Olivine stability

Nicholls 1. A. and Ringwood A. E. (1973) Effect of water on olivine stability in tholeiites and the production of silica-saturated magmas in the island-arc environment. J. Geol. 81, 285-300. 

The substitution of manganese for iron in LiMP04 would increase the voltage from 3.45 (pure LiFeP04) to 4.1 V (pure LiMnP04) while the other attractive properties of olivines (stability, safety, price, etc.) remain intact. Unfortunately, the past research has identified several problems with LiMnP04, especially the extremely low intrinsic. As LiMnP04 did not present any practical capacity in the early days to the exception of a very controversial paper, first communications reported electrochemical behaviour of the solid... 

A number of theoretical approaches can account for the fact that an enthalpy of formation of such a binary oxide or a ternary oxide is large and negative. The stability of a ternary oxide relative to the binary constituent oxides is, however, often small, as demonstrated in Table 7.1 using Mg2SiC>4 as an example [1], The enthalpy differences between the three different polymorphs of Mg2Si04 - olivine, /3-phase and spinel - are less than 2% of the enthalpy of formation of the polymorphs. These enthalpy differences are comparable in magnitude to the enthalpy... 

Recent experiments by Akamatsu et al. (1993) show that pressure has a significant elfect on Fe-Mg intracrystalline distribution (dK /dP = 2 Mbar ). At the upper pressure limit of stability of olivine (P 0.12 Mbar, corresponding to a depth of about 400 km p-wave seismic discontinuity, cf section 5.2.3), the increase in with respect to the P = 1 bar condition, Tand composition being equal, is about 0.24. For a Fogo olivine, this variation is similar to that induced by a T increase of about 250 °C (cf figure 5.11). 

Figure 5,25 Phase stability relations for natural pyroxenes in the quadrilateral. (A) Magmatic pyroxenes (not reequilibrated at low 7). (B) Three-phase region for pyroxenes of stratihed complexes and magmatic series. (C) Pyroxenes reequilibrated in subsolidus conditions. = Pbca orthopyroxene V = augite C2lc) A = pigeonite (P2jlc) 0 = olivine. From Huebner (1982). Reprinted with permission of The Mineralogical Society of America.

Burns R. G. (1975). On the occurrence and stability of divalent chromium in olivines included in diamonds. Contrih. Mineral. Petrol, 51 213-221. 

Transition metal ions most susceptible to large Jahn-Teller distortions in octahedral coordination in oxide structures are those with 3d4, 3d9 and low-spin 3(f configurations, in which one or three electrons occupy eg orbitals. Thus, the Cr2+ and Mn3+, Cu2+, and Ni3+ ions, respectively, are stabilized in distorted environments, with the result that compounds containing these cations are frequently distorted from type-structures. Conversely, these cations may be stabilized in distorted sites already existing in mineral structures. Examples include Cr2+ in olivine ( 8.6.4) and Mn3+ in epidote, andalusite and alkali amphiboles ( 4.4.2). These features are discussed further in chapter 6. 

Furthermore, the inverse fifth-power dependence of crystal field splitting on metal-oxygen distance, eq. (2.17), indicates that an increased CFSE may be acquired by most transition metal ions when they occupy smaller coordination sites. As a result of this enhanced stability, Ni2+, Cr2+, Co2+ and Fe2+ are predicted to have preferences for the olivine Ml and pyroxene Ml sites having smaller average metal-oxygen distances than the corresponding M2 sites (eq. (6.7) Appendix 7) and to display relative enrichments for the Ml sites in the order Ni2+ > Cr2+ > Co2+ > Fe2+ > Mn2+, Zn2+. This order closely resembles the measured site occupancies (Ghose et al., 1975 Rajamani et al., 1975 Walsh et al., 1976) shown in fig. 6.3. 

The Ni2+, Cr3+ and low-spin Co3+ ions do not acquire additional stabilization in distorted octahedral sites. They are expected to favour smaller sites that more closely approximate octahedral symmetry than other available sites in the crystal structures. As noted in 6.8.2, the high octahedral CFSE s acquired by these three cations in small octahedral sites in silicate and oxide structures accounts for the observed relative enrichments of Ni2+ in the olivine Ml and orthopyroxene Ml sites, the sole occupancy by Cr3+ of pyroxene Ml sites, and the occurrence and stability of low-spin Co3+ in Mn(IV) oxides. 

In the case of Cu2+ and Cr2+, compounds of which are susceptible to Jahn-Teller distortions ( 6.3 table 6.1), these cations are predicted to show strong preferences for the most distorted orthopyroxene M2 and amphibole M4 sites. A similar explanation accounts for the observed enrichments of Mn3+ ions in the distorted andalusite Ml, alkali amphibole M2, epidote M3 and, perhaps, epidote Ml sites (table 6.1). The presence of significant amounts of chromium in olivines from the Moon and as inclusions in diamond may be due to the presence of Cr2+ ions, and not Cr3+, in the distorted Ml and M2 sites of the olivine structure (Bums, 1975b), in which Jahn-Teller stability may be attained. A similar factor accounts for the stability and site occupancy of the Cr2+ ion in the orthopyroxene M2 site (table 6.1). 

The Co2+, Ti3 and V3 ions are expected to prefer either distorted or small octahedral sites. Thus, Co2+ should be slightly enriched in the orthopyroxene M2 and cummingtonite M4 sites, favour the pseudo-tetragonally distorted olivine Ml site, and be randomly distributed over the amphibole Ml, M2 and M3 sites. The V3+ and Ti3+ ions are expected to occupy the orthopyroxene Ml and alkali amphibole M2 sites, and to be enriched in distorted epidote M3 sites. As noted earlier, the occurrence and stability of Ti3+ ions in lunar and mete-oritic clinopyroxenes ( 4.4.1) may be explained by the availability of the distorted octahedal Ml site in the calcic clinopyroxene structure. 

Figure 7.3 Excess crystal field stabilization energy of mixing for Mg-Fe2+ olivines of the forsterite-fayalite series.

Bums, R. G. Sung, C.-M. (1978) The effect of crystal field stabilization energy on the olivine - spinel transition in the system Mg2Si04-Fe2Si04. Phys. Chem. Minerals, 2, 349-64 [see also Phys. Chem. Minerals, 2,177-97 (1978)]. 

Mao, H.-K. Bell, P. M. (1972a) Optical and electrical behaviour of olivine and spinel (Fe2Si04) at high pressure. Interpretation of the pressure effect on the olivine absorption bands of natural fayalite to 20 kb. Crystal field stabilization of the olivine-spinel transition. Ann. Rept. Geophys. Lab., Yearb. 71, 520-8... 

Reversible electrochemical lithium deintercalation from 2D and 3D materials is important for applications in lithium-ion batteries. New developments have been realized in two classes of materials that show exceptionally promising properties as cathode materials. The first includes mixed layered oxides exemplified by LijMn Nij, Co ]02, where the Mn remains inert to oxidation/reduction and acts as a framework stabilizer while the other elements carry the redox load. Another class that shows much potential is metal phosphates, which includes olivine-type LiFeP04, and the NASICON-related frameworks Li3M2(P04)3. 

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