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.     

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.     

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.     

香螺壳和毛蚶壳的相组成及组织形态分析

采用X射线衍射方法(XRD)对香螺壳和毛蚶壳由表至里逐层进行相分析，同时用扫描电镜(SEM)对两种壳体横断面各层显微组织进行观察。结果表明，香螺壳的外层(角质层)和中层(棱柱层)的相组成为斜方六面体(ohcdral)结构的方解石(bonate)型碳酸钙，而内层(珍珠层)则是由方解石和文石(aragonitc)两种晶体结构的碳酸钙共同组成的。而毛蚶壳各层均由方解石型碳酸钙组成。香螺壳各层的组织较毛蚶壳致密且生长取向各异。     

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.     

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.     

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.     

Time and metamorphic petrology: calcite to aragonite experiments

Although the equilibrium phase relations of many mineral systems are generally well established, the rates of transformations, particularly in polycrystalline rocks, are not. The results of experiments on the calcite to aragonite transformation in polycrystalline marble are different from those for earlier experiments on powdered and single-crystal calcite. The transformation in the polycrystalline samples occurs by different mechanisms, with a different temperature dependence, and at a markedly slower rate. This work demonstrates the importance of kinetic studies on fully dense polycrystalline aggregates for understanding mineralogic phase changes in nature. Extrapolation of these results to geological time scales suggests that transformation of calcite to aragonite does not occur in the absence of volatiles at temperatures below 200 degrees C. Kinetic hindrance is likely to extend to higher temperatures in more complex transformations.     

Hydrocalcite (CaCO3 * H2O) and Nesquehonite (MgCO3 * 3H2O) in Carbonate Scales

Hydrocalcite (CaCO(3) * H(2)O) with exactly one molecule of hydrate water is the main component of carbonate scales deposited from cold water in contact with air. When the magnesium content of the water is high, the hydrocalcite occurs together with MgCO(3) * 3H(2)O (nesquehonite). From the conditions under which hydrocalcite is transformed into calcite and aragonite, it appears that in some cases aragonite in nature may be formed by way of an intermediary of CaCO(3) * H(2)O.     

Subaerial diagenesis of carbonate sediments: efficiency of the solution-reprecipitation process

Interaction between percolating groludwaters and aragonitic carbonate sediments within the vadose zone of the coral cap of Barbados, West Indies, results in dissolution of aragonite and concurrent reprecipitation as low-magnesium calcite. Comparison of the ratios of strontium to calcium in groundwater, aragonitic carbonate, and recrystallized calcite indicates that locally the solution-reprecipitation process is operating at an efficiency greater than 90 percent.     

Methane-derived marine carbonates of pleistocene age

In some calcium carbonate-bearing sandstones from the edge of the continental shelf off the northeast United States, the deltaC(13) range is from -30 and -60 per mil for both aragonite and high-magnesium calcite. The deltaC(13) of co-existing shells of Modiolus sp. is normal (+ 1.7 to -2.7 per mil). The deltaO(18) values of around + 3.5 per mil in all samples suggest deposition at temperatures around 0 degrees C. Quaternary methane oxidized either chemically or microbiologically to carbon dioxide is the probable source of carbon in these carbonates.     

Strontium-calcium thermometry in coral skeletons

The strontium to calcium ratio of skeletal aragonite in three genera of reef-building corals varies as a simple function of temperature and the strontium to calcium ratio of the incubation water. The strontiumlcalcium distribution coefficients of coral aragonite apparently differ from the corresponding coefficient of inorganically precipitated aragonite. With some care, coral skeletons can be used as recording thermometers.     

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.     

Aragonite and Calcite as Constituents of Adult Oyster Shells

Adult oyster shells are composed mainly of calcite, but there are five small areas of aragonite: the resilium, the two pads at which the adductor muscle is inserted, and the two pads at which Quenstedt's muscles are inserted. Quenstedt's muscles are proposed as a name for the small modified, one-time pedal muscles of unknown function, discovered by Quenstedt.     

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.     

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.     

Ancient ice islands in salt lakes of the central andes

Massive blocks of freshwater ice and frozen sediments protrude from shallow, saline lakes in the Andes of southwestern Bolivia and northeastern Chile. These ice islands range up to 1.5 kilometers long, stand up to 7 meters above the water surface, and may extend out tens of meters and more beneath the unfrozen lake sediments. The upper surfaces of the islands are covered with dry white sediments, mostly aragonite or calcite. The ice blocks may have formed by freezing of the fresh pore water of lake sediments during the "little ice age." The largest blocks are melting rapidly because of possibly recent increases in geothermal heat flux through the lake bottom and undercutting by warm saline lake water during the summer.     

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.     

Modern cyanobacterial analogs of paleozoic stromatoporoids

Recent and subfossil calcareous structures resembling cystose and subclathrate Paleozoic stromatoporoids have been discovered in a sea-linked, stratified, alkaline crater lake on Satonda Island, Indonesia. The structures are produced by mats of coccoid cyanobacteria growing along the lakeshore from the water surface down to the O(2)-H(2)S interface located at a depth of 22.8 meters. Calcification of the mats is controlled by seasonal changes in calcium carbonate supersaturation in the epilimnion. The internally complex structures are a product of two different calcification processes: (i) periodic in vivo calcification of the surficial cyanobacterial layers by low-Mg calcite, and (ii) early postmortem calcification of the cyanobacterial aggregates below the mat surface by microbially precipitated aragonite. The finding supports the idea that Paleozoic stromatoporoids represent fossilized cyanobacteria (stromatolites). It also implies that the stromatoporoid-generating epicontinental seas during the early Paleozoic may have been more alkaline and had a higher carbonate mineral supersaturation than modern seawater.