Microstructure and crystallography of abalone shells

Biominerals are biogenic mineralized products comprising both mineral and organic components. Calcium-bearing minerals comprise about 50% of biominerals with the most common being polymorphs of calcium carbonate, e.g. calcite and aragonite. Abalone (Haliotis asinina Linnaeus, 1758; Haliotis rufescens Swainson, 1822 and Haliotis gigantea Gmelin, 1791) are marine snails, belonging to the Phylum Mollusca, class Gastropoda, family Haliotidae, genus Haliotis. They can be easily recognized by the row of apertures, which are closed, half-closed and open ones, along the spiral ridges and towards the shell longitudinal growth direction of the dorsal margin. This study considers the microstructure and crystallography of three species of abalone shells, Haliotis asinina from Australia, Haliotis rufescens from America and Haliotis gigantea from Japan; as well as the aperture infill of H. asinina. The microstructure and crystallography are analysed through Scanning Electron Microscopy (SEM) and Electron Backscatter Diffraction (EBSD) respectively. Abalone shells all have an outer prismatic and an inner nacreous layer. It is confirmed that the shell of H. asinina is comprised of aragonite in both prismatic and nacreous layers, with c-axis orientation of crystals throughout the prismatic and nacreous layers of shell. Towards the prismatic-nacreous interface, crystallographic alignment of prismatic layer becomes much more tightly constrained. In nacre, crystallographic continuity persists across several laminae. Along the longitudinal shell growth direction (from posterior part to anterior), there is a trend of gradually thickening tablets towards the shell interior. In addition to this trend from exterior to interior, the tablets formed at the anterior are thicker than those formed earlier at the posterior. It is also confirmed that the nacreous layer in H. rufescens and H. gigantea is aragonite. The prismatic layer of H. rufescens shell is composed of calcite, the c-axis of which is normal to the shell surface. The prismatic layer of H. gigantea shell consists of calcite and aragonite, the c-axis of aragonite is normal to the shell surface and that of calcite is parallel to the shell surface. In these three species, there is higher crystallographic constraint in the nacreous layer than in the prisms. Furthermore, the crystallographic continuity of the nacreous layer persists across as many as 40~50 laminae. H. asinina is about 800 µm thick, H. rufescens is the thinnest at around 600~700 µm and H. gigantea shell is the thickest at about 1 mm. Comparing the three species, the prismatic layer to the shell thickness of three species, the calcite prismatic layer (H. rufescens) takes up about 50% of the shell thickness compared with aragonite prismatic layer (H. asinina) of about 20~30%, and the prismatic layer with calcite and aragonite (H. gigantea) is between the two at over 30% of total shell thickness. Aperture infill of H. asinina is also composed of aragonite with both prismatic and nacreous layers. The crystallographic orientation of infill prismatic layer is parallel to the shell surface unlike that of the shell where the crystallographic orientation of the prismatic layer is perpendicular to the shell surface. There is a prism-like layer between aperture infill and the shell. The contact with the shell always occurs within the nacreous shell layer. Aperture infill grows on this prism-like layer and the growth rate of aperture infill is in- keeping with that of the shell growth rate. When individuals reach six months and older, shell growth and aperture infill occur at approximately the same speed

Airflow induced by pumping tests in unconfined aquifer with a low-permeability cap

Most analytical and numerical models developed to analyze pumping test data focus on saturated flow below the water table. Traditionally the soil above the initial water table prior to pumping has been thought to have little influence on the test results and has usually been ignored. It is hypothesized that, if the unsaturated zone is capped by low-permeability soil, airflow in the unsaturated zone may be developed during pumping and may have impact on the drawdown in the aquifer. A transient, three-dimensional and variably saturated flow model is employed to simulate the pumping-induced air and groundwater flows in both the saturated zone and unsaturated zone with a low-permeability layer. The results demonstrate that negative pressure in the unsaturated zone can be generated by pumping. The negative pressure begins to appear as the drawdown rate increases to a maximum, approaches a peak before the drawdown rate becomes zero, and then gradually disappears. Drawdown obtained from the capped aquifer is much greater because the water in the pores in the unsaturated zone is sucked by the negative pressure and the gravity drainage from the pores is hampered. Consequently, the drawdown versus time curve does not conform to the traditional S-shaped curve for an unconfined aquifer but is similar to that of a confined aquifer. If the airflow caused by the low-permeability cap is ignored, the error in estimated drawdown could be over 80% for the specific parameters and aquifer configuration used in the study. The possible errors in parameter estimation when airflow is ignored are explored. Overall, the hydraulic conductivity of the aquifer can be overestimated and the specific yield of the aquifer underestimated if airflow is ignored. The estimation error for specific yield tends to be greater than that in hydraulic conductivity. Copyright 2009 by the American Geophysical Union.published_or_final_versio

Quasi-Rip: A New Type of Rip Model without Cosmic Doomsday

The fate of our universe is an unceasing topic of cosmology and the human being. The discovery of the current accelerated expansion of the universe significantly changed our view of the fate of the universe. Recently, some interesting scenarios concerning the fate of the universe attracted much attention in the community, namely the so-called "Little Rip" and "Pseudo-Rip". It is worth noting that all the Big Rip, Little Rip and Pseudo-Rip arise from the assumption that the dark energy density $\rho(a)$ is monotonically increasing. In the present work, we are interested to investigate what will happen if this assumption is broken, and then propose a so-called "Quasi-Rip" scenario, which is driven by a type of quintom dark energy. In this work, we consider an explicit model of Quasi-Rip in detail. We show that Quasi-Rip has an unique feature different from Big Rip, Little Rip and Pseudo-Rip. Our universe has a chance to be rebuilt from the ashes after the terrible rip. This might be the last hope in the "hopeless" rip.Comment: 9 pages, 2 figures, 1 table, revtex4; v2: discussions added, Phys. Rev. D in press; v3: published versio

Prey capture behavior in \u3cem\u3eHeterometrus petersii\u3c/em\u3e (Thorell, 1876) (Scorpiones: Scorpionidae)

Prey capture by Heterometrus petersii (Thorell, 1876) (Scorpionidae) was observed in the laboratory. The behavior components displayed in prey capture were identified, compiled into a flow chart, analyzed and discussed