Radiaxial Fibrous Calcite as a Replacement after Syn-Sedimentary Cement
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The mechanical milling of biogenic aragonite and calcite was performed at room temperature with milling time of 1min~10hand the milled products were characterized with XRD and SEM.The results show that the reversible transformation between aragonite and calcite can be realized by milling at low temperature.In fact,the pure aragonite begins to transform into calcite after 15min milling.On further milling for about 1h,the calcite proportion amounts to the maximum of about 33%,however,gradually decreases to an equilibrium value with more than 1h milling.In comparison,the pure calcite begins to transform into aragonite only after 5hmilling.On further milling,the aragonite proportion increases rapidly to about 80%,then gradually increases to an equilibrium value after more than 6.5hmilling.The milled products always reach a similar equilibrium value(aragonite-88%,calcite-12%)after enough milling time,independent of the original material composition.Therefore,the milling-induced transformation between aragonite and calcite is consistent with Le Chatelier′s principle.
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9 popular species of modern bivalves living in Yellow Sea were selected for their shell mineral analyses. By means of X-ray powder diffraction method, the types of minerals in the shells were discerned and their components were calculated. The result shows that aragonite or calcite is the dominant mineral in the shells. According to the component of mineral, shells were divided into three sub-type, namely, aragonite shell, calcite shell and mixed shell with component of mineral by 98% aragonite, 90% calcite, 37% aragonite and 63% calcite, respectively. Species in Veneroida, Arcida had aragonite shell. Species in Pterioida had calcite shell. And species in Mytiloida had mixed shell. This research suggests that mineral component of bivalve shell closely relates to its living pattern. Bivalves that live in the sand sediments are apt to form aragonite shell. In contrast, bivalves that live by attaching themselves on the hard bottom are apt to form calcite shell or mixed shell. Furthermore, this research also suggests that at least in some species the shell size has no significant influence on the mineral component of the shell.
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Two calcium carbonate crystal polymorphs, aragonite and calcite, are the main inorganic components of mollusk shells. Some fossil evidences suggest that ara- gonite shell is more ancient than calcite shell for the Biv- alvia. But, the molecular biology evidence for the above deduction is absent. In this study, we searched for homo- logs of bivalve aragonite-related and calcite-related shell proteins in the oyster genome, and found that no homologs of calcite-related shell protein but some homologs of ara- gonite-related shell proteins in the oyster genome. We explained the results as the new evidence to support that aragonite shells are more ancient than calcite shells in bivalves combined the published biogeological and sea- water chemistry data.
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