Textures of experimentally deformed hematite ores with magnetite and wuestite

Transcription

1 RHEINISCH- WESTFÄLISCHE TECHNISCHE HOCHSCHULE AACHEN Textures of experimentally deformed hematite ores with magnetite and wuestite H Siemes, B Klingenberg, E Jansen, W Schäfer, G Dresen, E Rybacki, M Naumann Institut für Mineralogie und Lagerstättenlehre, RWTH Aachen, Germany Mineralogisch-Petrologisches Institut der Universität Bonn,Germany Geoforschungszentrum Potsdam, Germany Introduction siemes@rwth-aachende Preferred orientations (textures) of rocks are imprints of the geological history and inform about the anisotropies of physical properties of the earth's crust and mantle Textures, which are formed by deformation and recrystallization at a variety of pressure and temperature conditions, reflect the preferred orientation of the fabrics, which are characteristic for the different mineral components Here, the generation, the decomposition and building of preferred orientations of hematite ores at a variety of well defined experimental deformation conditions are described The natural material reveals a rather weak starting texture Since neutron diffraction permits texture investigations on large specimens, the pole figures represent volume textures with excellent grain distribution statistics Neutron diffraction experiments of the hematite specimens can be performed within their iron jackets of the deformation experiments

2 Measurement of pole figures The pole figure measurements were performed using the University of Bonn texture diffractometer SV7 at the FRJ-2 reactor in Jülich Since SV7 is equipped with a large linear detector covering a 2theta-range of 50 degrees, about 40 reflections of all mineral components were measured in one scan using a wavelength of 12 Ångstrøm The standard scan grid comprises about 500 sample positions The measuring time was about 20 hours In total, 33 specimens were measured and about 400 pole figures were extracted from the scans Transformation of the specimen orientation (phi,chi) on the Eulerian cradle into an equidistant pole figure grid (alpha,beta)

3 Neutron diffractograms Profile analysed diffraction patterns for the extraction of the pole figures before (top) and after deformation with reaction products (bottom)

4 Starting material From a hematite ore from South Africa consisting of layers of fine and coarse grains (see below) cylinders of 10 mm diameter and 20 mm length were cored in 3 orientations: S-specimens perpendicular to foliation S N-specimens parallel to foliation S P-specimens parallel to S and perpendicular to N Additionally, R-specimens from a Brazilian ore cored in the same orientation as N were used Experimental {003}-, {110}-, and {300} pole figures of the starting material: S- specimens (top row), N-specimens (center row), and P-specimens (bottom row )

5 Photomicrographs showing different grain sizes: 05 to 65 microns (left), 90 to 175 microns (right) Deformation experiments The deformation tests were performed using a Paterson apparatus of the GFZ Potsdam Iron jackets were employed to separate the specimens from the confining pressure medium Argon Deformation conditions confining pressure: 400MPa 600 C le temperature le 1100 C 10e-4/s ge strain rate ge 10e-6/s 71 % le maxstrain le 217 % 895 MPa ge maxstrength ge 24 MPa Stress/strain curves showing strain rate steps at 10e-6/s, 10e-5/s, and 10e-4/s

6 Hematite reacts with the iron jacket to magnetite and wuestite The crust thickness depends on the temperature, the duration of the test, and the separating foils (Au,Rh,Ag/Pd) which where placed between iron jacket and specimen to reduce the reaction process Stronger reactions to magnetite and wuestite at Rh-foil failure Strength of the specimens using different separating foils

7 Set of experimental pole figures Upper right corner: h=hematite, m=magnetite, w=wuestite Pole figures of the specimen S08 (1000 C, 20 %) as extracted from a single specimen scan at SV7

8 Hematite pole figures before deformation temperature: 700C, final strain: 215%, texture type 1 temperature: 800C, final strain: 210% texture type 2 temperature: 1070C, final strain: 155%, texture type 3 Modifications of the pole figures of N-specimens with increasing temperature

9 Magnetite pole figures specimen: S-08, temperature: 1000C, final strain: 200% specimen: R-19, temperature: 900C, final strain: 106% Wuestite pole figures specimen: S-06, temperature: 900C, final strain: 205% specimen: N-03, temperature: 1000C, final strain: 208%

10 Conclusions Deformation perpendicular to the foliation results only in minor texture modifications Deformation parallel to the foliation results in three different texture types of hematite: Type 1 (below 800C) dominance of the a{110}m<1-10> slip system Type 2 (above 800C) rotations due to the basal slip system c(001)a<110> Type 3 (above 1000C) increasing influence of diffusional flow processes Magnetite reveals a <220> fiber texture Wuestite reveals a <220>- fiber texture and a weaker component with <100> parallel to the compression axis Deformation map: large dots indicate the deformation conditions of this work

11 Acknowledgements Financial support by the DFG under contract no Si and by the BMBF under contract no 03KI5BO2 is gratefully acknowledged References Frost, HJ, Ashby, MF (1982) Deformation Mechanism Maps - The Plasticity and Creep of Metals and Ceramics Pergamon Press, Oxford, pp 166 Jansen, E, Schäfer, W, Kirfel, A (2000) The Jülich neutron diffractometer and data processing in rock texture investigations J Struct Geol 22, Kawamoto, T, Hirose, K (1994) Au-Pd sample containers for melting experiments on iron and water bearing systems Eur J Mineral 6, Paterson, MS (1990) Rock deformation experimentation In: Duba, AG, Durham, WB, Handin, JW, Wang, HF (Eds) The Brittle-Ductile Transition in Rocks The Heard Volume, American Geophysical Union Geophysical Monograph, 56 AGU, Washington, DC, pp Rosière, CA, Siemes, H, Quade, H, Brokmeier, H-G, Jansen, EM (2001) Microstructures, textures and deformation mechanisms in hematite J Struct Geol, 23, Schaeben, H (1994) Diskrete mathematische Methoden zur Berechnung und Interpretation von kristallographischen Orientierungsdichten DMG Informationsgesellschaft mbh, Oberursel, 137 S Will, G, Schäfer, W, Merz, P (1989) Texture analysis by neutron diffraction using a linear position sensitive detector Textures & Microstructures 10, Will, G, Merz, P, Schäfer, W, Dahms, M, (1990) Application of position sensitive detectors for neutron diffraction texture analysis of hematite ore In: Barret, C S, Gilfrisch, J V, Huang, T C, Jenkins, R, Predecki, P K (Eds) Advances in X-Ray Analysis 33 Plenum Press, New York, pp