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  • 1
    Publication Date: 2019-06-07
    Description: Calcite growing in biomimetic hydrogel environments incorporates the gel during its growth. The amount of occluded gel within the composite is mainly determined by the interaction between gel strength and crystallization pressure, with the latter being directly related to supersaturation and growth rate. In previous work we established a direct correlation between increased amounts of occluded gel with misorientations in the growing calcite crystals or aggregates. The presence of Mg2+ in the growth environment adds complexity to the internal structuring of the mineral.In this contribution we examine the effects of Mg2+ on the mechanical parameters of gelatin hydrogel and silica hydrogel by mechanical shear stress tests, we determine characteristics of the gel fabric occluded in the calcite using selective etching techniques and high-resolution field emission scanning electron microscope (FE-SEM) imaging, and we use electron backscatter diffraction (EBSD) to study co-orientation or misorientation in the calcite crystals or aggregates.We show that two independent mechanisms are responsible for the complex impact of Mg2+ in the growth medium on the calcite/gel composites. First, addition of 0.1 M Mg2+ reduces the yield-strength of the gels by about 50%. While gelatin gel shows continuous strain hardening in a similar way for Mg-bearing and Mg-free systems, the silica-gel weakens after reaching an ultimate shear strength, where the strain associated with the maximum in strength shifts by 350% to higher values. The decreased gel strength in the Mg-bearing systems leads to decreased amounts of occluded gel. Second, incorporation of Mg2+ in the growing calcite (i) increases its solubility and thus decreases crystallization pressure, and (ii) introduces small angle grain boundaries due to misfit strains which lead to “split growth”, i.e. misoriented subunits of the calcite or – ultimately – spherulitic growth. Our study further clearly shows that Mg not only influences the organization of the mineral component within the aggregate but also the fabric of the occluded gel matrix. The fabrics of the occluded gel change from compact gel membranes to finely dispersed networks with increasing Mg and, correspondingly, decreased crystallization pressure via increasing solubility as more Mg incorporates into calcite structure. This circumstance initiates the large variety of calcite crystal co- and misorientation patterns and hierarchical assemblies that we find in the investigated composites.
    Print ISSN: 0935-1221
    Electronic ISSN: 1617-4011
    Topics: Geosciences
    Published by Schweizerbart
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  • 2
    Publication Date: 2020-01-09
    Description: The fault gouge of the Alhama de Murcia fault (southeast Spain) shows a texture that resembles a mylonite, including a prominent foliation, S-C fabric, and isoclinal folds. It also embeds a large number of isolated pulverized quartz clasts (PQCs). Structural analysis indicates that the gouge fabric was mainly developed by slow frictional sliding along phyllosilicate-lined Riedel shear bands during continued shearing. In contrast, the PQCs show tensile fracture network features that are typically reported in seismically pulverized rocks found along seismogenic faults. This suggests that quartz-clast pulverization was due to a transient dilatational mechanism rather than shearing. We propose that the PQCs are the result of a rapid confined stress drop related to transient tensile stresses during coseismic ruptures that interrupt creep faulting along the gouge zone. The present study suggests that there is probably a large amount of evidence for paleoseismicity in fault rocks that is currently overlooked.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
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