Sea urchin spine calcite forms via a transient amorphous calcium carbonate phase

The skeletons of adult echinoderms comprise large single crystals of calcite with smooth convoluted fenestrated morphologies, raising many questions about how they form. By using water etching, infrared spectroscopy, electron diffraction, and environmental scanning electron microscopy, we show that sea urchin spine regeneration proceeds via the initial deposition of amorphous calcium carbonate. Because most echinoderms produce the same type of skeletal material, they probably all use this same mechanism. Deposition of transient amorphous phases as a strategy for producing single crystals with complex morphology may have interesting implications for the development of sophisticated materials.     

Ambipolar pentacene field-effect transistors and inverters

Organic field-effect transistors based on pentacene single crystals, prepared with an amorphous aluminum oxide gate insulator, are capable of ambipolar operation and can be used for the preparation of complementary inverter circuits. The field-effect mobilities of carriers in these transistors increase from 2.7 and 1.7 square centimeters per volt per second at room temperature up to 1200 and 320 square centimeters per volt per second at low temperatures for hole and electron transport, respectively, following a power-law dependence. The possible simplification of the fabrication process of complementary logic circuits with these transistors, together with the high carrier mobilities, may be seen as another step toward applications of plastic electronics.     

Intercalation of sea urchin proteins in calcite: study of a crystalline composite material

Sea urchin skeletal elements are composed of single crystals of calcite. Unlike their synthetic counterparts, these crystals do not have well-developed cleavage and are consequently much more resistant to fracture. This phenomenon is due in part to the presence of acidic glycoproteins occluded within the crystals. By means of x-ray diffraction with synchrotron radiation, it is shown that the presence of the protein in synthetic calcite only slightly decreases the coherence length but significantly increases the angular spread of perfect domains of the crystals. In biogenic calcite, the coherence length is 1/3 to 1/4 as much as that in synthetic calcite and the angular spread is 20 to 50 times as wide. It is proposed that the presence of macromolecules concentrated at mosaic boundaries that are oblique to deavage planes is responsible for the change in fracture properties. These results may be important in the material sciences, because of the unusual nature of this material, namely, a composite based on the controlled intercalation of macromolecules inside single-crystal lattices.     

Total alignment of calcite at acidic polydiacetylene films: cooperativity at the organic-inorganic interface

Biological matrices can direct the absolute alignment of inorganic crystals such as calcite. Cooperative effects at an organic-inorganic interface resulted in similar co-alignment of calcite at polymeric Langmuir-Schaefer films of 10,12-pentacosadiynoic acid (p-PDA). The films nucleated calcite at the (012) face, and the crystals were co-aligned with respect to the polymer's conjugated backbone. At the same time, the p-PDA alkyl side chains reorganized to optimize the stereochemical fit to the calcite structure, as visualized by changes in the optical spectrum of the polymer. These results indicate the kinds of interactions that may occur in biological systems where large arrays of crystals are co-aligned.     

An organic solid state injection laser

We report on electrically driven amplified spontaneous emission and lasing in tetracene single crystals using field-effect electrodes for efficient electron and hole injection. For laser action, feedback is provided by reflections at the cleaved edges of the crystal resulting in a Fabry-Perot resonator. Increasing the injected current density above a certain threshold value results in the decreasing of the spectral width of the emission from 120 millielectron volts to less than 1 millielectron volt because of gain narrowing and eventually laser action. High electron and hole mobilities as well as balanced charge carrier injection lead to improved exciton generation in these gate-controlled devices. Moreover, the effect of charge-induced absorption is substantially reduced in high-quality single crystals compared with amorphous organic materials.     

Biological control of crystal texture: a widespread strategy for adapting crystal properties to function

Textures of calcite crystals from a variety of mineralized tissues belonging to organisms from four phyla were examined with high-resolution synchrotron x-ray radiation. Significant differences in coherence length and angular spread were observed between taxonomic groups. Crystals from polycrystalline skeletal ensembles were more perfect than those that function as single-crystal elements. Different anisotropic effects on crystal texture were observed for sea urchin and mollusk calcite crystals, whereas none was found for the foraminifer, Patellina, and the control calcite crystals. These results show that the manipulation of crystal texture in different organisms is under biological control and that crystal textures in some tissues are adapted to function. A better understanding of this apparently widespread biological phenomenon may provide new insights for improving synthetic crystal-containing materials.     

Crystal orientation and plate structure in echinoid skeletal units

The submicroscopic morphology of magnesian calcite skeletal units of echinoids, revealed by scanning electron microscopy, was compared with crystal orientation data obtained by x-ray methods and with macroscopic morphology. The Perischoechinoidea and the Euechinoidea differ with regard to the shapes of their trabeculae. Nearly all plates and spines are single crystals. A variety of diJfferent directional relations of c- and a-axes to the main morphological directions are found for different species; adjacent plates with identical c-axis orientation differ strongly in orientation of their a-axes. Fracture surfaces of single trabeculae show cleavage planes and zonal layers attributed to changes in secretion conditions.     

Constraints on the formation of sedimentary dolomite

The experimental replacement of calcite and aragonite by dolomite under a variety of conditions indicates that dolomitization can take place in marine and lacustrine environments under two conditions: (i) low dissolved sulfate concentrations and (ii) insubstantial contemporaneous silica diagenesis. Common sites for dolomite formation are areas where the dissolved sulfate concentration is reduced by microbial sulfate reduction, through the mixing of seawater with large amounts of fresh water, or where low-sulfate alkaline lacustrine environments prevail. Even under these conditions, dolomite formation may be inhibited by the concurrent transformation of opal-A (amorphous silica) to opal-CT (disordered cristobalite and tridymite), whereas the subsequent transformation of opal-CT to quartz favors the formation of dolomite.     

X-ray Diffraction Studies of Echinoderm Plates

X-ray diffraction studies confirm that, with few exceptions, each skeletal element of echtinoderms is a single crystal of magnesium-rich calcite and that a relation exists between the shape of the element and the crystallographic a- and c-axes. The exceptions incluide the teeth of echinoids, and the calcareous ring as well as the anal teeth of holothurians. The tubercles of an echinoid plate begin their growth as parts of the single crystal of the plate; under the mechanical action of the spines that are attached to them, they become partly polycrystalline, as shown by scanning electron microscopy and by x-ray powder diffraction. The interface between inorganic crystalline and organic amorphous matter in the skeletal element appears to be the first example reported in nature of a periodic mninimal suirface.     

Single crystals of linear polyethylene crystallized from the glass

Isolated single crystals of polyethylene have been crystallized from the glassy state by annealing uniformly thick films of amorphous linear polyethylene just above their glass transition temperature. In agreement with previous results for polycarbonate, substantial molecular mobility at the glass transition temperature is.     

Elastomeric transistor stamps: reversible probing of charge transport in organic crystals

We introduce a method to fabricate high-performance field-effect transistors on the surface of freestanding organic single crystals. The transistors are constructed by laminating a monolithic elastomeric transistor stamp against the surface of a crystal. This method, which eliminates exposure of the fragile organic surface to the hazards of conventional processing, enables fabrication of rubrene transistors with charge carrier mobilities as high as approximately 15 cm2/V.s and subthreshold slopes as low as 2nF.V/decade.cm2. Multiple relamination of the transistor stamp against the same crystal does not affect the transistor characteristics; we exploit this reversibility to reveal anisotropic charge transport at the basal plane of rubrene.     

Skeletal low-magnesium calcite in living scleractinian corals

The skeletons of living specimens of the scleractinian coral Porites lobata have been found to contain up to 46 +/- 5 percent low-magnesium calcite even though free of gross detrital inclusions and boring or encrusting organisms. The calcite crystals occur in the interior of skeletal structures, have dimensions of 20 micrometers or less, and are surrounded by typical aragonite needles. Biogenic deposition seems to be the most likely source of the calcite, although the evidence does not rule out diagenesis of metastable.     

Polycrystalline echinoderm calcite and its fracture mechanics

Polycrystalline calcite was revealed by scanning electron microscopy of fractured skeletal ossicles of the sea star Echinaster spinulosus (Echinodermata, Asteroidea). Whisker-like calcite crystals were observed in specimens that were loaded in stress relaxation before being fractured; rapidly broken surfaces were smooth and glassy. The crystallites were 1300 angstroms wide and at least 3600 angstroms long and were packed together in lamellae. The lamellae were wound into spirals that formed the trabecular bars. All the crystallites in an ossicle appear to be aligned in the same direction. Geometric considerations indicate that the requirement for packing the crystallites smoothly may explain the high magnesium ion concentration of echinoderm calcite.     

Multilayer white light-emitting organic electroluminescent device

Organic electroluminescent devices are light-emitting diodes in which the active materials consist entirely of organic materials. Here, the fabrication of a white light-emitting organic electroluminescent device made from vacuum-deposited organic thin films is reported. In this device, three emitter layers with different carrier transport properties, each emitting blue, green, or red light, are used to generate white light. Bright white light, over 2000 candelas per square meter, nearly as bright as a fluorescent lamp, was successfully obtained at low drive voltages such as 15 to 16 volts. The applications of such a device include paper-thin light sources, which are particularly useful for places that require lightweight illumination devices, such as in aircraft and space shuttles. Other uses are a backlight for liquid crystal display as well as full color displays, achieved by combining the emitters with micropatterned color filters.     

Imaging the sublimation dynamics of colloidal crystallites

We studied the kinetics of sublimating crystals with single-particle resolution by experiments with colloidal spheres and by computer simulations. A short-range attraction between spheres led to crystallites one to three layers thick. The spheres were tracked with optical microscopy while the attraction was reduced and the crystals sublimated. Large crystallites sublimated by escape of particles from the perimeter. The rate of shrinkage was greatly enhanced, however, when the size decreased to less than 20 to 50 particles, depending on the location in the phase diagram. At this size, the crystallites transformed into a dense amorphous structure, which rapidly vaporized. The enhancement of kinetics by metastable or unstable phases may play a major role in the melting, freezing, and annealing of crystals.     

Thin-film transistor fabricated in single-crystalline transparent oxide semiconductor

We report the fabrication of transparent field-effect transistors using a single-crystalline thin-film transparent oxide semiconductor, InGaO3(ZnO)5, as an electron channel and amorphous hafnium oxide as a gate insulator. The device exhibits an on-to-off current ratio of approximately 106 and a field-effect mobility of approximately 80 square centimeters per volt per second at room temperature, with operation insensitive to visible light irradiation. The result provides a step toward the realization of transparent electronics for next-generation optoelectronics.     

Peptides enhance magnesium signature in calcite: insights into origins of vital effects

Studies relating the magnesium (Mg) content of calcified skeletons to temperature often report unexplained deviations from the signature expected for inorganically grown calcite. These "vital effects" are believed to have biological origins, but mechanistic bases for measured offsets remain unclear. We show that a simple hydrophilic peptide, with the same carboxyl-rich character as that of macromolecules isolated from sites of calcification, increases calcite Mg content by up to 3 mole percent. Comparisons to previous studies correlating Mg content of carbonate minerals with temperature show that the Mg enhancement due to peptides results in offsets equivalent to 7 degrees to 14 degrees C. The insights also provide a physical basis for anecdotal evidence that organic chemistry modulates the mineralization of inorganic carbonates and suggest an approach to tuning impurity levels in controlled materials synthesis.     

UNMINERALIZED FOSSIL BACTERIA

Unmineralized bacterial cells, mostly Micrococcus sp., but including also Streptococcus sp. and Actinomyces sp., were found in enormous numbers in lake beds of the Newark Canyon Formation of Early Cretaceous age, Eureka County, Nevada. The micrococci are black, and have an average diameter about 0.5 micro. Similar black micrococci (0.4 to 0.7 micro) were found in profusion in the bottom mud of Green Lake, New York. About 80 percent of this mud consists of minute idiomorphic calcite crystals and about 20 percent of these contain enormous numbers of the black micrococci. It is suggested that the Early Cretaceous bacterial cells owe their preservation to occlusion in calcite crystals that grew in a black, bacterial mud in a meromictic lake in which part of the Newark Canyon Formation accumulated.     

Micromorphological Features of Old Cultivated and Modern Soils in Guanzhong Areas, Shaanxi Province, China

Using the data obtained from the LGT soil profile, this article attempts to illustrate the process of modem soil formation in the Guanzhong areas and its micromorphological features. The micromorphology is observed under a petrographic microscope, and its image is quantitatively measured by LEICAL Qwin 2.6 software. Micromorphological observations of the thin sections show that the assemblage of minerals in different horizons is very similar, which is mainly composed of Q and P1. However, there are obvious differences in C/F15μm ratio, mineral content, and coarse features. The pedofeatures is mainly composed of clay, calcite, and amorphous Fe. Ap horizon is characterized by abundant needleshaped secondary calcite, secondary clay, and earthworm fecal pellet. BC horizon is characterized by a large quantity of secondary calcite with various shapes. Bt1and Bt2 horizons are characterized by abundant clay hypocoatings and a small quantity of secondary calcite. All the results of this research suggest that Earth-cumulic Orthic Anthrosols consist of both the upper Ap horizon, which cause loessal dung and eolian dust deposition, and cultivation occurs simultaneously during the process of Ap horizon-formation, and the lower BC horizon, which is aeolian sedimentary at the time of relative aridity during late Holocene.     

Attempt to date early South african hominids by using fission tracks in calcite

Calcite crystals extracted from marrow cavities of bones found in hominid-bearing breccias from Makapansgat and Swartkrans were studied for fossil tracks. The absence of the expected numbers of tracks in these and in calcites from Beds I and II, Olduvai Gorge, combined with the results of laboratory heating experiments, indicates that track annealing has occurred at ambient temperatures and precludes the widespread use of calcite for fission track dating.