Mechanisms of Argon Retention in Clays Revealed by Laser 40Ar-39Ar Dating.

A method for dating clays is important for studies of weathering, diagenesis, hydrocarbon migration, and the formation of major metalliferous deposits. However, many attempts have produced imprecise or inaccurate results. Data from shales show that, contrary to expectations, the (40)Ar-(39)Ar dating technique can be successfully used to determine the diagenetic age of ancient sediments because (39)Ar losses during irradiation are controlled by release from low retentivity sites in illite equivalent to those that have lost radiogenic (40)Ar in nature, rather than by direct recoil as is generally assumed.

Formation of magnetic single-domain magnetite in ocean ridge basalts with implications for sea-floor magnetism.

Although magnetic data are the primary evidence for ocean floor spreading, the processes by which magnetic phases in ocean floor basalts are formed remain poorly constrained. Scanning transmission electron microscopic observations show that magnetic single-domain magnetite in sheeted dike basalts of Deep Sea Drilling Project hole 504B formed through oxidation-exsolution of ilmenite, exsolution of ulvöspinel lamellae, and recrystallization of end-member magnetite by interaction with convecting fluids. The data suggest that the sheeted dike basalts, with single-domain magnetite as an efficient and stable magnetic carrier, contribute significantly to sea-floor magnetism.

Carbohydrate-controlled precipitation of apatite with coprecipitation of organic molecules in human saliva: stabilizing role of polyols.

Addition of common dietary carbohydrates to Millipore-treated human whole saliva either enhances or inhibits the formation of salivary precipitates, some carbohydrates showing no effect. The purpose of this study was to investigate the precipitation conditions more thoroughly and to elucidate the chemical nature of the precipitates formed. D-Xylose either enhanced precipitation (in long-term incubations) or had no appreciable effect (in 10 minute incubations). Other aldo- and keto-sugars and disaccharides (maltose, sucrose, lactose) generally enhanced precipitation, whereas all polyols (xylitol, D-sorbitol, mannitol, and maltitol) retarded the formation of turbidity in saliva. Xylitol inhibited formation of precipitates also in the presence of D-xylose, dextrans, and starch. Fast protein liquid chromatography (FPLC) of EDTA-soluble pellets obtained by centrifugation of the precipitates produced two major protein fractions (I and II) with a molecular weight of 112,000 and 46,000, respectively. The carbohydrates exerted a selective effect on the relative size of I and II in that polyol incubations resulted in a I to II ratio of 1:3, whereas control incubations (without added sugars) and incubations with other carbohydrates gave ratios of 1:6 to 1:10. Both peaks contained large amounts of acidic amino acids, proline, and glycine. The saliva precipitates contained a substantial portion of a crystalline phase that had the crystal structure of apatite, the individual crystallites being extremely small (less than 1 micron) with a Ca:P ratio of 1.46. The carbohydrates had a similar effect on the overall inorganic composition of the precipitates, but they had a clearly selective effect on the rate of formation of precipitates and on the relative amount of coprecipitating salivary proteins. This selectivity indicates that these carbohydrates, when consumed habitually, may exert different effects on the precipitation of Ca-salts at mineral-deficient enamel and dentine sites.

Orthopyroxene exsolution in augite: a two-step, diffusion-transformation process.

Orthopyroxene lamellae exsolved from augite on (100) are shown to grow by a two-step process involving (i) the diffusion of calcium and (magnesium, iron) to form clinohypersthene and (ii) the inversion of clinohypersthene to orthopyroxene, probably by glide twinning. If complete inversion is prevented by cooling or steep concentration gradients, the two-step process produces orthopyroxene with narrow margins of clinohypersthene. Measured elemental concentration gradients at (100) lamellar interfaces support this mechanism.

Biomineralization on crinoid echinoderms. Characterization of crinoid skeletal elements using TEM and STEM microanalysis.

Columnals of Neocrinus blakei, a modern species of stalked crinoid, were studied using a variety of analytical techniques. Analyses of the magnesium calcite of the crinoid stereom using powder X-ray diffraction and electron microprobe analysis yield a composition of Ca 88Mg 12C03. Scanning transmission electron microscopy (STEM) microanalytical data indicate that Mg incorporation into the calcite structure of the crinoid stereom is random and homogeneous to at least the 20 nm level. There appear to be no variations in composition at this level either within or between structural entities of the crinoid columnal stereom. TEM reveals a heterogeneity of contrast which may be due to incorporation of organic material or some other substance which is non-crystalline in character. Single-crystal X-ray diffraction data indicate that the individual skeletal plates are single crystals which yield diffuse and imperfect X-ray reflections due to a mosaic structure. Subsequent selected area electron diffraction (SAD) photographs via TEM, using various sizes of SAD apertures, indicate that the crystallites making up the mosaic structure are (order of magnitude) about 1.0 micrometer in size. The presence of mosaic structure in the single crystal skeletal elements may at least in part explain the lack of cleavage in fracture surfaces of echinoderm skeletal material. Based on these data, as well as data from skeletal elements of other deep water, stalked crinoids, we feel that these results may be applicable to crinoids in general, at least those existing in relatively constant temperature environments. The single-crystal nature of crinoid high magnesium calcite, and its remarkable homogeneity of composition suggest that a large "vital effect" (i. e., biologic control of skeletal deposition) mediates the mineralization process.

Critique of "an electron microscope study of crystal calcium carbonate formation in the mouse otolith".

Electron micrographs of otoconia of fetal mice, as obtained by Nakahara and Bevelander, provide morphological evidence that the otoconia consist of both organic material and calcium carbonate (calcite), contrary to their own conclusions. Calcite is an integral component of otoconia, apparently from their inception. The concept of mineralization by calcite of an already developed organic template ("preotolith") is shown to be in error.

Otoconial formation in the fetal rat.

The development of otoconia in the fetal rat was investigated by scanning and transmission electron microscopy and by x-ray elemental analysis. The transmission electron microscopical results indicate that primitive otoconia are highly organic appearing but are trigonal in cross section, indicating that they already possess a three-fold axis of symmetry and a complement of calcite. These otoconia develop into spindle-shaped units which accrue fibrous, organic material at an angle to their surfaces. Dumbbell-shaped otoconia, with distinct central cores and peripheral zones, result. These otoconia then mature to the adult crystal configuration having a more cylindrical body and pointed ends. The existence of trigonal, spindle- and dumbbell-shaped otoconia was verified by scanning electron microscopy of fresh-frozen material. Tissues prepared for transmission electron microscopy proved (by elemental analysis) to have been decalcified inadvertently, fortuitously revealing the arrangement of the organic material. Subsequent transmission electron microscopy of dumbbell-shaped otoconia not exposed to fluids during embedment showed that calcite deposits mimicked the arrangement of the organic material. X-ray elemental analysis demonstrated that calcium was present in lower quantities in the central core than peripherally. Findings are interpreted to indicate that organic material is essential to otoconial seeding and directs otoconial growth.

The nature and crystal growth of otoconia in the rat.

Several types of otoconia are present in the macular regions of young rats. These include multifaceted, transitional and rounded body forms, some variant otoconia and a few rhombohedrons. The adult form has typically rounded by nonsmooth body surfaces and pointed ends with three planar faces. The multifaceted and transitional otoconia fracture and etch more readily than do the adult type. The differences in properties of the otoconia are considered in the light of known facts concerning inorganic crystal nucleating and growth. This integrated approach indicates that many otoconia originate by seeding of multiple subunits on an organic substrate and develop by the mechanism of parallel growth. The basic structural unit is the rhombohedron. By analogy to inorganic crystals of calcite, it would seem that the typical otoconium grows on the end faces but growth on the side faces is suppressed by some unknown chemical factor. Some otoconia are exceptions, evidently seeding and growing in the pure rhombohedral form. Decalcification of cleaved otoconia shows that organic material is incorporated during growth. The observations are interpreted to indicate that organic substance influences growth and achievement of the adult otoconial form.