TSK 11 Göttingen 2006 Schmidt et al.
 Measurement of calcite
 crystallographic-preferred
 orientations by magnetic
 anisotropy and comparison
 to diffraction methods
 Vortrag
 Volkmar Schmidt1 Ann M. Hirt1
 Luigi Burlini2 Bernd Leiss3 Jens M.
 Walter4
 AMS as a petrofabric tool
 The anisotropy of magnetic suscepti-
 bility (AMS) of rocks reflects the pre-
 ferred orientations of minerals. There-
 fore AMS is a quick and easy way to
 characterize rock fabrics (Hrouda 1982,
 Borradaile 1988); the obtained result is
 also called the magnetic fabric of the
 rock. The method has been often used
 to measure the orientation of ferromag-
 netic minerals, mainly magnetite, but in
 recent studies it has been increasingly
 used to measure textures of paramag-
 netic minerals as phyllosilicates (Lüneb-
 urg et al. 1999, Cifelli et al. 2004).
 AMS of calcite textures A fur-
 ther application is the measurement of
 diamagnetic textures, especially calcite
 textures. Calcite is suitable for the AMS
 method, because it has a high magnetic
 anisotropy with the minimum suscepti-
 bility along the crystallographic c-axis.
 Therefore a preferred orientation of the
 c-axes, which can be induced by de-
 formation, generates a magnetic fabric.
 The relationship between AMS and de-
 formation of marbles has been investi-
 1 Institute of Geophysics, ETH Zurich, 8093
 Zurich, Switzerland 2 Geological Insti-
 tute, ETH Zurich, 8092 Zurich, Switzerland
 3 GZG, University of Göttingen, Göttin-
 gen, Germany 4 Forschungszentrum Jülich,
 Jülich, Germany
 Figure 1: Example of neutron diffraction
 pole figures (equal-area, lower hemisphere
 projection). Compaction direction is nor-
 mal to the pole figure. Lowest contour line
 is equal to 1.0 multiple of random distribu-
 tion (m.r.d.); contour interval is 0.25 m.r.d.
 gated in laboratory (Owens & Bamford
 1976) and field studies (de Wall et al.
 2000). However, the diamagnetic fabric
 of calcite is relatively weak and easily
 overprinted by paramagnetic and ferro-
 magnetic phases. In natural carbonate
 rocks the ferromagnetic and the para-
 magnetic subfabrics should be separated
 to assure a correct interpretation of the
 AMS. This separation can be made by
 torque measurements in high fields at
 different temperatures (see Schmidt et
 al., this volume).
 Textures of artificial calcite-
 muscovite aggregates
 To test how well AMS reflects the ac-
 tual mineral texture, we produced a se-
 ries of artificial calcite-muscovite aggre-
 gates. Powders made from Carrara mar-
 ble and muscovite single crystals were
 mixed in different proportions and com-
 pacted uniaxially at room temperature
 to obtain a texture. The samples were
 further compacted hydrostatically to re-
 duce the porosity and to improve the
 mechanical properties. The samples
 show a c-axis preferred orientation of
 1
Schmidt et al. TSK 11 Göttingen 2006
 the calcite and muscovite along the com-
 paction direction (Fig. 1). The textures
 were measured with different methods
 as AMS, X-ray diffraction and neutron
 diffraction and the results are compared.
 Sample volumes of more than 11 cm3
 were used to measure AMS as well as
 neutron diffraction; the same specimens
 could be used for both methods.
 Low- and high-field AMS measurements
 were made on all samples. The high-
 field measurements reflect more accu-
 rately the weak dia/paramagnetic fab-
 rics. A nearly perfectly oblate AMS el-
 lipsoid with minimum susceptibility axis
 sub-parallel to the compression direc-
 tion developed even at very low pres-
 sures, and the principle axes of the AMS
 ellipsoid are co-axial with the calcite
 fabric ellipsoids. A general increase in
 AMS is observed with an increase in
 compaction. The quantitative correla-
 tion between the strength of the texture
 and the AMS will be investigated in the
 future. Moreover the separation of mag-
 netic sub-fabrics will be tested on these
 artificial samples. The first results show
 that AMS can reflect the textures of the
 individual phases in a multiphase rock.
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