The system NiAl2O4Ni2SiO4 at high pressures and temperatures: Spinelloids with spinel-related structures

We show that many of the important features of polytypism are reproduced by a simple spin model, the axial next-nearest-neighbour Ising (ANNNI) model. The phase diagram of the ANNNI model contains sequences of long-wavelength-modulated phases. These correspond to the long stacking sequences developed in polytypic series, and are stabilized by a subtle balance between energy and entropy which results from short-range competing interactions. We discuss the extent to which this mechanism can describe polytypism in the spinelloids, pyroxenes, silicon carbide and other mineral phases.

Section: The Structure Of Spineiloid Polytypesmentioning

confidence: 99%

“…The phase relationships between spinelloid polytypes in the Ni2SiO4-NiA1204 system have been studied in detail by Akaogi et al (1982). They found that compositions close to Ni3AI2SiO8 can adopt one of three spinelloid structures.…”

Section: A Model For Polytypic Behaviourmentioning

confidence: 99%

The application of the ANNNI model to polytypic behaviour

Price¹,

Yeomans²

1984

“…The structural features of spinelloids have been described in a series of papers by Ma & Sahl (1975), Horioka, Takahashi, Morimoto, Horiuchi, Akaogi & Akimoto (1981), Horioka, Nishiguchi, Morimoto, Horiuchi, Akaogi & Akimoto (1981), Horiuchi, Akaogi & Sawamoto (1982) and Akaogi et al (1982). The family takes its name from its most commonly occurring member, the spinel structure, from which the other structures in the family can be derived.…”

Section: Spinelloid Structuresmentioning

confidence: 99%

“…Undoubtedly, many metastable phases have been synthesized in such polytypic systems. However, recent careful experimentation upon the SiC and spinelloid systems (Jepps & Page, 1983;Akaogi, Akimoto, Horioka, Takahashi & Horiuchi, 1982) has established that at least some polytypes have thermodynamically definable fields of stability.…”

Section: Introductionmentioning

confidence: 99%

The energetics of polytypic structures: a computer simulation of magnesium silicate spinelloids

Price¹,

Parker²,

Yeomans³

1985

A computer simulation technique is used to predict the lattice energies, structures and physical properties of magnesium silicate spinelloids from given interatomic potentials. Two types of potential models are considered, a fully ionic model, which includes electrostatic, short-range and dispersive terms, and a partially ionic model, in which fractional charges are allocated to the component ions and a Morse function is included to describe the effect of covalency in the Si-O bond. The calculated energies of the spinelloid polytypes are analysed in terms of the interaction energies between component structural units. The calculated spinelloid energetics are discussed in the light of recently developed models of polytypism. The predicted values of the interaction energy terms are tested by using them to calculate the energy of a 1/21101](010) stacking fault in the naturally occurring magnesium silicate spinelloid wadsleyite. The calculated stacking-fault energies are in excellent agreement with the value inferred from transmission electron microscopy.

“…All four spinelloid phases yielded dark-green crystals. The crystals of spinel were readily indentified by their truncated octahedral shape whereas the crystals of phases I, II and V could only be distinguished by recording X-ray precession photographs of reciprocal-lattice layers of the type {hk0} or {Ok/} (note that the spinelloid unit cells only differ by their b parameters -see, for example, Akaogi et al, 1982). The close structural relationship existing between the spinelloid phases also resulted in the formation of intergrowths, especially in samples cooled to 1798 K containing both phase I and phase V crystals.…”

Section: Introductionmentioning

confidence: 99%

Structures of the nickel gallosilicate spinelloids

Hammond¹,

Barbier²

1993

The structures of four spinelloid phases (spinel plus phases V, I and II), forming at 1 atm pressure in the NiGa204-Ni2SiO4 system, have been determined from X-ray data collected from single crystals grown from SiO2-rich melts in the temperature range 1798-1923 K. The results show that, under conditions of constant pressure, a strong correlation exists between the spinelloid structures, their atomic distributions and their chemical compositions. In particular, the Si contents of the tetrahedral sites are found to be strongly correlated with the amount of tetrahedral corner sharing: 6-12, 24-35 and 65% Si in the T(1), T(2) and T(3) sites with zero, one and two shared corners respectively. Consequently, the spinelloid structures accommodate increasing amounts of NizSiO4 in the order: spinel (6mo1%), phase V (27mo1%), phase I (33mo1%) and phase II (38 mol%). The crystal chemistry of the nickel gallosilicate spinelloids is discussed in comparison with that of their high-pressure aluminosilicate counterparts.