Mineral, accessory weathering

The Hostrock and Backfill Material. Most crystalline igneous rocks, including granite and gneiss, are composed of a comparatively small number of rock forming silicate minerals like quartz, feldspars (albite, microcline, anorthite etc.) micas (biotite, muscovite) and sometimes pyroxenes, amphiboles, olivine and others. Besides, there is a rather limited number of common accessory minerals like magnetite, hematite, pyrite, fluorite, apatite, cal cite and others. Moreover, the weathering and alteration products (clay minerals etc.) from these major constituents of the rock would be present, especially on water exposed surfaces in cracks and fissures. 

In nature this common set is typically further restricted over wide geographic areas because of the influence or otherwise of soil-forming factors, the most important of which are parent material and degree of weathering. Thus, a typical sample of soil will contain a suite of around six to ten different major minerals. A major mineral may be defined as one that is present at a concentration of a few percent or more, at which it will be readily detectable by routine techniques such as x-ray provider diffraction (XRPD). It is also known as energy-dispersive x-ray analysis (EDXA) or energy-dispersive analysis of x-ray (EDAX) or microscopic examination, either optical or electron. It is also not uncommon for several other minerals to be present in any given soil but usually in amounts that put them below the routine detection limits of many techniques. Nonetheless, these accessory, or trace, minerals can often be concentrated by some means that separates the soil sample into different physical or chemical fractions. Such procedures effectively lower... 

Burbine et al. (2002) tested an extreme case of a possible composition for the surface of Mercury. They made spectral observations of enstatite achondrites (igneous meteorites composed almost entirely of pure MgSi03, with some accessory minerals and essentially no FeO). The spectral features of enstatite achondrites (aubrites see Chapter 1.05) are similar to those for Mercury, but lack the spectral reddening observed in spectra of Mercury and have an additional feature at 0.5 p.m caused by troilite (FeS). This reddening (visible to UV ratio) is the result of space weathering, in which FeO is reduced to very small grains of metallic iron. Thus, the reddening indicates that some FeO must be present on Mercury to produce the nanophase iron. Alternatively,... 

Most toxicological studies focus on the toxicity or solubility behavior of a particular sample of a given mineral such as chrysotile, tremolite, or talc. They do not tend to examine the role that variations in morphology, trace element content, accessory minerals, and other characteristics between the same mineral from different samples in the same geologic locality, and between samples from different geologic localities, can play in in vitro and in vivo biodurability, and therefore toxicity. These parameters have been demonstrated to play important roles in the rate at which other minerals, such as sulfides, weather under environmental conditions (see summary in Plumlee, 1999), and so are also likely to be important for particle durability in vivo. 

Fig. 23. Contact between two different granites. The lower part of the picture represents the species with larger grain size and higher concentration of accessory minerals. Because of these factors it weathers more rapidly through the attack of plants.

Nature concentrates lanthanide minerals into commercially useful ores mainly by three processes. Minerals such as monazite are dilute accessories in common rocks, but are fairly resistant to weathering and have high densities. As their host rocks weather away, they remain intact and are moved by water and high-graded into placer deposits as less dense minerals are swept away from them. 

Zircon, ZrSi04, is a common but sparse accessory mineral in alkaline igneous rocks and some pegmatites. It is fairly resistant to weathering and metamorphic processes and can be concentrated in placer or beach sand deposits. Zircons accept a spectrum of lanthanide distributions as is evident from the two shown in fig. 21.32. 

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