Identifying Gypsum and Anhydrite in soils

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  Ahmed Mufty / 3 months
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X ray Diffraction: One of the best methods to identify the existence of Gypsum or Anhydrite in soils and rocks is to conduct an X-ray diffraction analysis on samples from the material due to its rigorous theoretical basis.  Every mineral has a special record of the interplanar spacing in every angle of the major axes. These distances between the crystal planes are called Basal Spacing and usually denoted as d, and they work as fingerprints of the mineral allowing fair distinction of the mineral from others. During the test on a very thin specimen between two transparent plates, X ray is generated by emission from a special element as Cobalt or Copper, and it is directed to the test specimen which is rotating slowly. With each time the reflected X-ray wave lengths from different atomic layers match the intensity of the received rays jump and recorded at that angle. The peaks of each match are plotted along with corresponding angle of rotation θ, and the mineral is identified by those peaks using Bragg’s law to calculate the Basal spacing. 2d sin θ = nλ Where n is the number of the wave lengths between two rays reflected from two successive atomic planes within the crystal, 1,2, 3, etc. But these numbers have no significance as all the integers are just repetitions, so n is taken as unity when calculation d. The parameter is the wavelength of the X ray generated in the test based on the element used and here are some wave lengths in Angstroms for different elements (Copper is the most widely used): Molybdenum 0.7107 Iron 1.9373 Copper 1.5418 Chromium 2.2909 Cobalt 1.7902     It is worth mentioning that the plotter has a speed of plotting mm/min equal to 2 per minute, i.e., double the angle of rotation. It is one of the successful methods to identify the minerals including Gypsum or Anhydrite by comparing the peaks obtained in the plot of intensity against angle of rotation with the standard tables of wave lengths or angles that peaks of different minerals occur. The table below presents the expected peaks of Gypsum and Anhydrite that should occur in X-ray diffraction test if they exist within the tested specimen. A typical resulted plot from X ray diffraction analysis for a highly Gypseous soil (from Al-Tharthar area in Iraq where Gypsum content may exceed 75% by weight) is shown below. Properties of the peaks of Gypsum and Anhydrite that take place in results of X-ray diffraction tests. a. Gypsum d (Å) 7.56 3.06 4.27 2.68 2.87 I/I1 100 55 50 28 25 2θ, copper 11.7 29.19 20.8 33.44 31.2 2θ, cobalt 13.6 34.0 24.2 39.0 36.34 2θ, chromium 17.43 43.97 31.12 50.60 47.0 b. Anhydrite d (Å) 3.5 2.85 2.33 2.08 1.87 I/I1 100 30 20 20 20 2θ, copper 25.4 31.4 38.6 43.5 48.7 2θ, cobalt 29.63 36.61 45.18 50.98 57.19 2θ, chromium 38.20 47.40 58.90 66.80 75.55 X-ray diffraction test results of a highly gypseous soil