Octahedral Site Preference Energy Research Articles

Spectra and coordination changes of transition metals in hydrothermal solutions: Implications for ore genesis

Optical absorption spectra of Fe(II), Co(II), Ni(II) and Cu(II) have been measured in aqueous solutions of up to 5 m NaCl at temperatures from 25°C to 300°C and at water-saturated vapor pressures. Ni and Co complexes exhibited a change from octahedral to tetrahedral coordination, this occurring at both higher temperatures and Cl − concentrations. Similar transitions to lower coordination number are predicted for Cu and Fe but were not directly observed because of interference with water overtone bands. The coordination changes in response to ligand type and concentration, pressure and temperature. Formation of lower coordination complexes is attributed to the decreased dielectric constant of the solvent, the predominance of electrostatic forces and a decrease in the octahedral site preference energy at elevated temperatures. Our data suggest that lower coordination complexes with lower or neutral formal charge, will result in minerals having a higher solubility. The molecular properties and changes in coordination of these complexes are important in determining the transport and deposition of hydrothermal minerals.

ESR and M�ssbauer studies of the precipitation process of various ferrites from silicate glasses

ESR and Mossbauer studies of the precipitation process of various ferrites from silicate glasses were made to characterize the precipitation mechanism. The changes in linewidth (ΔH1/2) and effectiveg-value (geff) in the ESR spectra of the precipitation process are well explained in terms of super-exchange interaction between magnetic ions and interparticle dipolar interaction between precipitated ferrites. The precipitation tendency of spinel type ferrites from silicate glasses was found to be in the following order: NiFe2O4≫CoFe2O4>Fe3O4≧ZnFe2O4, MnFe2O4. The above order coincided with the order of octahedral site preference energies of divalent transition metal ions.

Interaction of cations on octahedral and tetrahedral sites in simple spinels: A reply

The cation distribution in the partly inversed binary spinel is usually treated in terms of octahedral site preference energies. Such a model is based on the assumption of ideal cation mixing in each position. The present version of theory includes explicitly the interaction between altervalent cations on octahedral and tetrahedral sites by using a simple regular model of mixing. The values of octahedral site preference energies as well as the site interaction energies are readily obtained from experimental data on the temperature dependence of the inversion parameter.

Partitioning of Fe, Co, Ni, and Cu between sulfide liquid and basaltic melts and the composition of Ni-Cu sulfide deposits

The partitioning behavior of Fe, Co, Ni, and Cu between immiscible sulfide liquid and basaltic melts has been studied using evacuated silica-tube techniques at temperatures above 1,255 degrees C. The observed Nernst partition coefficients (D) are 274 + or - 34, 245 + or - 33, and 80 + or - 15 for Ni, Cu, and Co, respectively, between sufide liquid and a normal basaltic melt (with 8.3 + or - 0.3 wt % MgO) at 1,255 degrees C; the distribution coefficients (K D = (Me/Fe) (sub sulfide liquid) X (Fe/Me) (sub silicate melt) where Me = Ni, Cu, and Co) are 42 + or - 7, 35 + or - 6, and 15 for Ni, Cu, and Co, respectively. The composition of silicate melts was observed to have a significant effect on the partitioning of Ni and Cu. For andesitic silicate melts (with 4.5 wt % MgO), Ni is more sulfophile than Cu, whereas for a more basic melt (with 13.5 wt % MgO) Ni behaves as if it were less sulfophile than Cu. The behavior of Ni is explained by its high octahedral site preference energy and by the decreasing proportions of available octahedral sites for transition metal ions with a decreasing basicity of silicate melts (Burns and Fyfe, 1964).The composition of sulfide liquids, especially the concentration of Ni and Cu and Cu/(Cu + Ni) ratios in these liquids as a function of the composition of silicate magma, was estimated by extrapolating the present experimental results to ultramafic magmas and applying them to a typical natural komatiitic magma (Naldrett and Turner, 1977). The estimated compositions are in reasonable agreement with the reported compositions of natural Ni and Ni-Cu sulfide deposits. It is concluded that the composition of silicate magma determines not only the initial concentration of ore metals but also their partitioning behavior between immiscible sulfide liquid and silicate magma--the two important factors determining the composition of massive, magmatic Ni-Cu sulfide deposits.

Geochemistry and bonding of thiospinel minerals

The thiospinel group of minerals present a variety of interesting problems in mineral chemistry. Carrollite (CuCo 2S 4), linnaeite (Co 3S 4), siegenite [(Co, Ni) 3S 4], polydymite (Ni 3S 4) and violante (FeNi 2S 4) occur in ore deposits, daubréelite [(Fe, Mn, Zn)Cr 2S 4] is present in meteorites and greigite (Fe 3S 4) is found in lacustrine sediments. This paper illustrates how the crystal chemistry, geochemistry and certain physical properties may be interpreted by molecular orbital and band theories of the chemical bond. In the spinel structure, transition metal ions occur in tetrahedral and octahedral coordinations, and 3d electrons of cations bonded to sulphur atoms are distributed amongst non-bonding and anti-bonding molecular orbitals. The composition ranges, geochemistry, and variations of cell parameters, microhardness, reflectivities and relative stabilities of thiospinels are directly related to the numbers of electrons in antibonding molecular orbitals. Linnaeite, which is the most stable thiospinel mineral in the system Cu-Co-Ni-Fe-S, has the smallest number of antibonding electrons. It has the smallest cell edge, highest reflectivity and largest microhardness. With increasing electron occupancy of the antibonding orbitals, each of these physical properties, together with the thermal stabilities, decrease along the linnaeite-siegenite-polydymite, linnaeite-carrollite and violarite-polydymite series. In each of these minerals, the unusually small cell edge may be correlated with the occurrence of transition metal ions in low-spin states. The metallic conductivity and Pauli-paramagnetism of many of these minerals is related to electron delocalization in antibonding molecular orbitals. Iron cations occur in high-spin states in greigite, giving rise to increased numbers of electrons in antibonding orbitals. As a result greigite has a larger cell edge and lower thermal stability than other thiospinel minerals. The semiconducting and ferrimagnetic properties of greigite indicate that the 3d electrons are more localized on the cation than sulphospinels in the Cu-Co-Ni-Fe-S system. Absence of solid-solution between violarite and greigite is attributed to differing spin-states of octahedral Fe(II) ions, which are low-spin in violarite and high-spin in greigite. The high thermal stability of daubréelite is due to the large octahedral site preference energy of Cr 3+ in the structure. The preference of transition metal ions for octahedral coordination is reflected by the ease in which thiospinel phases transform to a cation defect NiAs structure at elevated pressures and temperatures. As a result, polymorphic transitions involving thiospinel phases are postulated in the mantle and in shocked meteorites.

The distribution of transition elements in the system CaMgSi 2O 6Na 2Si 2O 5H 2O at 1,000 bars pressure

This study describes the results of experiments relating to the distribution of trace amounts of first row transition metal cations between diopside crystals and coexisting silicate liquid in a synthetic hydrothermal silicate system. The phase relations were determined for the system CaMgSi 2O 6Na 2Si 2O 5H 2O at 1,000 bars pressure. This system is characterized by a large field of diopside + liquid + vapour which extends to very sodium disilicate-rich portions of the system. An unknown incongruently melting phase is also present. The partition of the various transition elements between diopside crystals and coexisting silicate liquid was determined using an electron microprobe analyser. It was found that three elements, V, Cr and Ni became strongly enriched in the diopside crystals with decreasing temperature. Titanium exhibited a small tendency towards enrichment in the diopside with falling temperature. Cobalt was enriched in the diopside but was, nevertheless, rather insensitive to changes in temperature. With decreasing temperature from 1,070 to 925°C, this element displayed a slight tendency to leave the diopside, however, with temperatures decreasing from 925°C, there was a slight tendency towards increased enrichment in the diopside. All these results have been accounted for in terms of ligand field effects, bearing in mind the structural environment of the various transition metal ions in the silicate liquid as determined from absorption spectra of glasses. Thermodynamic data calculated from the experimental results further indicate the dominant importance of ligand field effects. Values obtained for ΔH° for exchange reactions involving the substitution of Mg in diopside by transition elements reflect the relative octahedral site preference energies for the transition metal cations. Values of ΔS° 1198 for the exchange reactions indicate that Cr 3+ was present essentially in one type of site in the silicate liquids (i.e., in octahedral sites) whereas Co 2+ and Ni 2+ were distributed over a number of different sites (i.e., both tetrahedral and octahedral sites). It was concluded that transition element distribution in a silicate system may be adequately predicted if details concerning its environment in the melt and the ligand field properties are known. It was noted that octahedral site preference energies should be used with caution in very high temperature portions of some silicate systems.

Transition Metal Ions as Spectroscopic Probes for Detection of Network Structure in Novel Inorganic Glasses

Spectra of divalent transition metal ions in ZnCl2, KOAc-Ca(OAc)2, KNO3-Ca(NO3)2, and K2SO4-ZnSO4 glasses are presented. Ions with low octahedral site preference energies (e.g. CO2+) can be used as “probes” for divalent cations in glass and can indicate the presence or absence of network structures. Thus ZnCl2 glass has a tetrahedral network structure, analogous to vitreous BeF2, but KOAc-Ca(OAc)2 and KNO3-Ca(NO3)2 glasses contain potassium ions and discrete acetatocalcate(II) and nitratocalcate(II) “complex ions.” The structure of acetate and nitrate glasses is discussed in terms of the ideal glass concept.

The partitioning of selected transition element ions between olivine and groundmass of oceanic basalts

The concentration of Mn 2+, Fe 2+, Co 2+, Ni 2+ in the groundmass and olivines of oceanic basalts are presented. The partitioning of these ions between the two phases is interpreted with the use of crystal field theory. A direct relationship between the logarithm of the partition coefficient and octahedral site preference energy is observed.

Distribution of Cations in Spinels

The concept of octahedral site preference energy in spinels has been extended to include Madelung and short-ranged as well as crystal field terms. A set of site preference energies is formulated which can be used to predict the ionic distribution of spinels involving the nontransition as well as the transition ions. The agreement between predicted and experimentally determined ionic distributions is surprisingly good, and a number of heretofore puzzling distributions are explained.