Micas occurrence

Micas are primary phases in several types of igneous, metamorphic, and sedimentary rocks. Table 5.54 furnishes a rough scheme of main mica occurrences. 

Hydrogen isotope data for mantle xenoliths is usually acquired on hydrous minerals such as amphibole and mica. One problem is that early studies did not texturally characterize mica occurrences and so the information is of limited value. Perhaps the best-documented study is that... 

Occurrence. Its most important minerals correspond to alumino-silicates LiAlSi206 (spodumene), LiAlSi4O10 (petalite), lithium mica (lepidolite), LiAlSi04 (as the rare eukryptite). 

Occurrence. It is the commonest metallic element in the earth s crust. Aluminium occurs in many silicates such as micas and feldspars, hydroxo oxide (bauxite), cryolite (Na3AlF6). 

Occurrence. Thallium can be associated to heavy metals that occur in sulphidic ores (chalcophilic element behaviour) or to alkali metals in minerals such as car-nallite, sylvite, mica (the Tl+1 ion behaves as an alkali metal ion), or in true, but very rare, thallium minerals such as lorandite (T1AsS2), chalcothallite (Cu3T1S2). 

Occurrence. Silicon is the second most abundant element in the earth s crust, after oxygen (about 28% by weight). It occurs mainly in oxides (quartz, agate, opal, etc.) and a great variety of silicate minerals (feldspar, clay, mica, olivine, etc.). 

Table 5.54 Occurrences of main compositional terms of mica in igneous, metamorphic and sedimentary rocks.

Silicate minerals that usually occur as spherulitic aggregates of fibers have formed as a result of the alteration of the many minerals subsumed within the category of biopyriboles. Alteration of the micas under hydrothermal conditions produces compositional variants on recrystallization such as hydrous muscovite. Some of these samples have been labeled asbestiform, probably because they are found in veins that criss-cross rock masses. Fibrous micaceous minerals also occur as discrete disseminated particles, although few detailed analyses of crystallites from the disperse occurrences have been made. Fibrous mica found in veins usually grades (composition-ally) into members of the serpentine mineral group, the clays or the chlorites. 

The occurrence of iodine in igneous rocks was first conclusively demonstrated by Armand Gautier in 1901 (144). Since it had previously been detected in volcanic emanations and lavas and in the sludges from mud volcanos, and since it is often associated with boric acid, Gautier concluded that it must come from great depths and that therefore it ought to be possible to detect it in the most ancient rocks. His results showed that iodine, which exists in all the granites we have examined, seems not to form a constituent part of either their micas or of the apatites which... 

It would appear that the different types of potassic mica-like phases can be distinguished on the basis of alkali and iron content and geologic occurrence ... 

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. 

Weaver and Beck, 1971 Dunoyer de Segonzac, 1969 van Moort, 1971 and Hower, et al.. 1976). This is especially true for alkalis. Thus the occurrence of illite or mica is not a function of bulk composition but one... 

VELDE (B.), 1965b. Phengite micas synthesis, stability and natural occurrence. Amer. Journ. Sci. 262, 886-913. 

That the atomic weight of uranium lead is extremely variable has already been shown. In order to interpret this variability its sources must be studied both geologically and mineralogically. On the geologic side of the question the uranium ore can be divided in to three principal classes, which are sharply distinct. The definitely crystallized varieties of uraninite occur in coarse pegmatites, associated with feldspar, quartz, mica, beryl, and other minor accessories. The massive pitchblende is found in metalliferous veins, together with sulphide ores of copper, lead, iron, zinc, and so forth. As for camotite, that is a secondary mineral, found commonly as an incrustation on sandstone, and often, also upon fossil wood. There may be other modes of occurrence, but these are the most distinctive. 

Occurrence.—Many silicates contain traces of lithium, usually associated with large proportions of sodium and potassium. Lithium compounds are mainly derived from lepidolite or lithia-mica, a double fluosilicate of aluminium and an alkali-metal, containing 0 8—2-7 per cent, of lithium and petalite, a double silicate of aluminium and lithium with an admixture of sodium, containing 2 per cent, of lithium. 

Dawson J. B. and Smith J. V. (1975) Occurrence of diamond in a mica-garnet Uierzolite xenolith from kimberlite. Nature 254, 580-58. 

There have been tremendous strides made recently towards understanding how volatiles in general and water in particular is transported and stored in the mantle. This progress is based on research on a number of fronts studies of mantle-derived samples have provided insight into the nature and occurrence of hydrous phases such as amphibole, mica, and chlorite, and have provided constraints on the capacity of nominally anhydrous minerals (NAMs) such as olivine, pyroxenes, and garnet to contain water by a variety of substimtion mechanisms. Experimental studies on mantle-derived magmas have provided constraints on volatile contents in their source regions. Other studies have constrained the pressure, temperature, and composition conditions over which hydrous phases are stable in the mantle. 

Some glauconites have been identified in hydrothermally altered basalts together with celadonites (Alt et al., 1992 Clayton and Pearce, 2000). This material appears to form a mixed-layer mica-ferric smectite series. The formation of glauconite mixed-layer minerals is therefore not restricted to peloids under shallow-ocean-bottom conditions. However, the identification of glauconite as distinct from ferric Ulite is difficult. Perhaps these mineral occurrences should be given another name. 

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