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  • 1
    Electronic Resource
    Electronic Resource
    Time evolution of twin domains in cordierite: a computer simulation study (1999)
    Springer
    Physics and chemistry of minerals 26 (1999), S. 275-296 
    Details
    ISSN: 1432-2021
    Keywords: Key words Cordierite ; Twin structure ; Time evolution ; Domain ; Computer simulation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract  The time and temperature evolution of twinning in cordierite is simulated using three computer models. The orientation of walls between twin domains in natural cordierite follows mainly the ferroelastic pattern which minimises the strain energy of the walls between twin-related domains. Such ferroelastic twinning is simulated in an elastic three-states Potts model in which each structural six-membered ring is represented by a three state pseudo-spin. The resulting twin pattern in a sample with 3169 structural rings shows sector trilling and fine scale ferroelastic wall patterns which coarsen with increasing annealing time. The poorly defined wall directions observed in cordierite were found to be related to twin walls which do not minimise the strain energy. Instead, these walls are located along the corners of pseudo-hexagonal rings and appear as the consequence of local rather than global interatomic interactions. Simulations using two-dimensional (38028 atoms) and three-dimensional (408 228 atoms) structural models show a predominance of these topological walls over the strain walls at early stages in the ordering process. The domain structure in the simulation is patchy rather than corresponding to repeated stripe structures found in other ferroelastic and co-elastic materials. In all models, a strong tendency for sector trilling is observed. In kinetic tweed patterns a novel 60º tweed is found at atomic length scales while the usual strain-mediated 90º tweed appears at mesoscopic length scales. An unusual surface tension effect in domain formation and ’writhing’, fluid-like motion was found in the three-dimensional structural model. This motion, along with the existence of non strain-mediated walls may contribute to cordierite’s poorly defined domain wall directions at the early stages of domain coarsening.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002690050188
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  • 2
    Electronic Resource
    Electronic Resource
    Cation ordering in BaAl2Ge2O8-feldspar: implications for the phase transition in anorthite (1999)
    Springer
    Physics and chemistry of minerals 26 (1999), S. 354-366 
    Details
    ISSN: 1432-2021
    Keywords: Key words Phase transition ; Anorthite ; Ising-Model ; BaAl2Ge2O8
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract  BaAl2Ge2O8-Feldspar undergoes an order-disorder phase transition I2/c↔C2/m at T tr ≈1690 K. The thermodynamics of the Al,Ge cation ordering process is described in terms of the compressible Ising model in mean field approximation. The mean field potential predicts a first order character of the phase transition. This is compared to antiferromagnetic ordering in a two-dimensional square Ising model with NN-pair interactions and four-spin interactions on alternating squares. Calculated order parameters and short range ordering are in good agreement with the corresponding properties observed in BaAl2Ge2O8-feldspar by means of X-ray diffraction, hard mode infrared spectroscopy and TEM. Using known calorimetric data a similar model is postulated for Al,Si ordering in anorthite, CaAl2Si2O8, for which the derived potential describes a transition with slightly stronger first order character at T tr ≈1928 K.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002690050195
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    Paper (German National Licenses)
    Fulltext
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