Feldspars identification

These associations are noted by Hay (1966) as being found in sequences of sedimentary rocks or altered pyroclastics buried to depths greater than 3,000 meters and generally less than 10,000 meters. However, the limits are actually vague and the identifications imprecise. The relatively frequent occurrence and persistence of albite or potassium feldspar and alkali zeolite in such rocks leads one to believe that they can coexist stably in nature. This could be, however, a misleading conclusion based upon too few observations. The elimination of the silicic, alkali zeolites and the persistence of montmorillonite is known to exist in series of deeply buried rocks (Ii-jima, 1970 Moiola, 1970 Iijima and Hay, 1968). 

Density is one of the properties that may be used to separate minerals. There are many procedures for obtaining so-called heavy mineral fractions from soils. These fractions consist of minerals with densities greater than those of the more common minerals such as quartz and feldspars. In a previous section the classical use of density gradients to compare soils was mentioned. A logical further step is the precise identification of the various minerals in the different fractions, particularly the denser fractions, since these are likely to be the most diagnostic. 

A cover glass is mounted with epoxy resin (without hardener to facilitate removal later for polishing or staining, the latter technique is described helow and is particularly helpful in alkali feldspar and dolomite identification). The epoxy eventually hardens to make a permanent cover-glass hinder. 

Optical identification of kaolinite is very difficult. Biaxial (-), a= 1.556, P = 1.563, 7=1.565, 5 = 0.007,2H= 40° It is common rock-forming secondary mineral, forming after aluminous silicates, component of soils and replaces feldspar in rocks undergoing weathering. Associated mineral is quartz. 

The plagioclase feldspar minerals break down easily into clay minerals and can therefore occur as relict minerals in artists materials which are clay based however, no specific identifications currently known. 

Published experimental studies of mineral/cal-cium hydroxide reactions show that at low temperatures (below 110°C), the chief reaction products are calcium silicate hydrate (CSH) gels, while zeolites and feldspars are formed at higher temperatures and in the presence of alkalis NaOH and KOH. The phase identifications have however often been made by low resolution or bulk methods, neither of which are ideal for such material. Published results of numerical simulations are in broad agreement with those of experimental studies of cement/ rock interaction. These models predict that CSH gels will be replaced by zeolites and maybe feldspars as plume chemistry evolves. 

The recognition of secondary intragranular porosity associated with feldspar grains is not a problem, but any identification of intergranular pores as being of secondary origin is interpretative and subjective, and is based on textural criteria established by Schmidt and McDonald (1979). Calcite cement may be dissolved to create secondary intergranular porosity that... 

An extensive chemical and microscopic study of mica separated from 21 Piedmont soils from the Piedmont of southeastern United States was made by Denison et al. [1929]. The fraction analyzed was greater in size than that passed through a 200-mesh sieve. Weathering was evident in even these sand-size particles. In all soil profiles, biotite seemed to be altered to the same extent, the K2O content usually approximated 4%. On the other hand, muscovite, as described by the authors, had a wide variance in K2O content. This muscovite in some profiles contained less than 1 % K2O, whereas in other profiles, the mineral contained as much as 9% K2O. The authors proposed that there were two kinds of muscovite present—some secondary, having formed from the products of potassium-feldspar weathering, and some primary, which muscovite inherited from the parent material. The identification of material with 1 % K2O as muscovite would be questioned today. The use of X-ray diffraction for soil clay identification occurred after this microscopic work was done. It would have been useful to examine the < 200-mesh particles to supplement the work done with the microscope on particles of a size greater than 200 mesh. 

The significance of these recent structural results in soil studies is that it is probably insufficient to characterize a specimen as, say, K-feldspar if, in subsequent experimental studies, the detailed bonding forces around the K" ions are Ukely to be significant. Conversely, in certain pedological problems, the identification of an alkali feldspar may be useful, but more detailed information (if obtainable) about its structural state, submicroscopic twinning. 

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