Iron core compositional model

Known compositional variations of ferritin iron cores only Involve phosphate, which can range from as much as 80% (21) to as little as 5% of the iron (21) in normal mammalian liver or spleen, the amount of phosphate in the ferritin iron core is ca. 12% of the iron (24). When the phosphate content is high, the distribution of phosphate is clearly throughout the core rather than on the surface. However, interior locations for phosphate are also suggested when the phosphate content is lower, by data on an Fe(III)ATP model complex (P Fe 1 4) (25) or by phosphate accessibility studies in horse spleen ferritin (P Fe = 1 8) (24). Based on model studies, other possible variations in core composition could Include H2O or sulfate (26). 

Data for the content of lithophile elements in the Earth plus knowledge of the iron content of the mantle and core together establish a bulk Earth compositional model (McDonough, 2001). This model assumes chondritic proportions of Fe/Ni in the Earth, given limited Fe/Ni variation in chondritic meteorites (see below). This approach yields... 

The compositional model for the core has a light element composition that seeks to ht the density requirements for the outer core and is consistent with cosmochemical constraints. Signihcantly, along with iron and nickel the core contains most of the planet s sulfur, phosphorus, and carbon budget. Finally, this model composition is notable in that it is devoid of radioactive elements. The discussion that follows reviews the issues associated with compositional models for the core. 

In detail, an impact of this type, starting with two bodies both with iron cores and silicate mantles, would result in a planet surrounded by a disk of former mantle material. It is from this disk that an iron-depleted Moon would accrete (Canup, 2004). Models of this type are proving highly successful inasmuch as they have the capacity to simultaneously explain the masses of the Earth and Moon, the Earth-Moon angular momentum, and the unusual chemical composition of the Moon. 

St. Pierre, T. G. Chan, R Bauchspiess, K. R. Webb, J. Betteridge, S. Walton, S. Dickson, D. P. E. Synthesis, structure and magnetic properties of ferritin cores with varying composition and degrees of structural order models for iron oxide deposits in iron-overload diseases. Coord. Chem. Rev. 1996, 151, 125-143. 

Silicates provide further evidence for the unusual origin of lAB-IIICD. While differentiated silicates might be expected in association with iron meteorites, silicates in lAB-IIICD irons are broadly chondritic (Mittlefehldt et al., 1998 Benedix et al., 2000 see Chapter 1.11). Models for the origins of lAB-IIICD iron meteorites include crystallization of a sulfur- and carbon-rich core in a partially differentiated object (Kracher, 1985 McCoy et al., 1993), breakup and reassembly of a partially differentiated object at its peak temperature (Benedix et al., 2000), or crystal segregation in isolated impact melt pools on the surface of a porous chondritic body (Wasson and Kallemeyn, 2002). A recent compilation of the chemical compositions of lAB and IIICD iron meteorites may be found in Wasson and KaUemeyn (2002). 

It was at one time thought that even the terrestrial planets themselves formed directly by condensation from a hot solar nebula. This led to a class of models called heterogeneous accretion models, in which the composition of the material accreting to form the Earth changed with time as the nebula cooled. Eucken (1944) proposed such a heterogeneous accretion model in which early condensed metal formed a core to the Earth around which silicate accreted after condensation at lower temperatures. In this context the silicate-depleted, iron-enriched nature of Mercury makes sense as a body that accreted in an area of the solar nebula that was kept too hot to condense the same proportion of silicate as is found in the Earth (Lewis, 1972 Grossman and Larimer, 1974). Conversely, the lower density of Mars could partly reflect collection of an excess of silicate in cooler reaches of the inner solar nebula. So the... 

For molecular modeling of the lipoproteins, values for the partial specific volumes of the lipoprotein components are required. The partial specific volume of an aqueous egg yolk lecithin suspension is 0.984 ml/g (Hauser and Irons, 1972), and this provides a reasonable approximation for the partial specific volume of the phospholipid occupying the surface monolayer of a lipoprotein. The reciprocal of the density of liquid triolein (Small, 1986) yields its partial specific volume, 1.102 ml/g, and provides a reasonable approximation for triglyceride dissolved in the cholesteryl ester-filled core of the LDL. For cholesterol, the partial specific volume of 1.021 ml/g measured in benzene (Haberland and Reynolds, 1973) has been employed. The value of 0.740 ml/g employed for the partial specific volume of apoBlOO was determined from its amino acid composition (Lee et al., 1987). A value of 0.60 ml/g was used for the partial specific volume of the carbohydrate moiety. One important parameter, the partial specific volume of cholesteryl ester, remains to be determined. As will be shown below, its value is estimated to be 1.058 ml/g. 

A particularly interesting synthetic model for the ferritin core is the iron (III) polymer obtained by Spiro, Saltman, and coworkers from bi-carbonate-hydrolyzed ferric nitrate solutions (6). The polymer has an approximate molecular weight of 150,000 and is about 70 A in diameter. Analytical studies indicate a composition Fc403(0H)4(N03)2 I.5H2O for the polymer. In physical size and shape, the Saltman-Spiro ball is remarkably similar to the ferritin core. The structure that has been sug-... 

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