A Cretaceous scleractinian coral with a calcitic skeleton

It has been generally thought that scleractinian corals form purely aragonitic skeletons. We show that a well-preserved fossil coral, Coelosmilia sp. from the Upper Cretaceous (about 70 million years ago), has preserved skeletal structural features identical to those observed in present-day scleractinians. However, the skeleton of Coelosmilia sp. is entirely calcitic. Its fine-scale structure and chemistry indicate that the calcite is primary and did not form from the diagenetic alteration of aragonite. This result implies that corals, like other groups of marine, calcium carbonate-producing organisms, can form skeletons of different carbonate polymorphs.     

Amino acids in the proteins from aragonite and calcite in the shells of Mytilus californianus

Hydroxylysine and hydroxyproline are absent in the calcified proteins of Mytilus. The organic matrices from the calcite layers have a consistently higher ratio of acidic to basic amino acids than the aragonitic shell units. The uncalcified shell units periostracum and outer ligament, have very few acidic residues, which may in part account for the lack of mineralization.     

The oceanic carbonate system: a reassessment of biogenic controls

Fluxes of biogenic carbonates moving out of the euphotic zone and into deeper undersaturated waters of the North Pacific were estimated with free-drifting sediment traps. Short-duration (1 to 1.5 day) sampling between 100 and 2200 meters points to a major involvement in the oceanic carbonate system by a class of organisms which had been relegated to a secondary role-aragonitic pteropods. Pteropod fluxes through the base of the euphotic zone are almost large enough to balance the alkalinity budget for the Pacific Ocean. Dissolution experiments with freshly collected materials shed considerable light on a mystery surrounding these labile organisms: although plankton collections from net tows almost always contain large numbers of pteropods, these organisms are never a major component of biogenic materials in long-duration sediment trap collections. Their low abundance in long-duration collections results from dissolution subsequent to collection. Shortduration sampling showed significant increases in the ratio of calcitic foraminifera to aragonitic pteropods in undersaturated waters, indicating the more stable mineralogic form, calcite, was preserved relative to aragonite. Approximately 90 percent of the aragonite flux is remineralized in the upper 2.2 kilometers of the water column.     

High-magnesian calcite: leaching of magnesium in the deep sea

The high-magnesian calcite fraction of a shallow-water carbonate sand was converted to low-magnesian calcite after transport to the deep sea; strontium was also leached from the carbonate. Oxygen isotopic ratios indicate that loss of magnesium and strontium took place during recrystallization of the carbonate in the deep sea; this process did not alter textures of skeletal fragments. Previously, high-magnesian, calcite was thought only to dissolve in the deep sea.     

Carbonate compensation depth: relation to carbonate solubility in ocean waters

In situ calcium carbonate saturometry measurements suggest that the intermediate water masses of the central Pacific Ocean are close to saturation with resppect to both calcite and local carbonate sediment. The carbonate compensation depth, located at about 3700 meters in this area, appears to represent a depth above which waters are essentially saturated with respect to calcite and below which waters deviate toward undersaturation with respect to calcite.     

Magnesium interaction with the surface of calcite in seawater

Magnesian calcite overgrowth containing 4 (+/- 2) mole percent magnesium carbonate forms on calcite exposed to natural seawater near the ocean surface. This magnesian calcite is approximately 30 percent less soluble in seawater than pure calcite. The formation of the magnesian calcite of reduced solubility may have a major influence on calcite accumulation in deep sea sediments.     

Calcite: rates of dissolution in a vertical profile in the central pacific

Rate of dissoluton of calcium carbonate (calcite) was determined for a vertical profile 5000 mizeters deep in the central Pacific by measuring the loss of weight of optical calcite spheres arrayed on a mooring. Waters of the Pacific are undersaturated in all but the upper several hundred meters. A sharp increase in the rate of dissolution occurs at a depth of about 3700 meters, which corresponds closely to the depth below which calcite hias been dissolved from the sediments.     

The role of Mg2+ as an impurity in calcite growth

Magnesium is a key determinant in CaCO3 mineralization; however, macroscopic observations have failed to provide a clear physical understanding of how magnesium modifies carbonate growth. Atomic force microscopy was used to resolve the mechanism of calcite inhibition by magnesium through molecular-scale determination of the thermodynamic and kinetic controls of magnesium on calcite formation. Comparison of directly measured step velocities to standard impurity models demonstrated that enhanced mineral solubility through magnesium incorporation inhibited calcite growth. Terrace width measurements on calcite growth spirals were consistent with a decrease in effective supersaturation due to magnesium incorporation. Ca(1-x)Mg(x)CO3 solubilities determined from microscopic observations of step dynamics can thus be linked to macroscopic measurements.     

Etch patterns on calcareous sediment grains: petrographic evidence of marine dissolution of carbonate minerals

Scanning electron microscopy reveals that carbonate grains in sublittoral sediments in the North Sea have surfaces with a microscopic etch relief. In structurally complex skeletal grains, the relief grades into marginal corrosion zones. The etch patterns are caused by dissolution of calcium carbonate, taking place at the shallow sea floor. Carbonate phases affected are calcite, aragonite, and a spectrum of magnesian calcites.     

Gallstone of unusual composition: calcite, aragonite, and vaterite

A gallstone of almost perfect octahedral symmetry was composed of a mixture of crystallites of the three polymorphous forms of calcium carbonate: calcite, aragonite, and vaterite.     

钙质结核的微形态及矿物学特征

土壤中的钙质结核可分为软结核、硬结核和钙质硬盘３种形态，其化学组成以ＣａＣＯ３为主，矿质全量组成的各氧化物平均含量水平的顺序为：ＣａＯ〉ＳｉＯ２〉Ａｌ２Ｏ３〉Ｆｅ２Ｏ３〉ＭｇＯ〉Ｋ２Ｏ〉Ｎａ２Ｏ〉ＴｉＯ２〉ＭｎＯ〉Ｐ２Ｏ５。但在某些软结核中，ＳｉＯ２含量仍大于ＣａＯ含量。微形态分析结果表明，软结核为碳酸盐颗粒的胶结体，常嵌埋较多的土壤基质、石英、长石等粗颗粒；硬结核多为碳酸盐和方解石的胶结体，许多     

Seawater chemistry and early carbonate biomineralization

The first appearances of aragonite and calcite skeletons in 18 animal clades that independently evolved mineralization during the late Ediacaran through the Ordovician (approximately 550 to 444 million years ago) correspond to intervals when seawater chemistry favored aragonite and calcite precipitation, respectively. Skeletal mineralogies rarely changed once skeletons evolved, despite subsequent changes in seawater chemistry. Thus, the selection of carbonate skeletal minerals appears to have been dictated by seawater chemistry at the time a clade first acquired its mineralized skeleton.     

Pelagic sedimentation of aragonite: its geochemical significance

The relative importance of the pelagic flux of aragonite, as compared to calcite, to the deep-sea floor has been evaluated by means of a quantitative x-ray diffraction study of samples collected from sediment traps and from an unusually shallow portion of the open Atlantic Ocean (the Rio Grande Rise). The results suggest that the aragonite flux constitutes at least 12 percent of the total flux of calcium carbonate on a worldwide basis. The presence of high-magnesium calcite in several samples suggests that some of the calcareous material falling to the deep-sea floor may be derived from the long-distance transport of debris from shallow-water beenthic organisms as well as from the settling of planktonic remains. This observation supports the contention that 12 percent represents a minimum value. Aragonite and high-magnesium calcite transported laterally from shallow-water regions, upon dissolution during settling into deeper water, may contribute to the neutralization of excess anthropogenic carbon dioxide added to the oceans.     

Calcium Carbonate: Factors Affecting Saturation in Ocean Waters off Bermuda

Bermuda marine waters are not in equilibrium with the bulk carbonate sediments with which they are in contact, and they are supersaturated with respect to pure calcite. This apparent supersaturation seems to result from a metastable equilibrium between sea water and the most soluble solid phase available locally in excess.     

Calcareous deposits in the renal sac of a molgulid tunicate

Weddellite (calcium oxalate dihydrate) and calcite (anhydrous calcium carbonate phase) are components of concretions in the renal sac of the ascidian tunicate Molgula manhattensis. The presence of weddellite along with urate in the concretions suggests a resemblance to human kidney stones, although, unlike the latter, the concretions in Molgula do not seem to be pathologic deposits.     

Direct fabrication of large micropatterned single crystals

Micropatterning of single crystals for technological applications is a complex, multistep process. Nature provides alternative fabrication strategies, when crystals with exquisite micro-ornamentation directly develop within preorganized frameworks. We report a bio-inspired approach to growing large micropatterned single crystals. Micropatterned templates organically modified to induce the formation of metastable amorphous calcium carbonate were imprinted with calcite nucleation sites. The template-directed deposition and crystallization of the amorphous phase resulted in the fabrication of millimeter-sized single calcite crystals with sub-10-micron patterns and controlled crystallographic orientation. We suggest that in addition to regulating the shape, micropatterned frameworks act as sites for stress and impurity release during the amorphous-to-crystalline transition. The proposed mechanisms may have direct biological relevance and broad implications in materials synthesis.     

Calcite Dissolution: An in situ Study in the Panama Basin

The results of an in situ study of calcite dissolution in the Panama Basin indicate that the rate of dissolution in the water column increases suddenly below a water depth of about 2800 meters. This coincides with the depth at which the calcium carbonate content of surface sediments begins to decrease rapidly or the sedimentary lysocline. Since this level of increased dissolution both in the water column and on the sea floor does not appear to be related to the transition from supersaturation to undersaturation with respect to carbonate, there may be a kinetic origin for the lysocline in this region.     

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.     

Calcium carbonate concretions: cyclic occurrence in the hamster vagina

Three crystalline forms of calcium carbonate were identified in washings of the hamster vagina. Spherical concretions of vaterite and hexagonal concretions of calcite predominate on days 3 and 4 of the 4-day estrous cycle. Dumbbell-like concretions of aragonite predominate during pregnancy and pseudopregnancy. Each polymorph is associated with an acid-insoluble matrix. Concretions disappear after ovariectomy and reappear during daily injections of estrogen and progesterone.     

Thermodynamics of calcite growth: baseline for understanding biomineral formation

The complexity of biomineralized structures suggests the potential of organic constituents for controlling energetic factors during crystal synthesis. Atomic force microscopy was used to investigate the thermodynamic controls on carbonate growth and to measure the dependence of step speed on step length and the dependence of critical step length on supersaturation in precisely controlled solutions. These data were used to test the classic Gibbs-Thomson relationship and provided the step edge free energies and free energy barriers to one-dimension nucleation for calcite. Addition of aspartic acid, a common component in biomineralizing systems, dramatically affected growth morphology and altered the magnitude of the surface energy.