TEM in geological science
polymorphism, polytypism, polysomatism
F. Nieto
Dto. de Mineralogía y Petrología
Instituto Ándaluz de Ciencias de la Tierra
Universidad de Granada-CSIC
•Tectonic plate
•Geological map
•Outcrop
•Hand sample
•Optical microscopy
•SEM
•TEM
•HRTEM
–Megam
–km
–m
–mm
–m
–0.1 m
–10 nm
–Å
•Electron diffraction
identification, orientation and cell parameters
of minerals
•Images (low or high resolution)
Mosaic crystal, twins, dislocations, strain,
polytypism, polysomatism, phase
transformation, antiphase domains,
nanotextures, nanocrystals, exsolution, nonstoichiometry.
•Analytical electron microscopy
Quick mineral identification, high-spatial
resolution quantitative analysis.
MICROSTRUCTURE
Defects of various types and origins inside a monomineral
grain (definable as real structure in a strict sense) or the
diverse associations of several minerals in a polyphasic
grain, homogeneous on a macroscopic scale
( Mellini, 1985 )
The occurrence of a given microstructure mainly depends on the nucleation conditions
and/or on the post-crystallization sub-solidus evolution. They may be important
indicators of the thermobaric evolution of minerals and rocks.
Polymorphic transformations
•Reconstructive
transformations
•Displacive transformations
•Order-disorder
transformations
Polytypism
Polysomatism
Polymorphic transformations
•Reconstructive transformations
•Displacive transformations
•Order-disorder transformations
•Seven known polymorphs of TiO2
•Rutile, anatase and brookite have been
previously found in nature
An anatase crystal in the [010] projection
containing lamellae of a second mineral (X)
Banfield et al ( 1991 )
•The lamellae make up less than 1% of the
sample. They range from a few nm to
hundreds of nm across
[010]
[311]
•Well-defined orientation relationship
•The interplanar spacings of lattice parallel to
the interphase are not exactly equal.
Banfield et al ( 1991 )
Banfield et al ( 1991 )
•HR electron micrographs interpreted to determine
the positions of columns of Ti cations within the unit
cell (measured by electron diffraction)
•Model structure
tested by comparing computer-generated
images with experimental micrographs.
refined to adjust interatomic distances to those
in anatase and rutile.
•Polyhedral representation of the structure of
anatase, TiO2 (B), and their boundaries
•Black dots - Ti atoms
•This unnamed mineral has been reported previously
as the synthetic polymorph TiO2 (B)
Banfield et al ( 1991 )
•HR heating experiments carried out with
the electron microscope show that TiO2
(B) is converted to anatase at a furnace
temperature of ~ 700 ºC (~ 100 ºC below
the anatase-rutile transformation)
•A strip of anatase has replaced the TiO2 (B)
over a wide front
•Steps of anatase approximately 0.5 nm high
were observed growing along the interface at
high temperature.
Rutile is the most common form of TiO2 in nature. It is an
important accessory mineral in metamorphic rocks, particularly
high-pressure ones
•natural high-pressure phase of titanium
oxide with α-PbO2-structure
•it occurs as (<2 nm) lamellae between
multiple twinned rutile crystals
•coesite-bearing eclogite at Shima in the
Dabie Mountains, China
•orthorhombic lattice, corresponding to
α-PbO2- type TiO2 with space group
Pbcn.
•depth of more than 200 kilometers (7 Gpa)
•α-PbO2-type TiO2 could be an extremely
useful index mineral for ultrahigh pressure
Wu et al ( 2005 )
Polymorphic transformations
•Reconstructive transformations
•Displacive transformations
•Order-disorder transformations
High pigeonite has symmetrically
equivalent chains.
In low pigeonite there are two
symmetrically distinct Si-O tetrahedral
chains, each with a different degree and
sense of rotation.
With a decrease in temperature, the
polymorphic change produces the two
types of chains, but…the change does not
match up in the different areas of the
mineral.
Putnis ( 1992 )
Moore et al
( 2001 )
Therefore, when viewed on the [010] zone axis, there
are (100) layers that consist of A chains and (100)
layers that consist of B chains (P21/n).
Polymorphic transformations
•Reconstructive transformations
•Displacive transformations
•Order-disorder transformations
In omphacites (and in many other minerals) two cations
(or more) share a site. With a decrease in temperature,
each cation “chooses” its own exclusive site.
High temperature
High temperature
Low temperature
Such a differentiation of crystallographic sites may even
produce a change in the space group.
•low-temperature polymorph -- sub-group of
symmetry of the high-temperature one.
•The order-disorder
transformations can also
produce antiphase domains
Brenker et al ( 2003 )
Dark field electron micrographs using reflections of the
type h+k = odd (present in P2/n but absent in C2/c).
The mean antiphase domain size depends on
•peak temperature
•duration of peak metamorphism
•cooling rate
•composition
Polytypes
Kogure and Inoue ( 2005 )
Structural varieties based only on the
way in which basic layers pile up
(not unique to, but very typical of
phyllosilicates)
simulated HRTEM contrasts with
various layer orientations (S) in
kaolin minerals
Regular polytype
Kogure and Inoue ( 2005 )
Simulated HRTEM contrasts for
dickite recorded down [100] as a
function of the defocus value (nm).
(specimen thickness 2.5 nm)
Kogure and Inoue ( 2005 )
Selected-area diffraction patterns
from a disordered kaolin domain
Polysomatism
•
Serpentinites result from the
hydration of the oceanic upper
mantle from


ocean-floor spreading or
a wedge above the subducting
lithosphere
It is a well-known building stone
serpentine layers present partially
different dimensions along the a and
b directions; different mechanisms
provide solutions to their fitting
rolled microstructure
Baronnet and
Devouard (2005)
The antigorite structure results
from a structural modulation of the
serpentine layers along the a
direction
A selected area electron diffraction (SAED) pattern consists of the main diffraction
spots from the subcell, which are surrounded by satellite diffraction spots from the
modulated structure of antigorite
The m-values, which express the
modulation wavelength, range
from 13 to 50
Antigorite microstructures vary
from highly ordered to lower
periodic structures in the c
direction
Auzende et al., 2006
•fractured at different
scales (cm to nm)
shearing is caused by brittle
and/or ductile deformation mechanisms
an antigorite monocrystal
affected by a microfault
Auzende et al., 2006
grain boundary with intense antigorite
recrystallization
With increasing metamorphic grade, the brittle
behaviour gives way to pressure-solution
Serpentine microstructures can potentially preserve information
on metamorphic conditions
•Once the partial dehydration reaction is reached, antigorite may recrystallize
mainly by a pressure-solution mechanism. The required fluids would be derived
from the progressive dehydration of antigorite. When pore pressure becomes
too high, a major fracturing event may occur (Dobson et al., 2002)
•Embrittlement accounts for deep earthquakes within the subducting
lithosphere (e.g. Jung et al., 2003)
•Pressure solution is the most effective mechanism to accommodate
deformation; its recurrent association with eclogite facies rocks in paleosubduction zones (the Alps, the Himalayas and the Caribbean) is consistent with
the possible role of antigorite serpentinites in the preservation and exhumation
of HP-LT rocks
•Serpentinites can localize the deformation within a subduction/exhumation
channel, thus making it possible to preserve eclogites from depths of about
100 km below the Earth’s surface.
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