Mixed-mineral assemblages

Figure 10.7 Reflectance spectra of mixed-mineral assemblages (modified from Singer, 1981). Left orthopyroxene (En86Fs14) - clinopyroxene (Wo41En51Fs7) mixtures right orthopyroxene - olivine (Fo85Fs15) mixtures. Mineral proportions are expressed as wt per cent. Note how pyroxene dominates the mineral-mixture spectra. Olivine causes broadening of the pyroxene 1 pm band but another olivine feature persists near 1.25 pm.

Fenton, B.R., Waite, T.D., 1996. A kinetic study of cation release from a mixed mineral assemblage implications for determination of uranium uptake. Radiochim. Acta 74, 251-256. 

One should notice the possibility of producing single-phase illite materials by the same type of process. If, for reasons unknown at the moment, the path of chemical change leads to aluminous illite instead of iron glauconite, i.e., parallel to the K axis with low initial iron content, one could produce single phase illite or mixed layered mineral assemblage. These are apparently rare, but such an explanation could be used to explain the illite and mixed layered mono-mineral layers of "metabentonite" deposits which cannot be explained as recrystallization of an eruptive rock. Mono-mineral layers in carbonate rock the so called... 

It is instructive to compare the natural mineral assemblages with those in the simplified system which has been studied experimentally. The system muscovite-pyrophyllite is useful because it contains phases analagous to natural minerals. The system is dominated by two factors (1) the stability of the expandable phase (potassic beidellite) either alone or as a mixed layered phase (2) the stability of kaolinite in the presence of quartz. It appears from the data presented by Velde (1969) that the... 

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.

Pelitic rocks investigated in the same areas where corrensites are formed during alpine metamorphism (Kiibler, 1970) revealed the absence of both montmorillonite and kaolinite but the illite or mica fraction was well crystallized as evidenced by measurement of the "sharpness" of the (001) mica reflection (Kiibler, 1968). This observation places the upper thermal stability of the expandable and mixed layered trioctahedral mineral assemblages at least 50°C. above their dioctahedral correlevants. This is valid for rocks of decidedly basic compositions where no dioctahedral clay minerals are present. 

Because the vent fluid compositions are determined largely by fluid-rock interactions that take place in the source region, the equilibrium mineral assemblage that is precipitated during mixing will depend to a large extent on the chemistry of the source rocks. Fluids that are buffered to lower values produce a... 

Fig. 8.3. a Correlation of the temperature-dependant clay mineral assemblages in shales and sandstones, Saharan basins b Distribution of mont-morillonites, illites and mixed layers (I/M) within the compositional triangle pyrophyllite -muscovite - celadonite (glauconite)... 

It will be evident that considerable difficulties may arise in the positive identification of clay vermiculites, particularly in distinguishing them from montmorillonites and clay chlorites. These difficulties will be enhanced when clay vermiculite is but one component of a mixed-layer assemblage, and many examples of such minerals in sedimentary and soil clays have been now recorded (cf Chapter 8). As we have seen in the preceding section, laterally mixed structures also add to the complexities of the situation and make it essential that the detailed expansion-contraction characteristics of the lattice be recorded if maximum information is to be conveyed. 

Dealing with spectral responses from samples that contain mixed assemblages is a significant challenge. If one of the alteration minerals is highly reflective, its... 

In our initial use of VIRS methods, we have chosen a diverse set of projects. These include epithermal-style precious metal mineralization (eastern and central Newfoundland), mesothermal gold mineralization (Central Newfoundland), syngenetic VMS mineralization and related alteration (Central Newfoundland), porphyry-style Mo-Cu mineralization (southern Newfoundland) and uranium mineralization in the Central Mineral Belt of Labrador. Space does not permit detailed discussion of results, but in all cases we were able to recognize distinctive alteration species and document their distribution. In some cases, results provided surprises, eg., the recognition of topaz in quartz-alunite alteration and possible Li-rich micas in VMS alteration influenced by magmatic fluids. In some cases, the resolution of species in mixed assemblages proved to be difficult, but the overall spectral patterns could still be used to discriminate alteration facies and demonstrate their superposition. 

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