Enstatite activation

This difference is apparent in olivine and enstatite activities (Table 7). As is appreciated from Figure 3, this makes for a considerable difference in the intersection of the curves. Presumably one would conclude that the more iron-rich material is the more plausible residua of the source region. 

Roy and Romo (1957) and Boettcher (1966) performed high pressure experiments on natural vermiculites. They observed the production of a 14 X chlorite between 300 and 550°C, talc + enstatite and an unidentified phase above 650°C. The experiments on natural minerals indicate that vermiculite will occur when alkali content or activity in solution is low. This trioctahedral expanding phase is relatively stable at high pressures and temperatures as are interlayered minerals which are composed in part by such layers. It is not stable relative to montmorillonite at low emperature. 

Other Minerals Up to this point, reference has been made to other luminescing phases within meteorites but in all cases no systematic studies have been reported to 1) determine the cause of the CL or 2) to relate the CL to the genesis of the mineral or meteorite. The following comments are made to draw attention to the possible significance of CL observations in other meteoritic minerals most observations are from the author s personal experience and have not been documented. Oldhamite is a rare mineral in the enstatite chondrites but is known to carry appreciable quantities of rare earth elements (REE). The CL spectra have not been studied in detail but visual CL is yellow and a CL emission at 580nm was reported (H) but with rapid intensity change under the electron beam. A REE activator of the CL is possible. Hibonite is a relatively common... 

Under heating, activated talc is transformed into enstatite ... 

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. 

According to DTA and TG data, endo-effect at 365°C, caused by Mg(OH)2 decomposition, remains constant in the first case, becomes very small in the second case and is practically absent in the third case. This means that the interaction of initial components occurs at the stage of mechanical activation. DTA curves also exhibit endo-effect at 865°C, being maximal for the second mixture. It is due to the formation of forsterite, Mg2Si04, in the first mixture and due to formation of a mixture of forsterite with enstatite in the second and the third mixtures. 

Aranovich L. Y. and Newton R. C. (1998) Reversed determination of the reaction Phlogopite -I- quartz = enstatite 4- potassium feltspar -I- H2O in the ranges 750-875 °C and 2-12 kbar at low H2O activity with concentrated KCl solutions. Am. Mineral. 127, 261-271. 

As summarized earlier, Oelkers (2001b) has proposed that the effect of dissolved cations derived from the dissolving silicate can be modeled using equations such as (25). For example, Oelkers and Schott (2001) reported that the rate of enstatite dissolution at pH 2 was independent of silica activity but decreased with increasing magnesium activity according to the following equation ... 

The other main component in the support, sepiolite, can also undergo transformation to enstatite above 830°C [14]. The XRD patterns of the support pre-treated at temperatures below 1000°C (Figure 4) showed no enstatite peak. In order to further study if this transformation could be responsible for the effect of heat pretreatment on the activity, the TG-... 

The activity of the FeSiOj component of the MgSiOj-FeSiOj (enstatite-ferrosilite) solid solution was obtained by using as experimental charges compositions in this series plus silica plus Fe metal, in the furnace atmosphere having the controlled /o as described above. The relevant reaction is... 

Saxena and Nehru (1975) Independent equations of state for diopside and orthoenstatite solutions are assumed to obey regular solution models (one-constant Margules equations where Wqi= Wg2) Tbe W parameter for each solution is calculated from assumed differences in the standard free energies of fonnation for end-member phases and published experimental data on the compositions of coexisting phases. Temperatures for natural assemblages of enstatite and diopside are estimated from Margules expressions for homogeneous mixtures in which activities are presumed to be related to individual site occupancies. 

The low value of (dP/dT) for the reaction (L) is due to the very large volume term involving HO for this reaction. The activity of SiO has a profound effect on the stability of phlogopite as has been pointed out by Luth (1967) and Yoder and Kushiro (1969). A whole series of stability curves exist for phlogopite in which the activity of SiO is successively buffered by quartz, enstatite-forsterite, leucite-sanidine, and leucite-kalsilite. 

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]. 

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