Enstatite crystal structure

Enneagold heteronuclear cluster compounds. X-ray crystallography, 39 375-376 Enniatin-B, 42 119-120 Enolase, arsonomethyl analogue, 44 205-206 Enstatite, crystal structure of, 4 51 Enthalpy... 

In summary, we calculate that the low clinoenstatite is not stable under hydrostatic conditions and enstatite has a comparatively small stability field. The energy differences are so small that they are within the reliability of the simulations and thus the precise positions of the phase boundaries are not well located. The primary reason for this problem is the reliability of the potential models. Hence, calculating phase relationships represents the most difficult challenge for free energy minimization techniques. However, the simulations do provide valuable insights into the mechanisms of phase transitions and the effect of pressure and/or temperature on the crystal structures and the relative phase stabilities. 

Griffin et al. showed the Mg NMR parameters have been determined for two polymorphs of enstatite (MgSi03), an important magnesium silicate phase present as a major component of the Earth s upper mantle. The crystal structures of both polymorphs contain two crystallographically distinct magnesium sites however, only a single resonance is observed in... 

Ghose S., Schomaker V., and McMullan R.K., "Enstatite, Mg2Si20g A Neutron Diffraction Refinement of the Crystal Structure and a Rigid-Body Analysis of the Thermal Vibration," Z. Kristallogr., 176, 159-75 (1986). 

Burgers vector and dislocation line analysis of naturally deformed aug/tc-enstatite crystals indicate activation of many slip systems such as (100)[001], 110 1<110>, 110 1<112>, (100)[010], (010)[100], (010)<101>, and 110 < 111 >, the first two being the most active [311,312]. The study demonstrates that most dislocations are dissociated and stacking faults are produced that can be interpreted based on the complex structure of these chain silicates. In naturally deformed augite from a pyroxenite with lamellar exsolution [lOl] dislocations in (010) combine single to double chains, yielding so-called chain multiplicity faults [316]. 

The chemical classification of pyroxene components disregards to some extent the represented structural groups, being based essentially on the chemistry of major cations. As an example, table 5.29 reports the classification of Deer et al. (1978), which subdivides pyroxene components into five groups (Mg-Fe, Ca, Ca-Na, Na, and Li pyroxenes). In this classification, the term enstatite identifies a component crystallizing in different structural classes Pbca, P2Jc, Pbcn). 

Table 5.33 lists the structural characters of various pure components of pyroxene crystallizing in the stable state, according to the synthesis of Smyth and Bish (1988). Note the doubling of cell edge a in the transition from monoclinic to orthorhombic structure, composition being equal (i.e., clinoenstatite-enstatite clinoferrosilite-ferrosilite). This effect may be represented by the equation... 

As might be anticipated for minerals with chain structures, pyroxenes commonly occur in columnar, prismatic, rodlike, and acicular forms. Enstatite has been found in the form of rosettes of fine-fibrous crystals. Special names such as victorite, chladnite, and shepardite were assigned to different occurrences in this distinctive morphology, possibly because the fibrous aggregates were located in iron meteorites. However, the composition and crystal... 

Proof that the anions of the substances mentioned really consist of long chains of P04 tetrahedra has been supplied by complete structural analyses of crystals. Four different types of such chain-like anions, differing in shape and inner periodicity, have so far been recognized. In the low temperature form of lithium polyphosphate, which has the same type of structure as lithium polyarsenate and diopside [Ca, Mg(Si03)2]x these are extended chains which (see Fig. 9a) as in enstatite have a Zweierperiodizit at (138) (see also ref. 66 in Table XVI). Rubidium polyphosphate has quite similar Zweierketten (55) (see Fig. 9b and ref. 63 in Table XVI) except that the P04 tetrahedra are somewhat rotated with respect to Form a. In the high... 

Silicon nitride crystallizes in two hexagonal forms (a- and fl-SisN and also a cubic y-modification with spinel structure that is stable at high pressure and temperature (>15GPa, >2000 K Zerr et al, 1999 Leitch et al, 2004). The a-phase occurs in nature as a rare mineral (nierite) in ordinary and enstatite chondritic meteorites (Lee et al, 1995). 

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