Lath-shaped Crystals Research Articles

Biotite Alteration in Deeply Weathered Granite. I. Morphological, Mineralogical, and Chemical Properties

AbstractBiotite in deeply weathered granitic rocks in southwestern Australia has altered to exfoliated grains composed of biotite, mixed-layer clay minerals, kaolinite, vermiculite, gibbsite, goethite, and hematite. Discrete vermiculite and vermiculite-dominant mixed-layer clay minerals are not major weathering products. Oxidation of octahedral iron in biotite is associated with ejection of octahedral cations, loss of interlayer K, and a contraction of the b-dimension of the biotite sheet. Si, Mg, Ca, Mn, K, and Na are lost from biotite during weathering, and Ti, Al, Ni, and Cr are retained. Fe and water have been added to the grains during weathering. Much Fe occurs as aggregates of microcystalline, aluminum-rich goethite particles on flake surfaces and within etchpits, with smaller amounts occurring as hexagonal arrangements of lath-shaped crystals of goethite on flake surfaces.

The Pliocene Glenns Ferry Oolite: Lake-margin Carbonate Deposition in the Southwestern Snake River Plain

ABSTRACT An extensive fresh-water oolitic carbonate representing the remnant deposits of a Pliocene low-salinity lake occurs at the base of the Glenns Ferry Formation along the western margin of the Snake River Plain. This oolite crops out in a 45 kilometer-long belt that treads northwest-southeast and parallels the southwestern shoreline of fossil Lake Idaho. It consists almost entirely of ooids ranging in size from 0.5 mm to 6.0 mm in diameter. Optical and scanning electron microscopy reveal a random mixture of radial laths and equant blocky grains of low-magnesian calcite in the ooid cortices. The primary mineralogy of the lath-shaped crystals is interpreted as having been aragonite and the radial orientation of these crystals as having been synsedimentary based upon comparison with crystal in the cortex of Great Salt Lake ooids. Crenulation of concentric laminae is indicative of significant algal influence during ooid formation, suggesting accretionary processes more typical of algal oncoids. Cortex deposition took place in a shallow agitated environment on a nearshore lacustrine bench, while ooid deposition took place lakeward on the deeper water bench slope.

Controlled flocculation of coarse suspensions by colloidally dispersed solids I: Interaction of bismuth subnitrate with bentonite

AbstractDeflocculated suspensions of coarse powders tend to cake as the individual particles settle out and form compact, cohesive sediments. Limited flocculation results in looser sediments because the settled-out flocs incorporate large amounts of the liquid suspending medium. Controlled flocculation of bismuth subnitrate suspensions was achieved by the addition of small amounts of bentonite. The interaction of the coarse, positively charged bismuth subnitrate particles in aqueous suspension with negatively charged, colloidally dispersed bentonite was investigated by measuring electrophoretic mobility, sedimentation volume, and viscosity. Gradual addition of bentonite dispersion to bismuth subnitrate suspensions first reduced the ζ-potential of the bismuth subnitrate particles from +28mv to zero, then inverted it, and finally caused it to level off at −20mv for bismuth subnitrate-bentonite weight ratios below 200. Owing to the much greater specific surface area of bentonite, the surface of the bismuth subnitrate lath-shaped crystals was completely covered by 0.5% of its weight in clay platelets. Adhesion was promoted by electrovalences between surface bismuthyl ions and cation-exchange sites of the clay and by secondary valences. The charge neutralization of bismuth subnitrate by bentonite was a heterocoagulation process: the addition of small amounts of the clay flocculated the bismuth subnitrate suspensions and eliminated caking. While the ζ-potential of the bismuth subnitrate particles leveled off when their surface was saturated with bentonite platelets, sedimentation volume and viscosity continued to increase when the clay concentration was increased further while maintaining the bismuth subnitrate concentration constant. The excess, nonadsorbed bentonite formed the characteristic house-of-cards structure, incorporating the bentonite-coated bismuth subnitrate particles as cornerstones.

Structure and Thermal Behavior of Pressure-Crystallized Polypropylene

Samples of isotactic polypropylene have been crystallized at a pressure of 3.86 kbar and under a variety of crystallization conditions. These include isothermal crystallization at five different degrees of supercooling ranging from ΔT=70°C to ΔT=34°C and crystallization by slow cooling from 248°C. Each sample was studied by means of x-ray diffraction at both room and elevated temperature, by differential scanning calorimetry, and by electron microscope observation of replicas of the fracture surface. The x-ray diffraction results show that all the pressure-crystallized samples have formed in the γ, or triclinic, modification. Depending upon crystallization conditions, this phase may be stable or metastable. For high values of ΔT, the triclinic phase shows a partial conversion to the α, or monoclinic, phase under elevated temperature annealing. For low values of ΔT, the γ phase does not convert to α even at elevated temperatures. The DSC measurements indicate that the melting temperatures of the pressure-crystallized samples are dependent upon the degree of supercooling. For a heating rate of 40°C/min, the melting temperature is at 157°C for ΔT=70°C; it rises with decrease of the degree of supercooling and occurs in two stages at 175° and at 182°C for ΔT=34°C. Fracture-surface replicas of the slowly cooled sample show that the bulk polymer contains well-delineated, lath-shaped crystals, similar in morphology to those obtained from dilute-solution crystallization. For the samples isothermally crystallized, the fracture-surface replicas show the presence of long thin fibril-like entities. Consideration is given to the possibility that these fibrillar elements are single-crystal lamellas of low-molecular-weight polymer that have crystallized during cooling of the whole polymer from the crystallization temperature.

The study of crystal growth with the electron microscope II. The observation of growth steps in the paraffin n -hectane

Examination of the paraffin n -hectane in the electron microscope has given evidence of two different growth mechanisms for crystals of this compound. The one corresponds to the spiral growth mechanism already observed for n -hexatriacontane (Dawson & Vand 1951), and the orthorhombic lozenge-shaped crystals of n -hectane show the typical spiral growth steps already described. The other growth mechanism gives rise to lath-shaped crystals and operates when growth takes place by the slow evaporation of a cold solution of n -hectane in xylene. The laths are invariably twinned across the diagonal of the orthorhombic cell, and growth in one direction is greatly accelerated by the occurrence of an indestructible step at the twin boundary.

The Petrology of the Warren House Series, Malvern

THE Warren House Series consists of lavas which belong to a sodic suite, being rhyolites, keratophyres, and spilites. The rhyolites have a fine-grained micrographic groundmass, and some contain phenocrysts of quartz, orthoclase, anorthoclase, and plagioclase felspar. Epidotization of the rocks has occurred, but it is the felspathic and femic constituents of the groundmass which have suffered alteration and not the felspar phenocrysts, the action not being intense enough to affect the latter. The keratophyres consist essentially of small lath-shaped albite crystals which give a trachytic texture to the rock. The spilites are more basic, vesicular, and variolitic or sub-ophitic in texture, while some of them show pillowstructure in the field. The keratophyres and the spilites also exhibit varying degrees of epidotization. Pyroclastic rocks are not very common, but a few interesting varieties have been noted. Intrusions of dolerite occur within the lavas. One type is an ophitic dolerite, which when fresh probably contained olivine. Another type is now an epidiorite. Both these types have been epidotized, the epidiorite to such an extent that it passes into an epidote-uralite rock in places.