Phase assemblage

There are three key variables in the design of a glass-ceramic the glass composition, the glass-ceramic phase assemblage, and the nature of the crystalline microstmcture. 

The glass-ceramic phase assemblage, ie, the types of crystals and the proportion of crystals to glass, is responsible for many of the physical and chemical properties, such as thermal and electrical characteristics, chemical durabiUty, elastic modulus, and hardness. In many cases these properties are additive for example, a phase assemblage comprising high and low expansion crystals has a bulk thermal expansion proportional to the amounts of each of these crystals. 

The extra pieces of information describe the extent of the system - the amounts of fluid and minerals that are present. It is not necessary to know the system s extent to determine its equilibrium state, but in reaction modeling (see Chapter 13) we generally want to track the masses of solution and minerals in the system we also must know these masses to search for the system s stable phase assemblage (as described in Section 4.4). 

Table 6.6. Calculated saturation indices of various minerals in seawater and the mass of each precipitate in the stable phase assemblage...

It is tempting to place significance on the relative magnitudes of the saturation indices calculated for various minerals and then to relate these values to the amounts of minerals likely to precipitate from solution. The data in Table 6.6, however, suggest no such relationship. Thirteen minerals are supersaturated in the initial fluid, but the phase rule limits to ten the number of minerals that can form only two (dolomite and quartz) appear in the final phase assemblage. 

Brown, T. H. and B. J. Skinner, 1974, Theoretical prediction of equilibrium phase assemblages in multicomponent systems. American Journal of Science 274, 961-986. 

Linear regions (constant slope, hence constant potentials) define the composition interval over which a two-phase assemblage is stable. Because the minimum Gibbs free energy curve of the system is never convex, the chemical potential of any component will always increase with the increase of its molar proportion in the system. 

Figure 6.9 shows how the calculated phase boundaries compare with the experimental observations of Bowen and Schairer (1935) on the same binary join. The satisfactory reproduction of phase assemblages clearly indicates that the... 

Two phase assemblages of any of these minerals are known. It should be noted that aluminous phases, such as kaolinite, have never been reported with corrensite neither are sedimentary phyllosilicates such as 7 8 chlorite or glauconite. Non-phyllosilicates in association with corrensite frequently include diagenetic quartz, albite and dolomite. Pelitic rocks, specially associated with those containing corrensite, contain allevardite and fully expanding montmorillonite (dioctahedral). 

In each of the different parageneses outlined here, the instability of a mineral can be denoted by its replacement with one or usually several minerals. The rocks in these facies are typified by multi-phase assemblages which can be placed in the K-Na-Al-Si system. This is typical of systems where the major chemical components are inert and where their masses determine the phases formed. The assumptions made in the analysis up to this point have been that all phases are stable under the variation of intensive variables of the system. This means that at constant P-T the minerals are stable over the range of pH s encountered in the various environments. This is probably true for most sedimentary basins, deep-sea deposits and buried sedimentary sequences. The assemblage albite-potassium feldspar-mixed layered-illite montmorillonite and albite-mixed layered illite montmorillonite-kaolinite represent the end of zeolite facies as found in carbonates and sedimentary rocks (Bates and Strahl,... 

Figure 41. Phase diagram for the extensive variables R -R -Si combining the data for synthetic magnesian chlorites and the compositional series of natural sepiolites and palygorskites. Numbers represent the major three-phase assemblages related to sepiolite-palygorskite occurrence in sediments. Chi = chlorite M03 = trioctahedral montmorillonites M02 = dioctahedral montmorillonite Sep = sepiolite Pa = palygorskite Kaol = kaolinite T = talc.

An interesting possibility arises when this four-phase assemblage... 

We shall attempt the working hypothesis that in the intermediate system equilibrium would require the maximum number (seven) of phases. This is not entirely unreasonable since the composition of sea water seems quite stable. Hence, which are the seven phases, and what evidence exists concerning the stable-phase assemblage ... 

---