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Silicate minerals
About: Silicate minerals is a research topic. Over the lifetime, 1794 publications have been published within this topic receiving 67064 citations.
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TL;DR: In this paper, the authors use phase diagrams to illustrate possible paths of crystallization of initial CO-2-bearing silicate haplomagmas, and place limits on the compositions of immiscible carbonatite magmas which can be derived from silicate parent magmas.
Abstract: The synthetic system Na_2O–CaO–Al_2O_3–SiO_2–CO_2 has been widely used as a model to show possible relationships among alkalic silicate magmas, calciocarbonatites, and natrocarbonatites. The determined immiscibility between silicate- and carbonate-rich liquids has been strongly advocated to explain the formation of natural carbonatite magmas. Phase fields intersected at 1.0 GPa by the composition joins NaAlSiO_3O_8–CaCO_3 (Ab–CC, published) and NaAlSiO_4(Ne)_(90)Ab_(10)–CC (new), along with measured immiscible liquid compositions, provide pseudoternary phase relationships for the composition triangles Ab–CC–Na_2CO_3(NC) and Ne_(90)Ab_(10)–CC–NC. Interpolation between these, and extrapolation within the CO_2-saturated tetrahedron Al_2O_3–SiO_2–CaO–Na_2O, provides pseudoquaternary phase relationships defining the volume for the miscibility gap and the surface for the silicate–carbonate liquidus field boundary. The miscibility gap extends between 10 and 70 wt % CaCO_3 on the triangle Ne–Ab–CC at 1.0 GPa; it does not extend to the Na_2O-free side of the tetrahedron. The liquidus minerals in equilibrium with both silicate- and carbonate-rich consolute liquids are nepheline, plagioclase, melitite, and wollastonite; with increasing Si/Al the liquidus for calcite reaches the miscibility gap. We use these phase relationships to: (1) illustrate possible paths of crystallization of initial CO_2-bearing silicate haplomagmas, (2) place limits on the compositions of immiscible carbonatite magmas which can be derived from silicate parent magmas, and (3) illustrate paths of crystallization of carbonatite magmas. Cooling silicate–CO_2 liquids may reach the miscibility gap, or the silicate–calcite liquidus field boundary, or terminate at a eutectic precipitating silicates and giving off CO_2. Silicate–CO_2 liquids can exsolve liquids ranging from CaCO_3–rich to alkalic carbonate compositions. There is no basis in phase relationships for the occurrence of calciocarbonatite magmas with ∼99 wt % CaCO_3; carbonate liquids derived by immiscibility from a silicate–CO_2 parent (at crustal pressures) contain a maximum of 80 wt % CaCO_3. There are two relevant paths for a silicate liquid which exsolves carbonate-rich liquid (along with silicate mineral precipitates): (1) the assemblage is joined by calcite, or (2) the assemblage persists without carbonate precipitation until all silicate liquid is used up. The phase diagrams indicate that high-temperature immiscible carbonate-rich liquids must be physically separated from parent silicate liquid before they can precipitate carbonate-rich mineral assemblages. Path (1) then corresponds to the silicate–calcite liquidus field boundary, and a stage is reached where the carbonate–rich liquids will precipitate large amounts of calcite and fractionate toward alkali carbonates (not necessarily matching natrocarbonatite compositions). In path (2) the high-temperature immiscible carbonate liquid precipitates only silicates through a temperature interval until it reaches the silicate–carbonate liquidus field boundary, where it may precipitate calcite or nyerereite or gregoryite. Sovites are readily explained as cumulates, with residual alkali-rich melts causing fenitization. We can see no way in phase diagrams for vapor loss to remove alkalis and change immiscible natrocarbonatite liquids to CaCO_3–rich liquids; adjustments to vapor loss would be made not by change in liquid composition but by precipitation of calcite and silicate minerals. The processes illustrated in this model system are applicable to a wide range of magmatic conditions, and they complement and facilitate interpretation of phase relationships in the single paths represented by each whole- rock phase euilibrium study.
49 citations
TL;DR: In this paper, the authors investigated the partitioning behavior of rare earth elements (REE) between immiscible silicate liquids and found that REE preferentially partition into the end-member liquids that are rich in network-modifying cations.
48 citations
01 Jan 1999
TL;DR: The surface area is a key parameter in predicting weathering rates using geochemical models (e.g., Jonsson et al. as discussed by the authors ), and the surface area may vary over several orders of magnitude as functions of grain size, mineralogy, oxide coating, weathering history, or biological effects.
Abstract: The rates of many heterogeneous reactions are dependent upon the mineral-water interfacial area. Examples include release of nutrients from primary minerals, rate of growth of authigenic minerals, adsorption and desorption of metal and organic contaminants on soil and sediment grains, neutralization of acid deposition by weathering reactions, oxidation and reduction of mmetal-containing phases and solutes, clumping of colloids or bacteria by electrostatic attraction, and photocatalytic degradation of organic pollutants at metal oxide surfaces (Davis et al., 1993; White and Brantley, 1995). Several workers have also shown that the surface area is a key parameter in predicting weathering rates using geochemical models (e.g. PROFILE) and soil chemistry under the influence of acid rain (Jonsson et al., 1995; Hodson et al., 1996, 1997a). Along with the permeability, the surface area is one of the most difficult physical parameters to quantify in extrapolating from the laboratory to the soil plot to the watershed (White and Peterson, 1990). Most models of solute transport in aquifers and in soils simply ignore the mineral-water surface area term by combining it with the kinetic rate constant into one fitting parameter, despite the fact that the specific surface area may vary over several orders of magnitude — 102–106 cm2/g as functions of grain size, mineralogy, oxide coating, weathering history, or biological effects, or as a combination of these factors. Despite the importance of the mineral surface area in many areas of geochemistry, little systematic effort has been expended to understand or predict this term for primary silicates (more work has been completed on the surface area of clays and simple oxides).
48 citations
TL;DR: In this paper, the authors used suppression subtractive hybridization (SSH) technology to determine the mechanisms by which fungi weather potassium-bearing minerals on a molecular level, and they found that the factors that could accelerate weathering of Kbearing minerals involved organic acids, carbonic acid, and redox participant molecules.
48 citations
TL;DR: In this paper, the authors considered the Bigadic, Emet and Kirka lacustrine basins of western Turkey may be considered as Tibet-type graben structures that were developed during the Miocene within the Izmir-Ankara suture zone complex.
Abstract: The Bigadic, Emet and Kirka lacustrine basins of western Turkey may be considered as Tibet-type graben structures that were developed during the Miocene within the Izmir-Ankara suture zone complex. The volcanic-sedimentary successions of these basins are made up of mudstone, carbonate (limestone and dolomite) and detrital rocks, and also of crystal or vitric tuffs about 135 to 200 m thick. The Degirmenli (Bigadic), Emirler (Bigadic) Kopenez (Emet) and Karaoren (Kirka) tuffs constituting the zeolite deposits are situated beneath four borate deposits (colemanite, ulexite, borax). The most abundant diagenetic silicate minerals are K- and Ca-clinoptilolites in the zeolite deposits, and Li-rich trioctahedral smectites (stevensite, saponite and hectorite) and K-feldspar in the borate deposits. In the Degirmenli, Emirler. Kopenez and Karaoren deposits, the following diagenetic facies were developed from rhyolitic glasses rich in K and poor in Na: (glass + smectite), (K-clinoptilolite + opal-CT), (Caclinoptilolite + K-feldspar ± analcime ± quartz) and (K-feldspar+analcime+quartz). K-feldspar which is also rarely associated with phillipsite (Karaoren) and heulandite (Degirmenli and Karaoren), succeeds clinoptilolite and precedes analcime in these diagenetic facies where dioctahedral smectites, opal-CT and quartz are the latest minerals. No diagenetic transformations exist between clinoptilolite, K-feldspar and analcime that were formed directly from glass. The lateral facies distributions resulted from the differences in salinity and pH of pore water trapped during deposition of the tuffs, but vertical distributions in vitric tuffs seem to have been controlled by the glass/liquid ratio of the reacting system and the permeability or diffusion rate of alkali elements. The Bigadic, Emet and Kirka zeolite deposits which were formed in saline basins rich in Ca and Mg ions, show similar chemical changes, i.e. loss of alkalis and gain in alkaline-earth elements that have taken place during the diagenetic transformation of rhyolitic glasses to dioctahedral smectites or clinoptilolite. The absence of sodic zeolites such as mordenite, erionite, chabazite and silica-rich phillipsite is mainly due to the very high K/Na ratio of the starting materials rather than initial alkaline conditions or high Na content in lake waters.
48 citations