The rock samples that are collected can be either analyzed petrographically or geochemically. The samples are crushed and grinded according to the type of analysis to be conducted.
For the geochronological studies and study of major and trace elements the samples are powdered and for petrological studies thin sections are prepared. Methods followed are; Hydraulic crusher- in this the bigger fraction of samples are crushed into smaller fragments (~4-5cm). Some pieces of samples are kept for preparing thin section and other are further processed. Jaw crusher- the crushed samples are further crushed for smaller fragment of desirable size (1cm). It consists of a vibrating feeder, movable and immovable jaw which are made up of chrome steel. Tema-Mill- the crushed samples are powdered to desirable size.
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For XRF analysis the powder should be of ?300 mesh and for geochronological studies it should be of ?500 mesh. It consists of concentric rings made up of tungsten carbide. The rings collide among them to powder the samples. Tema-mill is time consuming and percentage of contamination is very less. Before adding samples it should be cleaned with acetone to avoid contamination. Sample preparation for XRF analysis-XRF is used for determination of major and trace element chemistry of rock samples.Pellet preparation: Pressed pellets are prepared using 40 mm aluminum cups filled with Boric acid crystals as binder.
Finely powdered sample (-300 mesh) is sprinkled over boric acid and pressed in a 40 ton hydraulic press to produce a circular 40 mm disk. The pressed powder pellets allows trace element determinations, with limits of detection up to 1 ppm for selected elements. The elements determined presently are K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Ba, Ce, Pb, Th.
Light elements like Si, Al, Mg and Na are less precise by this method. All major and trace elements are determined on sample pellets for which fused glass disk cannot be made (for e.g. river and marine sediments, soils etc.).Fused glass disk is prepared on a Claisse Fluxy instrument. One gram of finely powdered sample is mixed with 5 gram of flux (LiT/LiM/LiBR 49.
75/49.75/0.50, Pure) and fused in a platinum crucible. The fluxer is microprocessor controlled and uses LPG and in controlled manner raises the temperature up to 1000°C. It has built in programmes for different sample types. Rotating and stirring of crucible for uniform mixing and pouring the fused liquid on to a platinum mold to produce 30 mm glass disk is done automatically. It is also possible to create need based programmes with different gas flow, crucible tilt, mixing speed and length of each step.
Fused disks are excellent for analyzing major elements as it reduces matrix effects; eliminates particle size effects; and provides a homogeneous specimen. Samples which contain higher than normal concentrations of elements like lead, tin, arsenic and/or antimony (as they can seriously damage the platinum ware) and samples which contain organic matter are not suitable for this method of sample preparation. Elements determined by this method are Na, Mg, Al, Si, P, K, Ca, Ti, Mn, and Fe.For Geochronology Sieve analysis- sieving in done by water gravity process. In case of rock samples the powder of ~500mesh is sieved and for sediments Wilfley gravity separation: it is a shaking table in which gravity separation is done on the basis of densities of mineral. It separates high specific gravity minerals from low specific gravity mineral.
The table consist of a rectangular shape deck with riffles, a feeder through which samples are added. The table shakes in forward and backward direction. The slope of the table is adjusted in such a way that the heavy minerals will concentrate at one end due to the shaking movement. A thin film of water is allowed to flow through table, so that the lighter minerals can move and the heavy minerals can concentrate. The samples are collected in three forms, the light fraction, the mixed and the heavy fraction. The samples are then dried in the oven.
Frantz Magnetic separation-Magnetic separation depends on the magnetic susceptibility of the minerals to be separated. It simply follows the principle of electromagnetism i.e.; when current is passed through the coil, magnetic field is created which in turn increases the strength of the magnet in the coil. As a result of which the magnetic and nonmagnetic sediments are separated which is collected in two different boxes. The separator can be rotated in both directions of slope i.
e. forward slope and sideward slope. The separation depends on (1) the tilt of the chute, (2) the amperage applied to electromagnet and, (3) the slope and rate of feed to the chute. At the lower end of the chute the particles are separated into two boxes, one consisting of grains of higher susceptibility (Magnetic) and the other consisting of grains of lower susceptibility (non-magnetic).
The dried samples are taken for separation. Hand magnet is used to separate strongly magnetic minerals like magnetite and pyrrhotite. Heavy liquid separation Bromoform separation- Bromoform (CHBr?) with specific gravity=2.89 g/cc. Bromoform liquid is mostly used to separate the heavy minerals having specific gravity >2.
89g/cc. Di-iodomethane separation- Diiodomethane (CH?I?) with specific gravity=3.32 g/cc. the minerals having specific gravity more than 3.32g/cc are separated.The separation of heavy mineral using heavy liquid is on the basis of their different densities. The non-magnetic minerals collected from the magnetic separator are used for separation.
The separation in carried out inside a lamellar fume hood. The samples are separated in using a separating funnel, the samples are added into the funnel and heavy liquid was poured to it. The mixture was properly stirred using glass rod and was kept undisturbed for sometimes, so that the heavy minerals get settled properly. The heavy minerals get settled at the bottom and the light minerals use to float at the top.
The heavy minerals are collected and kept for drying.
