Transcript Нейтронографические и акустические исс

The study of Quartz Textures in Multiphase rocks using
Neutron Diffraction Texture Analysis
at the JINR, Dubna
G.F. Ndlovu1, T.I. Ivankina2, R.N. Vasin2
Council for Scientific and Industrial Research (CSIR), Pretoria, South Africa
Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research, Dubna, Russia
1
2
JINR, Summer Student Practice, September 2009
Main topics
• Why Quartz
• Why measure texture
• Goals
• Methods
• Results
• Experimental
• Theoretical
• Conclusions
• Acknowledgements
Why Quartz
•
•
•
Most common compound in the Earth’s Crust (SiO2 ) and most useful
Occurs in all environments and all rock types - sedimentary, metamorphic or igneous
Its piezoelectric properties make it highly useful in modern technology
OKU 818
Composition
- quartz (~40%)
- mica (~30-40%)
- plagioclase (~20-30%)
Outokumpu Deep Drilling Project depth - 2516 m
Why do geologists measure texture?
• The modelling of physical anisotropies (seismic wave velocities, heat conductivity,
thermal expansion, magnetic, piezoelectric, etc.) of rocks
 Deconvolution of the deformation history of rocks on the basis of the symmetry of the
mineral textures, which commonly reflect the symmetry of deformation
Single crystal
Texture
Property
Rock sample (marble)
Crystallographic texture
Many materials are polycrystalline bodies, i.e. they consist of grains (crystallites) with a different size and
orientation.
Crystallographic texture is the lattice (or crystallographic) preferred orientation of crystallites of the same
phase (mineral) in the chosen coordinate system: LPO (or CPO).
Random orientation:
NO crystallographic
texture
Aligned grains:
One-component
crystallographic texture
Multi-component
crystallographic texture
The properties of the polycrystal are anisotropic and depend upon texture
Objectives
•
Learn about the operation of the SKAT diffractometer
– Measure the diffraction spectra of geological samples (quartz-bearing)
•
Use AutoIndex, GeoTOF, Pole Figure plot and Beartex programs
–
–
–
Indexation of spectra
Extracted experimental pole figures from spectra
Obtain quantitative 3D orientation distribution function (ODF)- quantitative measure of texture
Methods
Experimental
using neutrons creates completely new possibilities, some of which are unique and inaccessible by x rays
Main Advantages of Neutron Diffraction Technique
• low absorption of neutrons in matter »large sample volumes accessible
• TOF » complete diffraction patterns can be recorded
• application of multiple detectors » measurements are fast
• excellent spectral resolution » suitable for polyphase geological samples with many diffraction lines
• unique scattering angle 2 of all detectors » minimum of intensity corrections required
Pole figure raster
Pole sphere
A
S
S
A
S
A
Schematic view of the SKAT’s
detector system. 19 detector
modules named from A to S,
with S in the center of the pole
figure.
The line on the unit
sphere corresponds to
the scattering vectors
of detector ring, line in
the XY plane is its
stereographic
projection.
The grid of the measured
pole figure. Small circle
corresponds to the plane
projection of the scattering
vectors, dots shown where
the data on pole density are
situated.
Texture diffractometer SKAT operates in the
beam of the reactor IBR-2 (JINR, Dubna,
Russia). 19 detectors are placed completely
on the assembly ring maintaining the axial
symmetry with respect to the neutron beam
Theoretical Methods
The traditional method for the representation of preferred
orientations is pole figures, i.e.,stereographic projection of
the normals to the planes (hkl). A pole figure gives an
answer to the question:Which volume fraction of the sample
have a orientation for which the lattice plane normal
coincide with a sample direction Z
Spherical coordinates of normal to
crystalographic plane (001) ( pole P2)
Neutron diffraction quartz
PF (11-20)
stereographic projection of
pole Р 2
Mathematic description of the crystallographic texture: orientation distribution function
(ODF) f(g), where (g) corresponds to the rotation to align the coordinate system of the sample
Ka with the coordinate system of the crystallite Kb.
Za
β
(Xa,Ya,Za): Ka – right-handed, Cartesian sample coordinate system.
Zb
(Xb,Yb,Zb): Kb – right-handed, Cartesian crystal coordinate system.
Quantitatively the orientation of the certain crystallite (g) is described by
three Euler angles g={α,β,γ}. All the possible orientations (0 ≤ α,γ ≤ 2π; 0
≤ β ≤ π) form the orientation G-space.
Xb
α
Xa
Yb γ
Ya
f (g) describes the volumetric fraction of crystallites with the orientation
g+dg. It is normalized to 1:
Data processing
Normalized diffraction spectra
Experimental pole figures (measured simultaneously
due to application of TOF-method)
Calculation of the ODF (WIMV method)
Recalculation of pole figures
using the ODF
• (0001) - absent reflection
• (11-20),
• (10-11), (01-11) –
overlapped.
ODF characteriztion:
texture index F2,
construction of the
ODF-histogram and
ODF-spectrum
Crystallographic textures that are characteristic of quartzites
A.N. Nikitin, T.I. Ivankina, K. Ullemeyer, R.N. Vasin, 2008, published in Kristallografiya, 2008, Vol. 53, No. 5, pp. 859–866
Dauphine twins, type I: two
crystals, one rotated around
[0001] on 180°. Pole figures,
stereographic projection,
linear scale contours.
PFs exhibit strong, symmetrically dependent peaks of high pole density
Model texture, type II
(type I + misorientation)
Pole figures, stereographic
projection, linear scale
contours.
Differ from the first-type textures by diffused peaks and lower pole density
Analogous quartz textures in different rocks
Model texture, type III
(rotation around normal to
(02-23))
Pole figures,
stereographic projection,
linear scale contours,
rotation axis direction: left
to right.
The (001) PF exhibits a diffused pole-density peak with a tendency to waist
distribution of c axes along small-circle arcs
Model texture, type IV
(rotation around normal to
(02-21))
Pole figures, stereographic
projection, linear scale
contours, rotation axis
direction: left to right.
Results
(10-10)
(10-11)
(11-20)
(10-12)
(11-21)
Intensity, a.u
Fig. 1. Diffraction spectrum and pole figures corresponding to indexed reflections for the OKU 818 quartzite sample
TOF-chanels
Experimental pole figures
Recalculation of pole figures using the ODF
•
(11-21),(01-11) - absent reflection
•
(10-11), (01-12) – overlapped
•The main objective of texture analysis is to obtain information on the crystal orientation distribution in the sample
•Incomplete pole figures and regions of the diffraction spectrum containing overlapping peaks
Recalculated pole figures of the principle crystallographic planes
001
0.3
1.6
101
110
0.5
1.3
0.6
1.3
011
0.6
1.6
• The texture of a polycrystalline sample is a statistical ensemble of crystallites.
• A statistically representative number of crystallites or grains is needed to obtain reliable information.
• Obtaining reproducible pole figures requires 104–105 grains
Conclusions
•
Experimental PFs were used to reconstruct ODFs, on the basis of which PFs were calculated for the principal
directions in quartz bearing rock samples
•
The rock sample under study exhibit a strong quartz texture (the maximum pole density > 1.56)
•
Pole figure data processing yielded the complete texture for quartz
•
In addition to the mineral textures factors like oriented, microcracks and grain boundaries control the elastic
properties of rock samples
•
Useful in studying how quartz grains interact with or are affected by other minerals during deformation
Remarks
–
Improve the intensity/background ratio and increase the flux of thermal neutrons at the sample position
Acknowledgements
• Many thanks to the following Organisations and Personnel
– JINR, Dubna
– FNL, JINR, Dubna
• Dr. Tatyana Ivankina
• Dr. Roman Vasin
– The NRF
• Dr. Noel Jacobs
– The CSIR & Univ of Free State
• Prof. Thembela Hillie
• Prof. Wiets Roos
Thank You!