TSK 11 Göttingen 2006 Hilgers et al.
 Microstructures of fibrous
 halite veins Poster
 Christoph Hilgers1 Gill Pennock2
 Zsolt Schléder1 Stansilaw Burliga3
 Janos L. Urai1
 Introduction
 Halite veins hosted in clastic sedimen-
 tary rocks are frequently observed next
 to evaporite layers. Their microstruc-
 ture can be enhanced by gamma ir-
 radiation and etching, which can be
 used to infer the deformation mecha-
 nism of halite (e.g. Howard & Kerr
 1960, Schleder & Urai 2005). In this
 study, we present results from gamma-
 decorated vein microstructures of fi-
 brous halite veins hosted in claystone.
 Data
 Samples were taken in the Klodawa salt
 mine in Poland and the Hengelo mine
 in the Netherlands. Both locations ex-
 pose halite veins with a white and or-
 ange colour. The halite host rock, which
 is the source of halite precipitated in
 the vein, is located centimeters to me-
 ters away from the veins. The vein mi-
 crostructure is fibrous with variable fi-
 bre diameters, and slightly widens to-
 wards the vein-wall interface. This in-
 dicates antitaxial vein growth from the
 centre towards the wall and precipita-
 tion of new material on both sides of
 the vein. Fibre grain boundaries are
 straight and not serrated. Irradiated
 samples show that some fibres consist of
 subgrains, which formed during growth.
 1 Geologie-Endogene Dynamik, RWTH-
 Aachen, D-52056 Aachen, Germany
 2 Utrecht University, Faculty of Earth
 Sciences, 3508 TA Utrecht, The Netherlands
 3 Wroclaw University, Department of Struc-
 tural Geology, ul. W. Cybulskiego 32, 50-205
 Wroclaw, Poland
 Figure 1: Gamma-irradiated fibrous halite
 veins show different fibre widths ranging
 between millimeters to microns. Micron-
 scale fibres are growth subgrains, while de-
 formation subgrains are absent in the vein.
 Klodawa mine, Poland.
 Overall, the vein is devoid of defor-
 mation subgrains. Repeated solid- or
 fluid inclusion bands have not been ob-
 served in the veins, which would point to
 the crack-seal mechanism causing vein
 opening (see also Hilgers & Urai 2005
 for similar conclusion on antitaxial gyp-
 sum and calcite veins).
 EBSD analysis suggests a random tex-
 ture (Fig. 2), which is also apparent in
 colored maps showing the crystal direc-
 tions of the grains. Thus, significant
 growth competition is absent and grains
 continue to grow regardless of their crys-
 tallographic orientation. Growth in an
 1
Hilgers et al. TSK 11 Göttingen 2006
 Figure 2: The fibrous halite vein from Hengelo shows a random texture (137 grains
 measured).
 open vug would result in euhedral crys-
 tals and significant growth competition,
 while randomly oriented fibrous grains
 require contact with the wall rock dur-
 ing growth (Hilgers et al. 2001).
 Conclusion
 Fibrous antitaxial halite veins formed in
 extension fractures in claystone. Mi-
 crostructural indicators for repeated
 crack-seal mechanism are absent, and
 thus veins may rather have formed dur-
 ing a continuous growth process. The
 bulk permeability of the fractured rock
 was low even if the vein aperture reaches
 several centimetres, because a random
 crystallographic texture of the vein mi-
 crostructure requires close contact be-
 tween the growing vein and the host
 rock.
 References
 Hilgers C, Koehn D, Bons PD & Urai JL (2001)
 Development of crystal morphology during
 unitaxial growth in a progressively widening
 vein: II. Numerical simulations of the evo-
 lution of antitaxial fibrous veins. Journal of
 Structural Geology, 23, 873–885
 Hilgers C & Urai JL (2005) On the arrange-
 ment of solid inclusions in fibrous veins and
 the role of the crack-seal mechanism. Jour-
 nal of Structural Geology, 27(3), 481–494
 Howard & Kerr, RC (1960) Blue halite. Sci-
 ence, 132(3443), 1886–1887
 Schleder Z & Urai JL (2005) Microstructural
 evolution of deformation-modified primary
 halite from the Middle Triassic Röt Forma-
 tion at Hengelo, The Netherlands. Interna-
 tional Journal of Earth Sciences, 94, 941–955
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