Enstatite crystallization

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... 

Olivine occurs mainly as inclusions in enstatite and is also found in albite, spinel, and troihte (Brearley, 1989). At least three kinds of poly-crystalline clusters appear to have formed separately (i) enstatite-rich units with forsterite inclusions, (ii) coarser versions with albite and olivine inclusions, and (iii) forsterite-anorthite units with no enstatite. Enstatite crystals are intergrowths of ortho- and clinopyroxene and their microstructures indicate cooling from >1,000 °C at -1,000 °Ch Brearley (1989) suggests that the Kakangari matrix formed from amorphous or partly crystalline particles <10 p,m in size that were annealed at 1,100-1,200 °C or possibly higher, and then rapidly cooled in an hour. The chondrules in Kakangari could have formed from similar material that was heated to higher temperatures, partly melted, and quenched at comparable rates, provided that the chondrules acquired lower concentrations when molten. 

Shallowater and Mt. Egerton are unique meteorites that may be closely related to the aubrites. Shallowater is unbrecciated, contains higher abundances of both metal and troilite than other aubrites (Keil et al., 1989 Watters and Prinz, 1979), and the pyroxene is ordered orthopyroxene, rather than disordered pyroxene common to the group (Reid and Cohen, 1967). Shallowater also contains xenoliths composed of twinned clinoen-statite, forsterite, plagioclase, EeNi metal, and troilite that comprise 20 vol.% of the rock (Keil et al., 1989). Mt. Egerton is also unbrecciated and composed of centimeter-sized enstatite crystals with 21 wt.% interstitial EeNi metal. 

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]. 

Landwehr et al. (2001) extended the model of Wood and Blundy (1997) to include and Th". They measured experimentally Aj and Z)xh in a wide variety of synthetic clinopyroxene compositions in order to evaluate the crystal compositional dependence of U-Th fractionation. Their observations confirm the predictions of Wood et al. (1999), namely that as the M2 site becomes smaller, so D-m becomes smaller than Du (Figs. 1 and 11). The M2 site becomes smaller as the enstatite component of the clinopyroxene increases and Ca on M2 is replaced by Mg. Enstatite solubility in clinopyroxene increases with increasing temperature, consequently clinopyroxene coexisting with orthopyroxene will show higher Du/Dih at higher temperature. For this reason, DuIDjb increases with increasing pressure along the mantle solidus, as discussed above. 

Mossbauer Effect in Similar Glasses and Crystals. Expression of the Mg-Fe content of tektites in terms of enstatite-ferrosilite percentages suggests a comparison with silicate minerals which contain Fe and... 

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... 

Rouxel and Verdier (1996) studied the viscoplastic forming and the crystallization ranges of SiC particle reinforced Y-Mg-Si-Al-O-N glass composites produced by hot-pressing at 1050°C. Crystallization starts beyond 1050°C, with spinel, MgAl204, enstatite, MgSi03, and y- and 8-Y2Si207 as... 

Single chains formed by tetrahedral SiC>4 groups sharing two vertices adopt various configurations in the crystal. There are different numbers of tetrahedra in the repeat unit for different minerals for example, both diopside CaMg[Si03]2 and enstatite MgSiCL have two repeat units, as shown in Fig. 14.4.9(e). 

These assignments of the crystal field bands may be used to construct the 3d orbital energy level diagrams illustrated in fig. 5.17 for Fe2+ ions in the Ml and M2 sites of ferrosilite, Fsgg 4. The polarized absorption spectra of this ferrosilite (fig. 5.15b) show that two of the M2 site Fe2+ bands are centred near 10,700 cm-1 and 4,900 cm-1. The lower-level splittings of 2,350 cm-1 and 354 cm-1 listed in eq. (5.11) for enstatite axe assumed to apply to ferrosilite. This information is... 

The crystal field splitting estimated for Fe2+ in the enstatite M2 site is in reasonable agreement with the A0 value of 6,522 cm-1 calculated by a theoretical point charge model (Goldman and Rossman, 1977a Rossman, 1988). 

High-temperature treatment within the range 550-800°C brings to crystallization of products. In non-activated mixtures, heated at 800°, the crystallization of MgO and insignificant amounts of forsterite Mg2Si04 are observed. In samples activated for a short time, a mixture of forsterite and enstatite is formed, while after activation for 3 h, only enstatite MgSiOa is formed. 

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