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Click Here For Best Selection Of High Quality Polarizing Microscope

Click Here For Best Selection Of High Quality Polarizing Microscope

Orthoclase feldspars supply us with a good example of mineral, which is liable to decomposition on exposure to atmospheric conditions. If the rock from which the section has been cut is ‘fresh,’ this mineral is quite colorless and transparent. But when decomposed, it has lost its transparency, and has become a mere aggregate of countless grayish semi-opaque particles of the hydrated mineral, kaolin. Some fragments are in an intermediate condition – altered merely in parts. The slides containing the minerals that show different degrees of alteration should be examined first. The case of the orthoclase is especially important because the tendency to decomposition is often useful in distinguishing this mineral from quartz, which it may strongly resemble.

 

            Olivine, also, is unstable under atmospheric conditions, and is frequently seen more or less altered to the green mineral serpentine. The mode of alteration is very characteristic. Olivine, when fresh, is a colorless mineral, usually traversed by irregular cracks that are usually visible when the mineral is evaluated with the aid of the geological polarizing microscope, and it is along these that the alteration product first develops. Later stages show that these ‘channels’ of serpentine have become wider, the ‘islands’ of fresh olivine have become smaller, and the last stage shows the mass of serpentine having the form of the olivine from which it was derived. The serpentine is commonly associated with granules of black iron oxide, another product of the decomposition of the original olivine.

 

            Biotite is another unstable mineral, the product in this case being the green mineral chlorite. Excellent examples showing different stages of alteration are obtainable in different sections, ranging from the fresh brown biotite to the pseudomorph in chlorite.



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Wednesday, February 13th, 2008 at 3:28 am
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Minerals Characteristics in Ordinary Transmitted Light
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