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Nindiyasari, Fitriana (2015): Crystallization of carbonate and sulfate minerals in organic matrices: examples from biomimetic and biological polymer-mineral composites. Dissertation, LMU München: Faculty of Geosciences



Biological carbonate hard tissues, such as the shell of the bivalve Mytilus edulis, are composites of biopolymers and minerals. M. edulis has two distinct layers, the outer layer consists of fibrous calcite and the inner layer is composed of nacreous aragonite. Close to the interface between nacreous aragonite and fibrous calcite, a 1-2 micrometer wide zone exists that consists of granular aragonite. Aragonite granules and tablets as well as calcite fibrous are embedded into matrix biopolymers. In order to understand the composite nature of these hard tissues, biomimetic experiments using hydrogels were carried out. Hydrogels are able to model biogenic matrix environments due to their ability to confine space and to determine diffusion rates, local concentrations and supersaturation of the solutes. Hydrogels have local crystallization microenvironment that is distinguished from that in solution by confinement of solutes in the hydrogel pores. However, hydorgels only mimic biological extracellular matrices to some extent as the hydrogel fiber organization lacks any order, unlike it is in the case of the cholesteric liquid phase, e. g. chitin. The hydrogel strength is adjustable by changing its solid content. It further increases local hydrogel fiber co-aligments that to some extent will mimic organic matrices in biological hard tissues. Different kinds of hydrogels were used to study calcite crystallization (silica, agarose, gelatin). As each hydrogel has different characteristics, hydrogels can act differently in promoting or inhibiting crystallization. Hydrogels have an ability to mechanically impede the growth of a crystal depending on the strength of the hydrogel. Gelatin hydrogel is a poly-peptide material derived from natural collagen through hydrolytic degradation. The hyrolitic degradation breaks the triple-helix structure of collagen into single-strand molecules. Gelatin contains both acidic and basic amino acids with isoelectric point values near ∼5 and with predominance of acidic moieties. Agarose hydrogel is a linear polysaccharide extracted from marine red algae. It consists of beta-1,3 linked D-galactose and alpha-1,4 linked 3,6-anhydro-alpha-L-galactose residues. Gelatin and agarose hydrogels are composed of a fibrous structure that have varying mesh void dimensions depending on the hydrogel solid content. Hydrogel with 2.5 wt % gelatin solid content exerts less pressure against the growing calcite crystal aggregate than a hydrogel with 10 wt % gelatin solid content. Silica hydrogel does not exert strong pressure against the growing calcite crystal aggregate due to its nature as it is composed of minute (less than 20 nm) sized spherical particles that do not appear to form a network. The hydrogel strength together with the growth rate of the crystal defines the amount of incorporated hydrogel into the growing calcite crystal aggregate such that a strong hydrogel will incorporate more gel into the calcite crystal than a weak hydrogel. Calcite grown in Mg-free silica hydrogels has a rhombohedral shape and is elongated on the c-axis. It grows as dumbbell-shaped aggregates in the presence of Mg. Silica hydrogel either Mg-free or Mg-bearing does not give a major influence on the co-orientation of the obtained crystal aggregate. Calcite grown in Mg-free agarose has two morphologies: rhombohedron-shaped calcite crystals and calcite radial aggregates. Calcite grown in Mg-bearing agarose has sheaf-like and peanut like morphologies. The presence of Mg in agarose influences the co-orientation of calcite crystals within calcite Mg-bearing agarose composites. The calcite/Mg-free agarose composite has several large crystal subunits while the calcite/Mg-bearing agarose composite shows a spherulitic microstructure. In the case of gelatin hydrogel, the precipitate consists of calcite aggregates that have a variety of features i.e. the formation of mosaic crystals and mesocrystal-like subunits in one aggregate, the formation of aggregates with a fan-like distribution of the c-axis orientation and the formation of spherulitic aggregates. The formation of aggregates with different characteristic in the subunits can be explained as a result of a combination between local differences in gelatin matrix arrangement and physicochemical conditions such as the change in Mg/Ca ratio, pH, saturation, etc. The development of a fan-like distribution of the c-axes orientation in the calcite aggregate subunits can be explained as a result of Mg intrasectorial zoning. A different degree of Mg incorporation in different growth steps will accumulate misfit strain in the lattice. This misfit strain could be released through the formation of dislocations at regular intervals, such that small-angle boundaries develop. This growth further leads to the extreme split growth and the formation of fan-like and spherulitic crystal aggregates. The etching experiments of calcite/hydrogel composites reveal the structure of the incorporated hydrogel within the calcite crystals and aggregates. In the case of Mg-bearing silica hydrogel more silica hydrogel is incorporated into the calcite crystal than in the case of Mg-free silica hydrogel. Thick hydrogel membranes are observed when Mg-free gelatin and agarose hydrogels are used. These membranes do not occur when Mg is present. The formation of these membranes in Mg-free gelatin and agarose hydrogels is a result of an accumulation of the hydrogel fibers that are driven back by growing crystals or aggregates. The stiffness of the gelatin and agarose hydrogel fibers increase as Mg is added into the hydrogel. The hydrogel becomes stiffer and exerts more pressure against the growing aggregates. No hydrogel membranes are observed in aggregates grown in Mg-bearing gelatin and in agarose hydrogels. On the basis of biopolymer and mineral composites, gypsum (CaSO4)/cellulose fiber composites were prepared. The purpose of the addition of cellulose fiber to gypsum was to create a composite with a high ecological value and interesting mechanical properties such as high Young’s modulus, high bending strength and high compression strength. The cellulose fiber affects the mechanical property of the composites depending on the fiber characteristics, e.g. the nature of the cellulose (natural or synthetic), water retention value, degree of swelling, etc. Lyocell fiber, a synthetic fiber, is found to be able to increase the Young’s modulus of the final composite.