Showing papers on "Silicate minerals published in 1981"

A negative feedback mechanism for the long‐term stabilization of Earth's surface temperature

Abstract: It is suggested that the partial pressure of carbon dioxide in the atmosphere is buffered, over geological time scales, by a negative feedback mechanism, in which the rate of weathering of silicate minerals (followed by deposition of carbonate minerals) depends on surface temperature, which in turn depends on the carbon dioxide partial pressure through the greenhouse effect. Although the quantitative details of this mechanism are speculative, it appears able to partially stabilize the earth's surface temperature against the steady increase of solar luminosity, believed to have occurred since the origin of the solar system.

Diffusion in silicate minerals and glasses: A data digest and guide to the literature

Abstract: The rates of diffusion of cations, oxygen and hydrogen in silicates are of fundamental importance to a wide range of petrological and geochronological problems. In order that the Earth Scientist may readily apply the available experimentally-determined diffusion rates to such problems, a compilation of data for diffusion in silicate minerals and glasses is presented along with a “working guide” for its use. Published data and experimental conditions are listed in tables, and these are preceded by an outline of diffusion mechanisms, terminology, equations and units, diffusion regimes, factors controlling diffusion rates, measurement techniques, and diffusional behaviour.

Sulfur isotopic studies of the Dunka Road Cu-Ni deposit, Duluth Complex, Minnesota

Abstract: The Dunka Road deposit is one of a series of Cu-Ni occurrences located within troctolitic rocks of the southern portion of the Duluth Complex, Minnesota. Sulfide minerals, principally pyrrhotite, chalcopyrite, cubanite, and minor pentlandite occur interstitially with silicate minerals and average about 2 volume percent in ore-grade horizons. Areas of sulfide concentrations in excess of 1 to 2 percent occur erratically throughout the troctolitic sequence but are in general concentrated near the basal contact with rocks of the metasedimentary Virginia Formation.The delta 34 S values of sulfides in the troctolites range from 0.2 to 15.8 per mil, with an average value near 7.5 per mil. Pyrrhotite in the underlying Virginia Formation hornfels and pyrite from unmetamorphosed Virginia Formation are characterized by a similar range in delta 34 S values. The high positive delta 34 S values and similarity to values found in the Virginia Formation strongly suggest that most of the sulfur in the Dunka Road deposit was derived from the breakdown of pyrite in the Virginia Formation. The sulfur isotopic compositions of minerals in the troctolites show no consistent lateral or vertical trends and are in general characterized by a high degree of variability. Such fluctuations in delta 34 S values indicate that sulfur isotopic homogenization was not attained throughout the troctolitic sequence.Several mechanisms may have contributed to the variations in delta 34 S values found within the troctolites, including mixing of sedimentary and igneous sulfur, assimilation of sedimentary sulfur characterized by variable delta 34 S values, fractionation caused by fluctuations in oxidation states during sulfur volatilization and assimilation, and periodic introduction of isotopically distinct magma batches. In light of the erratic sulfide mineral and delta 34 S distributions, the predominant mechanism is thought to be the introduction of both isotopically and spatially variable country rock sulfur into crystallizing troctolitic magma.

The crystal chemistry of the uranyl silicate minerals

Abstract: The uranyl silicate minerals have been divided into three groups on the basis of their uranium to silicon ratios. The 1:1 group includes uranophane, beta-uranophane, boltwoodite, sodium boltwoodite, kasolite, sklodowskite, and cuprosklodowskite. A structure refinement of uranophane, a structure determination of boltwoodite, and previously reported structure determinations of most of these minerals indicate that they are composed of uranyl silicate chains made of edge-shared uranium pentagonal bipyramidal groups and silicate tetrahedra. These chains have the composition ((UO/sub 2/)(SiO/sub 4/))/sub n/sup /sup -2/n/ and are crossliked by a bridging oxygen atom to form a uranyl silicate sheet. These sheets are crossbonded by the additional cations in the structure. The uranyl minerals with a uranium to silicon ratio of 1:3 include weeksite and haiweeite. A partial structure analysis of weeksite suggests that the structure type for this group consists of uranyl silicate chains, similar to those found in the 1:1 group, that are crosslinked by the additional silicate tetrahedra in the structure. The uranyl mineral group with a uranium to silicon ratio of 2:1 contains only the mineral soddyite. This structure is composed of uranyl silicate chains that are crossbonded by sharing a common silicon to give a three-dimensional framework structure. Amore » new triclinic uranyl silicate mineral was discovered during this study, although there is not enough sample to describe it adequately. The locations of the uranium atoms in this structure indicate that it may not be composed of uranyl silicate chains such as those found in all the other uranyl silicate minerals.« less

The dissolution kinetics of brucite, antigorite, talc, and phlogopite at room temperature and pressure

Abstract: Dissolution experiments were done on a series of layer-type Mg minerals (brucite, antigorite, talc, and phlogopite) of progressively more complex structure and chemical composition. Twenty-five gram portions of these minerals were dry-ground to <400 mesh and then subjected to controlled dissolution in distilled water at 25"C and I atm CO2 partial pressure. For antigorite, talc, and phlogopite, Mg from the octahedral sheets was released more rapidly than was Si from the tetrahedral sheets, r.e., the dissolution was incongruent. It appears that the solubility of these layer-type minerals is related to the relative number of octahedral to tetrahedral sheets present in the structure, i.e., the dissolution kinetics of layer-type silicate minerals is controlled bv the rate of destruction of the tetrahedral silica sheets of the mineral.

Gangue mineral 87 Sr/ 86 Sr ratios and the origin of mississippi valley-type mineralization

Abstract: The 87 Sr/ 86 Sr ratio measurements in Mississippi Valley-type gangue minerals, and host marine carbonates ranging in age from upper Cambrian to Mississippian, show that in northwestern Ohio, where the mineralization is sparse, the minerals precipitated from fluids in local isotopic equilibrium with the host carbonates in which the minerals were emplaced In contrast, most gangue minerals from districts of major mineralization in Illinois, Kentucky, Virginia, and Tennessee contain strontium with higher 87 Sr/ 86 Sr ratios than the carbonate hosts and appear to have crystallized from fluids containing strontium from silicate minerals in the sedimentary succession No evidence has been found for a magmatie component in the ore fluid strontiumData from Elmwood, Tennessee, show that the early ore fluids, like those of northwestern Ohio, were in isotopic equilibrium with the host carbonates but that they were mixed with and displaced by introduced fluids containing more radiogenic strontiumThese findings are consistent with the two-fluid mixing model for the Mississippi Valley mineralization, where a fluid indigenous to the host carbonates and containing reduced sulfur was mixed with introduced fluids analogous to metal-bearing and low sulfide oil field brines, with an extensive history of chemical exchange with clays and other silicate minerals in the sedimentary succession

Dissolution kinetics: Catalysis by strong acids

Abstract: The dissolution of silicate minerals in mixtures of hydrofluoric acid and strong acids such as HC1 is catalyzed by H + ions adsorbed at the mineral surface. Studies of the catalyzed dissolution of kaolinite, quartz, potassium feldspar, and sodium montmorillonite have shown that the rates of dissolution can be increased by as much as 900%. The dissolution process can be modeled as simultaneous catalyzed and uncatalyzed reactions, where the surface concentration of H + ions is determined by a Langmuir adsorption isotherm. The degree of catalysis is a strong function of the nature of the silicate surface, a fact which has application in the area of selective dissolution within mixed mineral systems.

Petrography of the uranium-bearing minerals of the Liberty Hill Pluton, South Carolina; phase assemblages and migration of uranium in granitoid rocks

Abstract: Optical petrography, fission track U mapping, and electron microprobe analysis of granitoid rocks from the Liberty Hill pluton, South Carolina, show a nonuniform distribution of U in the rocks. In order of abundance, U occurs: in accessory minerals, with U concentrations decreasing in the order: uraninite, coffinite, uranoan thorite, zircon, monazite, titanite, allanite, apatite, zeolites; on altered sulfaces of Fe/sup +2/- and S-bearing minerals; at microcracks filled with carbonate, zeolite, and Fe-Mn oxyhydroxide minerals; on clay minerals of altered feldspars; and in major minerals as a widely dispersed minor component. Thorium has been identified in the accessory minerals thorite, thoroan uraninite, coffinite, allanite, and monazite. Uranium and Th distribution is believed to be of magmatic origin within the major U-Th-Zr phase assemblage: uranoan thorite +zircon /plus or minus/ uraninite. The mechanisms of U retention are similar to those found in sedimentary U deposits. The U-Th-Pb isotope studies of the Liberty Hill pluton show that these mechanisms are effective in retaining the initial concentrations of U. Refs.

Germanium-Silicon Interactions in Biological Systems

Abstract: Germanium and silicon* co-occur in group IV-A of the Periodic Table. The two elements have similar outer shells and their ionic radii are also very similar (Ge = 0.47 A; Si = 0.42 o). Ge therefore resembles Si in its chemical properties (Jolly, 1966), and is known to substitute for Si in a number of silicate minerals (Goldschmidt, 1958). Both Si and Ge have been reported in all plants and animals examined.

Exploration of burundi nickeliferous laterites by electrical methods

Abstract: The laterites in Burundi, which are formed by weathering of ultrabasic rocks, show a complete profile with the following horizons: canga , the ferruginous crust capping, ferralite , consisting essentially of iron hydroxides, and saprolite , which contains a large quantity of hydrosilicate minerals Nickel bearing minerals occur in the saprolite and the lower portion of ferralite Resistivity well-logging and resistivity sounding indicated that the electrical properties of rocks depend upon their composition: Canga and ferralite showed high resistivities of 6,500 Ωm and 800 Ωm, respectively The resistivity of saprolite was found to be much lower, between 10 Ωm and 20 Ωm The laterite is underlain by resistive peridotite The chargeability of saprolite was found to be lower than that of the upper horizons and the bedrock Electrolytic conductivity of laterite, which depends on the geometry of the deposit, was found to be low, because the laterite contains moisture and ground water, which are highly resistive The relatively high conductivity of saprolite is caused by nickeliferous hydrosilicates, which exhibit the electrical properties of clay minerals, with an apparent maximum conductivity of 025 S/m The conductivity of saprolite corresponds to a concentration between 30% and 50% of conductive silicate minerals distributed in the pore space of deposit A nickel enrichment of up to 6% was estimated from the resistivity of the saprolite Prospecting for laterites by electrical sounding showed that the development of laterite horizons in a nickel deposit correlates with the surface morphology of weathered ultrabasic massif Thus the method can be used in preliminary exploration of such deposits

Electron microscope studies of the surface microstructures of layer-lattice silicates

Abstract: Microstructural features of crystal surfaces of the silicate minerals muscovite, margarite, montmorillonite, beidellite, and kaolinite have been investigated by studying the way in which gold atoms, deposited from the vapor phase, coalesce on these surfaces. Although detailed interpretation of the experimental data is in some cases open to speculation, the results obtained from these investigations show marked differences between the surface structures of the different silicate minerals and reveal microstructural features which would be difficult to detect by any other means. It would seem that this experimental technique can be used to obtain quantitative information concerning the composition of silicate surfaces provided the precautions are taken to preclude postdepositional migration and coalescence of the metal particles. The coalescence of gold particles partially embedded in a carbon film is promoted by aqueous salt solutions and acids. This phenomenon imposes serious practical restrictions on the way in which this type of study can be carried out in order to obtain quantitative surface structural information and does not seem to have been taken into account in many previous experimental investigations.

Melting Property of MgO Containing Sinter

Abstract: The solution property of MgO containing materials into the CaO-Fe2O3 system and the melting property of MgO containing sinter during reduction were investigated. The results obtained are summarized as follows:(1) The formation of high viscosity silicate and magnetite prevents the homogenization of slag and therefore, Cr slag and Ni slag remain unslagged. The formation of dicalcium ferrite prevents the dissolution of periclase into the CaO-Fe2O3 system.(2) During reduction unslagged MgO makes the shell of MgO-FeO solid solution with FeO in glassy silicate and its solution is negligibly low.(3) For both normal sinter and pellet, all silicate minerals dissolve above about 1240°C.(4) MgO addition in sinter is effective to decrease the slag quantity during reduction but the melting property of sinter is not so improved as that of pellet. The proper basicity for sinter and pellet is about 1.4.

CHAPTER 25 – Iron

Abstract: Publisher Summary Silicate minerals vary in iron content, and this variation is reflected in the iron content of silicate rocks. Hortonolite and other similar basic rocks may contain 35–40 per cent iron, while many acidic rocks contain no more than 1 per cent total ferric and ferrous oxides. Sulfide minerals containing iron are of considerable economic importance. This chapter discusses the methods of determination of metallic iron and ferrous iron in silicate rocks. The ease with which ferrous iron is converted into the ferric state by atmospheric oxidation is well known and during the course of the preparation of the rock sample and at every subsequent stage of the determination, care must be taken to ensure that such oxidation is kept to the minimum. Pratt method is commonly used for the determination of ferrous iron in some laboratories. The chapter discusses the Pratt method and the Wilson method. In the Wilson method, the ferrous iron, which is liberated on dissolving the silicate rock in hydrofluoric acid, is immediately removed from the solution by reaction with ammonium vanadate. An alternative method of removing the free ferrous iron is by combining with a suitable chelating reagent.

An Examination of Sedimentary Rocks and Constituent Minerals with the Scanning Laser Acoustic Microscope

Abstract: Optical characteristics of sedimentary rocks and their constituent minerals are routinely observed and quantified with the petrographic (polarizing) microscope and reported in the literature; a quantitative acoustic examination at comparable magnification (i.e., about 85 times) is reported here for the first time. The ultrasonic velocities of selected individual grains of quartz and feldspar are presented and compared with published values for these minerals obtained using macroscopic techniques. Qualitative interpretations of the acoustic photomicrographs include an examination of silicate minerals (quartz, feldspar), sedimentary rocks (sandstone. mudstone), and some common features of sedimentary rocks (laminar, bedding, pores, cement, fractures). A comparison of acoustic and optical characteristics of specific features, such as twinning and fractures of sedimentary rocks and minerals is made.

The Mineralogy of Çaldağ (Manisa-Turgutlu) Lateritic Iron, Nickel - Cobalt Deposits

Abstract: The physical features, mineralogical and chemical composition of the Caldag Lateritic iron, nickelcobalt deposits indicate that the ore occurs in various zones. The limonite zone, which constitutes the uppermost of the Laterite, shows iron enrichment, but high Cr and Ni levels may create some technical problems during the use of iron as an ore. The nickel content increases downwards from the limonitic zone and reaches the highest level at a zone enriched in asbolane. It decreases further downwards and shows normal values in serpentine at the bottom. As far as this study concerned, ore mineralisation in the area is smaller in size when compared to the similar deposits of the world. Nickel is commonly found in the form of silicate minerals in other lateritic deposits where as it is related to the Ni-Co-asbolane in the Caldag iron, nickel-cobalt deposits. The ore deposit in the area was formed as a result of the lateritic processes during the period of Upper Cretaceous - Eocene, but the lateritization did not advance and the area was covered by the Eocene formations.