Expandable mineral, fully

In zones of hydrothermal alteration it is apparent that the formation of dioctahedral montmorillonites is limited by temperature. They almost never occur in the innermost zone of alteration, typically that of sericitization (hydro-mica or illite), but are the most frequent phase in the argillic-prophylitic zones which succeed one another outward from the zone where the hydrothermal fluid is introduced in the rock. Typically, the fully expandable mineral is preceded by a mixed layered phase (Schoen and White, 1965 Lowell and Guilbert, 1970 Fournier, 1965 Tomita, et al., 1969 Sudo, 1963 Meyer and Hemley, 1959 Bundy and Murray, 1959 Bonorino, 1959). However, temperature is possibly not the only control of expandable clay mineral occurrence, the composition of the solutions and the rock upon which they act might also be important. It is possible that high magnesium concentrations could form chlorite, for example, instead of expandable minerals. 

Experimental work in the systems K-Mg-Si-Al-Fe- O concerning celadonites has also produced expandable minerals (Velde, 1972 Velde, unpublished). In both the muscovite-MgAl celadonite and MgFe-MgAl celadonite compositional series, fully expandable phases were produced below 300°C at 2Kb pressure. These expandable phases can coexist with a potassic feldspar (Figure 23). Their (060) reflection near 1.50 X indicates a dioctahedral structure which can apparently be intimately... 

As we have seen in the previous section, the bulk chemical compositions of montmorillonites taken from the literature are dispersed over the field of fully expandable, mixed layered and even extreme illite compositions. Just what the limits of true montmorillonite composition are cannot be established at present. We can, nevertheless, as a basis for discussion, assume that the ideal composition of beidellite with 0.25 charge per 10 oxygens and of montmorillonite with the same structural charge do exist in nature and that they form the end-members of montmorillonite solid solutions. Using this assumption one can suppose either solid solution between these two points or intimate mixtures of these two theoretical end-member fully expandable minerals. In either case the observable phase relations will be similar, since it is very difficult if not impossible to distinguish between the two species by physical or chemical methods should they be mixed together. As the bulk chemistry of the expandable phases suggests a mixture of two phases, we will use this hypothesis and it will be assumed here that the two montmorillonite... 

The two series of phase relations deduced above result in, at a first approximation, two "facies" for the expandable dioctahedral minerals— that of low temperature where fully expandable minerals exist and where the tie-line or association beidellite-montmorillonite persists. More elevated conditions produce a kaolinite-illite tie-line characteristic of sequences of buried rocks. 

We have seen that experimental data suggest high temperatures for dioctahedral montmorillonite stabilities, especially the Na-Ca types with beidellitic substitution. Yet in most studies of montmorillonite stabilities under natural conditions, the fully expandable phase is lost rather early, near 100°C. This phase appears to be succeeded by an interlayered expandable-non-expandable mineral. Apparently two sets of information do not agree. 

Because the compositions are basic, the expanding minerals are trioctahedral and they are apparently associated in all facies with chlorite. The occurrence of a regularly interstratified montmorillonite (saponite) -chlorite mineral, corrensite, is typified by an association with calcic zeolites and albite. Temperature measurement in the "hydrothermal" sequences at several hundred meters depth indicate that the ordered, mixed layered mineral succeeds a fully expandable phase between 150-200 C and this ordered phase remains present to about 280°C. In this interval calcium zeolites disappear, being apparently replaced by prehnite. The higher temperature assemblage above corrensite stability typically contains chlorite and epidote. 

If we consider three components, the phases will be arranged as in Figure 48a at conditions of initial burial. The solid solution series are somewhat abbreviated for simplicity. The phase relations are dominated by fully expanding and mixed layered minerals which cover a large portion of the compositional surface. Notably two dioctahedral expandable minerals exist as does a large undefined series of trioctahedral phases designated as expanding chlorite, vermiculite and trioctahedral montmorillonite. 

Vermiculites have a 2 1 layer structure similar to smectites, but expand less freely in water, presumably because of the higher layer charge in the former minerals. Most of this structural charge resides in the tetrahedral layers of the vermiculite platelets. Even when fully wetted, vermiculites do not expand beyond the two water-layer stage ( " 1.5 nm c-spacing). 

The mineral types familiar in sediments and sedimentary rocks are present micas, mica-like phases, fully expandable phases and mixed layered series. In a sense, celadonite mica is isolated from dioctahedral mica by a multiphase zone where montmorillonite is stable with a feldspar and mica. It is evident that the only way to. produce celadonite mica under high potassium concentrations is by having a proper bulk composition toward that of celadonite. The possibility of producing celadonite in a potassium deficient system, i.e., where montmorillonite coexists with a non-alkali bearing phase, has not yet been studied experimentally. 

It would appear that their frequence decreases in older rocks, especially the Paleozoic (Weaver, 1959). The assembled studies of Perry and Hower (1970), Dunoyer de Segonzac (1969), Muffler and White (1969), Browne and Ellis (1970), Weaver (1959), Weaver and Beck (1971), Burst (1959), van Moort (1971) and Iijima (1970) demonstrate that the conversion of montmorillonite to other minerals in sequences of deeply buried sedimentary rocks is independent of time or geologic age and appears to be a function of the geothermal gradient which the rocks have experienced. These studies indicate that fully expandable dioctahedral montmorillonite is not stable above 100°C at depths of two kilometers or more. The occurrence of these minerals in sedimentary rocks can be considered to be controlled by their orogenic history. 

Figure 28. Depth-temperature plot of natural mineral assemblages for the fully expandable phases (Mo), random and ordered 30-80% mixed layered (ML) and superstructured, ordered 30-20% mixed layered (All) minerals. Data from Steiner (1968), S Muffler and White (1969), M Perry and Hower (1970, 1972), P Iijima (1970), I Browne and Ellis (1970), B Dunoyer de Segon-zac (1969), D and Weaver and Beck (1971), W. I-C illite, chlorite paragenesis. Tertiary or younger sediments are represented in these studies.

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). 

High pressure studies using natural sepiolite and palygorskite (Frank-Kameneckiji and Klockova, 1969) indicate that these minerals can contain variable quantities of silica because they exsolve quartz while retaining their basic structural and mineral identity. These experiments also demonstrate that the natural minerals are compositionally intermediate between talc or montmorillonite and quartz. These latter phases are formed upon the thermal breakdown of sepiolite and palygorskite under conditions of 1 and 2Kb total pressure. Both sepiolite and palygorskite appear to remain stable in sequences of buried rocks, at least up to the depth where fully expandable dioctahedral montmorillonite disappears (Millot, 1964). 

Sulphur-gas geochemistry is not, ideally, a "stand alone" technique. It should be used in conjunction with other geochemical, geophysical and geologic studies to understand the processes that cause and influence sulphur-gas anomalies around and over mineral deposits. When the processes become more fully understood, predictive models may be developed for the occurrence of volatile sulphur compounds over different types of mineralisation and over buried mineralisation. Future work will then expand the use of sulphur-gas anomalies from studies of specific mineral deposits to regional and reconnaissance studies. 

Most other metals present in pharmaceuticals are present in sufficient concentrations that high sensitivity is not imperative and they may therefore be determined by flame atomic absorption spectroscopy. These products are extremely variable in composition but nonetheless yield easily to this type of analysis, which is generally unaffected by compounding agents such as binders or expanders. Thus, the elements Na, K, Mg, Ca, Mn, Fe, Co, Cu, Zn, and Mo are among those determinable by flame (51-53) and, recently, furnace (54) atomic absorption in multivitamin-mineral tablets. Chemical interactions between some metals dictate the use of an internal standard when several elements are present simultaneously. It should be noted here that a spark emission or ICP spectrometer equipped with an appropriate polychromator would have the advantage of simultaneous and therefore more rapid analysis in these multielemental products. These techniques have probably not been fully utilized in this regard. 

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