Chemically induced fracturing in alkali feldspar |
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Authors: | K S Scheidl A -K Schaeffer E Petrishcheva G Habler F D Fischer J Schreuer R Abart |
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Institution: | 1. Department of Lithospheric Research, University of Vienna, 1090, Vienna, Austria 2. Institute of Mechanics, Montanuniversit?t Leoben, 8700, Leoben, Austria 3. Institute for Geology, Mineralogy and Geophysics, Ruhr-University Bochum, 44801, Bochum, Germany
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Abstract: | Fracturing in alkali feldspar during Na+–K+ cation exchange with a NaCl–KCl salt melt was studied experimentally. Due to a marked composition dependence of the lattice parameters of alkali feldspar, any composition gradient arising from cation exchange causes coherency stress. If this stress exceeds a critical level fracturing occurs. Experiments were performed on potassium-rich gem-quality alkali feldspars with polished (010) and (001) surfaces. When the feldspar was shifted toward more sodium-rich compositions over more than about 10 mole %, a system of parallel cracks with regular crack spacing formed. The cracks have a general (h0l) orientation and do not correspond to any of the feldspar cleavages. The cracks are rather oriented (sub)-perpendicular to the direction of maximum tensile stress. The critical stress needed to initiate fracturing is about 325 MPa. The critical stress intensity factor for the propagation of mode I cracks, K Ic, is estimated as 2.30–2.72 MPa m1/2 (73–86 MPa mm1/2) from a systematic relation between characteristic crack spacing and coherency stress. An orientation mismatch of 18° between the crack normal and the direction of maximum tensile stress is ascribed to the anisotropy of the longitudinal elastic stiffness which has pronounced maxima in the crack plane and a minimum in the direction of the crack normal. |
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