2O6 and CaMgSi2O6, Na and Ca mix on M2 and Al and Mg mix on M1.
Differently-charged cations prefer to order at low temperatures (this lowers the Coulomb energy of the crystal). Omphacite is an ordered phase with intermediate
composition.
Pyroxene ChemistryPyroxene Chemistry
The general pyroxene formula: The general pyroxene formula:
WW1-P1-P (X,Y) (X,Y)1+P1+P Z Z22OO66
WhereWhere W = W = CaCa Na Na X = X = Mg FeMg Fe2+2+ Mn Ni Li Mn Ni Li Y = Al FeY = Al Fe3+3+ Cr Ti Cr Ti Z = Z = SiSi Al Al
Anhydrous Anhydrous so high-temperature or dry conditions so high-temperature or dry conditions favor pyroxenes over amphibolesfavor pyroxenes over amphiboles
Pyroxene ChemistryPyroxene Chemistry
The pyroxene quadrilateral and opx-cpx solvusThe pyroxene quadrilateral and opx-cpx solvusCoexisting opx + cpx in many rocks (pigeonite only in volcanics)Coexisting opx + cpx in many rocks (pigeonite only in volcanics)
DiopsideDiopside HedenbergiteHedenbergite
WollastoniteWollastonite
EnstatiteEnstatite FerrosiliteFerrosiliteorthopyroxenes
clinopyroxenes
pigeonite (Mg,Fe)(Mg,Fe)22SiSi22OO66 Ca(Mg,Fe)SiCa(Mg,Fe)Si22OO66
pigeonite clinopyroxenes
orthopyroxenes
SolvusSolvus
12001200ooCC
10001000ooCC
800800ooCC
PyroxenoidsPyroxenoids““Ideal” pyroxene chains with Ideal” pyroxene chains with
5.2 A repeat (2 tetrahedra) 5.2 A repeat (2 tetrahedra) become distorted as other become distorted as other cations occupy VI sitescations occupy VI sites
WollastoniteWollastonite (Ca (Ca M1) M1) 3-tet repeat3-tet repeat
RhodoniteRhodoniteMnSiOMnSiO33
5-tet repeat5-tet repeat
PyroxmangitePyroxmangite (Mn, Fe)SiO(Mn, Fe)SiO33
7-tet repeat7-tet repeat
PyroxenePyroxene2-tet repeat2-tet repeat
7.1 A12.5 A
17.4 A
5.2 A