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  • ChapterChapterChapterChapter----3333

    Analytical Techniques, Analytical Techniques, Analytical Techniques, Analytical Techniques, Methodology and ResultsMethodology and ResultsMethodology and ResultsMethodology and Results

  • CHAPTERCHAPTERCHAPTERCHAPTER----3333 ANALYTICAL TECHNIQUES, METHODOLOGY AND RESULTS

    43

    CHAPTERCHAPTERCHAPTERCHAPTER----3333

    ANALYTICAL TECHNIQUES, METHODOLOGY AND RESULTS

    In the previous chapter, sampling methods, details of samples, field

    observation and petrography of selected samples have been described. This

    chapter describes all the process and the methods as well as instruments which

    were used for analyzing the samples. Sample preparation is very crucial part to

    get a good and an accurate data with little error. Different methods of sample

    preparation are required for different analysis. Sample preparation involves size

    reduction, phase separation, splitting and homogenization of samples etc.

    3.1. Preparation of Samples

    Geochemical analysis involves accurate and precise determination of

    major and trace elements. It can be obtained up to desirable limits by negating

    the human error as much as possible. In laboratory, due care was taken to

    deduce better interpretation while preparing the samples particularly to avoid all

    possible sorts of cross contamination. A total of eighty samples from Shillong

    plateau which belong to Kyllang Pluton, Moudoh Pluton and Basement Gneiss

    around Kyllang and Moudoh have been selected for the geochemical analysis

    based on the criteria of freshness or least altered rocks.

    Samples were collected during the field work, at least 5 kg weight samples,

    and representative of rocks type have been collected from each location. It should

    be remembered that the sample size is important consideration and is a function of

    the grain size in any rock. The coarser and more porphyritic grain, then sample

    should be larger. The collected rock specimens were broken into two parts on the

    outcrop itself (in the field) to avoid any contamination during breaking in the

    laboratory. After having done the megascopic studies, one part is kept as

    reference and the other part is used in making thin sections for petrographic

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    44

    studies and the remaining part was broken into thumb nail chips which are cleaned

    with water to remove surface contamination. The cleaned samples are air dried

    before grinding and chipping. After chipping, homogenizations of chips were done

    several times to get representative but small quantity of chips for grinding.

    Figure 3.1. Flow Chart for Geochemical Methodology

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    45

    3.1.1. Sample Cleaning and Chipping

    1. The sample are firstly cleaned by de-ionized water and dried by using air

    blower to remove all the impurities lying on the surface of the sample.

    2. The cleaned and dried sample has been taken for chipping. The sample is

    broken into small elongate or thumb nail size chips of about 2cms in

    length and about 0.5 to 1cm thick.

    3. Homogenization of the chips has been done by using coning and

    quartering process to get a representative small quantity of chips for

    grinding.

    4. The homogenized chips were put in grinding machine (jaw crusher) to

    reduce the size of the homogenized chips upto -60 mesh. Then the

    samples were put in Tema Mill for powdering to reduce the grain size upto

    -230 mesh.

    5. Homogenization, coning and quartering have done several times at every

    stage. Re-homogenization before weighing for analysis is also must.

    3.1.2. Jaw Crusher Mechanism

    The Jaw Crusher crushes chipped samples by compression without

    rubbing. Hinged overhead and on the centerline of the crushing zone, the

    swinging jaw meets the material firmly and squarely. There is no rubbing action

    to reduce capacity, to generate fines or to cause excessive wear of jaw plates.

    Crushing is done by compression between two surfaces, with the work

    being done by one or both surfaces. Jaw crushers with the compression method

    are very suitable for reducing the size of extremely hard and abrasive rock. In

    this step the chipped samples are put into a Jaw Crusher.

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    46

    Figure 3.2. A small picture showing the Jaw Crusher Mechanism.

    The Jaw crusher grinds the chips into a coarse powder. The coarse

    powder is generally of the size of -60 to -80 meshes from the first run. This step

    of grinding in Jaw Crusher is repeatedly run for a couple of times to get finer

    powder. After these runs the powder is taken out and emptied into an air tight

    poly-bag. The Jaw Crusher is cleaned inside out with organic solvents like

    acetone or alcohol and also with air blower before putting the next sample to

    avoid any sort of contamination while running the next sample.

    3.1.3. Laboratory Disc Mill Mechanism (TEMA Mill)

    The powdered sample coming from the Jaw crusher (previous step) is

    then taken for grinding in a TEMA Mill. The Laboratory Disc Mill is quick, dust

    free grinding machine which can reduce the grain size of the powder of minerals

    and rock samples to -200 meshes for analytical purposes. By means of

    predominantly horizontal vibrations, the material is ground by impact and friction,

    usually in minutes and at the same time homogenized the sample is more

    strongly homogenized. The grinding apparatus is called Disc mill which

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    47

    comprises eccentric rotation of the heavy metal disc in the sample- disc

    chamber. The sample-disc chamber assembly comprises a heavy metal disc at

    the center of the chamber encircled by a metal ring. The sample is placed in

    between the metal disc and metal ring while running the machine. During the

    operation the disc hits the wall of metal ring and the closed container thus it

    grinds the material stuffed in between the disc and the circular metal ring within

    the container.

    Figure 3.3. A picture showing the TEMA mill apparatus.

    The whole apparatus should be cleaned properly before and after running

    every different sample to get rid of cross contamination of the samples.

    3.2. Loss on Ignition (LOI)

    Loss on ignition is carried out to give a general indication of the "volatile"

    species in a sample. Loss on ignition refers to the weight loss experienced by a

    sample when it is heated in a furnace at a specific temperature and for a specific

    time period.

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    48

    The LOI is generally carried out by heating the sample in a furnace at a

    temperature range of 850 - 950C for one hour to 90mins. The weight loss

    caused by heating reflects all the volatile species that are lost at the given

    temperature and time. The volatile species include surface water moisture as

    well as water of crystallization and organic carbon species if present. In addition,

    carbonates decompose to oxides with the loss of carbon dioxide and sulphates

    decompose (but usually only partially) to oxides with the loss of sulphur trioxide.

    Fluorides may also be partially lost. As a result of the complex nature of these

    reactions, the LOI value is best used as a general indicator of the amount of

    volatile species present. It cannot be used to determine the presence of

    individual species. Some base metals will also be lost in this process.

    Procedure followed for calculating the Loss on Ignition:

    1. Start the digital weighing machine and leave it alone for about an hour, so

    that it equilibrates with the conditions and then set it to zero.

    2. Weigh the silica crucible (W1) and note down the reading on the weighing

    machine.

    3. Now add 2gm of the powdered rock sample very carefully into the crucible

    without taking it out of the weighing machine after step 2 and note down

    the reading on the weighing machine (W2). W2 should be approximately

    W1 + 2gms.

    4. Repeat the same for all the samples. And store them in desiccators to

    avoid any moisture entering the sample.

    5. Now switch on the furnace and put the crucibles in it and set it at a

    temperature of 850 for a time interval of 120mins. Remember the pattern

    (serial number) in which the samples were kept because any sort of exterior

    labelling on the crucible is going to evaporate at that temperature.

    6. After the 120mins, switch off the furnace and leave the crucibles in it for

    13 14 hours so that it reaches the room temperature.

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    7. Keep a desecrator nearby and swiftly shift the crucibles from the furnace

    to the desecrator.

    8. Now weigh the crucible (W3).

    9. Therefore the LOI (Loss on Ignition) is = W2 - W3.

    10. Then convert it into percentage.

    3.3. Sample Dissolution for Inductively Coupled Plasma-Mass Spectrometry

    (ICP-MS)

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