Interstitial materials

Fig. 1. A "barred olivine" chondrule from the Allende-type CV chondrite that feU in Mexico in 1979. The transmitted polarized light image of the 0.5 mm-diameter chondrule was taken from a poHshed thin section. The bars are composed of oHvine, (Mg,Fe)2Si04. The interstitial material is glass quenched...

Fig. 4. Photomiciogiaph of polished and etched sections of Poidand cement clinkers. The C A appears as dark interstitial material, the C AF as light...

Mineral wax yellow to dark brown, solid substances that occur naturally and are composed largely of paraffins usually found associated with considerable mineral matter, as a filling in veins and fissures or as an interstitial material in porous rocks. 

The concentration of uranium in the sediments of roll-front deposits is typically in the 1000 to 2000 ppm range (8). The uranium occurs as coatings on grains and as interstitial material the predominant uranium mineral is uraninite Coffinite is often... 

While the procedure for this experiment is provided with a fair amount of detail, students can have input into the design of the calorimeter and, although a list of materials is provided, different materials can be made available and they can choose the ones they think would be most effective. The inquiry aspect of this experiment lies primarily in the detailed analysis of their results required to suggest an appropriate interstitial material and the need to assess sources of error. If, for example, students have a large error in the determination of the specific heat capacity of copper, they must decide what could contribute to the error and then try to redesign their calorimeter or their technique in order to limit the error. 

Production clinkers have been found to contain cubic or orthorhombic forms of the aluminate phase, alone or in combination. The monoclinic modification has not been observed. The orthorhombic modification is also known as the prismatic, dark interstitial material, and is sometimes pseudo-tetragonal. It can arise only if sufficient alkali is available, but its formation appears to be favoured also by rapid cooling and by bulk compositions potentially able to yield a relatively high proportion of aluminate phase (M12). 

Sulphate-resisting Portland cements have relatively high ratios of iron to aluminium, and the ferrite phase cannot have the composition given above if it contains most of the iron. Tables 1.2 and 1.3 include a tentative composition and atomic ratios corresponding to it, based on scanty data for the interstitial material as a whole (G3,G4) and the requirement of reasonable site occupancies. 

At 1200-1300°C, high-temperature light microscopy showed movement of the silica cores and CS crystals the amorphous material was softening. Where the belite shell had burst, clusters of belite crystals with a central pore usually replaced the original quartz grain, but where it had not, these clusters only formed at a higher temperature and were more compact, with very little interstitial material. Results at higher temperatures are described in Section 3.4.4. 

AR. For lower ARs, the interstitial material consists essentially of ferrite of higher iron contents, and in white cements it consists of aluminate very low in iron, possibly together with glass. In each case, small amounts of silicate phases are probably also present. 

Other than in their mean compositions. Slow cooling produces relatively large crystals of each phase, while fast cooling produces close intergrowths textures are discussed further in Section 4.2.1. Zoning occurs readily in the ferrite (Section 2.3.1), and may also occur in the aluminate. The distribution of atoms between octahedral and tetrahedral sites in the ferrite depends on the temperature at which internal equilibrium within the crystal has been achieved (Section 1.5.1). The degree of crystallinity of both phases appears to vary with cooling rate (V3). All these effects, and perhaps others, may affect the behaviour of the interstitial material on hydration. 

All the effects described above indicate that rapid cooling is desirable the aluminate phase reacts more slowly with water when finely grained and intimately mixed with ferrite, making it easier to control the setting rate (S24), decrease in alite content either from reactions involving the interstitial material or from decomposition is avoided, a higher MgO content can be tolerated, and the clinker is easier to grind. 

Fig. 4.2 Reflected light micrograph of a polished and etched section of a Portland cement clinker, showing crystals of alite (dark, angular) and belite (less dark, rounded) embedded in a matrix of interstitial material, itself composed mainly of dendritic ferrite (light) and aluminate (dark). Courtesy Materials Science Department, British Cement Association.

There should be little free lime. What there is should occur as rounded grains, typically 10-20 pm in size, and associated with alite and interstitial material. Lime appears cream in sections etched with HF vapour. Its presence may be confirmed by a microchemical test using White s reagent (5 g of phenol in 5 ml of nitrobenzene + 2 drops of water) long, birefringent needles of calcium phenate are formed. The test also responds to CH. Alkali sulphates occur in the clinker pore structure they are etched black with HF vapour, and inhibit the etching of silicate phases with which they are in contact. 

The bulk composition affects the relative amounts of phases, the LSF, SR and AR affecting primarily the ratios of alite to belite, silicate phases to interstitial material and aluminate to ferrite phase, respectively. An LSF that is too high, either absolutely or in relation to the burning conditions, also gives rise to an excessive content of free lime. Belite may occur in such clinkers only as inclusions in alite. 

Fig. 4.3 Backscattered electron images of polished sections of (A) a Portland cement clinker and (B) grains of a Portland cement in a fresh paste. In both sections, alite is the predominant clinker phase. In (A), the relatively large, darker areas are of belite, and the interstitial material consists of dendritic ferrite (light) in a matrix of aluminate (dark) cracks and pores (black) are also visible. In (B), the belite forms well-defined regions, which are rounded, striated and darker than the alite the interstitial material, present, for example, in a vertical band left of centre within the larger grain, consists mainly of ferrite (light) and aluminate (dark). Scrivener and Pratt (S28).

Taken as a whole, the evidence indicates that all four major phases can be satisfactorily determined by QXDA if an adequate experimental procedure is employed (e.g. Ref. G30) or by calculation. Alite, belite and the total content of interstitial material can also be determined by microscopy. With any of these methods, assuming the best available procedures to be used, the absolute accuracy is probably 2-5% for alite and belite, and l-2 /o for aluminate and ferrite. 

Milfimeter-to-submillimeter lumps of matrix-rich material are present in many chondrites. Lumps that are mineralogicaUy similar to nearby matrix occurrences are probably pieces of rims or aggregates of interstitial material. However, heavily altered matrix-rich lumps called dark clasts with distinctive mineralogies probably have a different origin (Section 5.7.10). In CH and CB chondrites, aU the matrix material is present as dark clasts there are no matrix rims or interstitial material. 

CR2 chondrite matrices are predominantly composed of olivine with grain sizes of 0.2-0.3 p.m accompanied by intergrowths of serpentine and saponite with dimensions of 20-300 nm and minor sulfide and magnetite grains 0.1-25 p.m in size (Zolensky et al., 1993 Endress et al., 1994). CR2 chondrites also contain 3 vol.% of more altered matrix lumps mostly 0.1-1 mm in size, which are similar in bulk composition to the other matrix material (Endress et al., 1994). However, phyllosilicates, magnetite and chondrule fragments are more abundant in the matrix lumps than in interstitial material, and sulfides are richer in nickel suggesting that the lumps are chondritic clasts that formed in a totally different environment. 

Matrix material in CV3 chondrites occurs as rims on chondrules and other components as well as interstitial material, which is coarser grained. The volume of a matrix rim is approximately equal... 

Ureilites are ultramafic rocks composed dominantly of olivine and pyroxene, with <10% interstitial material rich in elemental carbon. Modal abundances of the major silicates are quite variable, with pyroxene varying from 0% to 90%. Roughly one-half of ureilites contain pigeonite as the sole pyroxene, but in the others, pyroxene assemblages include various combinations of pigeonite, orthopyroxene, and/or augite. 

This step is intended to reduce residual moisture to levels allowing no microbial growth or chemical reactions of the end product. The amount of residual moisture present in a product depends on its desorption isotherms. Such isotherms in turn depend on various factors including the product temperature, pressure chamber, partial vapour pressure in the container and nature of the interaction of the water vapour with the interstitial material formed in the freezing step. The computer should be fed with information on the target sample component. For example, if the component of interest is a protein, then overdrying may alter its configuration and decrease the potency of the end product. Consequently, the computer should control not only the final product temperature but also the partial water vapour pressure and the duration of the desorption step. 

Closely packed systems, in which deformable particles make up by far the largest volume fraction, whereby they deform each other. The interstitial material can be a liquid or a weak gel. 

---