X-Ray Diffractogram and Identification of Minerals

ACSS-510

Deptt. Agril. Chemistry and Soil ScienceBidhan Chandra Krishi Viswavidyalaya,

E-mail: pabitramani@gmail.com, Website: www.bckv.edu.in : 91-33-25822132, 91-9477465968,

Dr. Pabitra Kumar ManiAssoc. Professor

X-Rays Wavelengths used for XRD

Schematic of an atom, depicting electron shells and the energy transitions for Kα, Kβ, and Lα characteristic radiation. Kα arises from the replacement of K-shell electrons by electrons from the L shell; Kβ, by replacement of K-shell electrons by M-shell electrons, and Lα, by replacement of L-shell electrons by M-shell electrons.

(B) Generalized depiction of an X-ray spectrum, showing peaks in intensity at wavelengths (energy levels) corresponding to characteristic radiation. The highest-energy (shortest wavelength) characteristic radiation shown is Kβ. Peaks marked Kα1and Kα2, which are seldom resolved in XRD data, arise from contribution of electrons from two sublevels in the L shell.

• Crystalline substances (e.g. minerals) consist of parallel rows of atoms separated by a ‘unique’ distance

• Diffraction occurs when radiation enters a crystalline substance and is scattered

• Direction and intensity of diffraction depends on orientation of crystal lattice with radiation

10

X-ray diffraction

• The distance of atomic planes d can be determined based on the Bragg’s equation.

BC+CD = nλ , nλ = 2d·sinθ , d = nλ /2 sinθ

where n is an integer and λ is the wavelength.

• Different clays minerals have various basal spacing (atomic planes). For example, the basing spacing of kaolinite is 7.2 Å.

Mitchell, 1993

Bragg’s law: The two rays will constructively interfere if the extra distance ray I travels is a whole number of wavelengths farther then what ray II travels.

X-ray diffraction is now a common technique for the study of crystal structures and atomic spacing.

X-ray diffraction is based on constructive interference of monochromatic X-rays and a crystalline sample.

These X-rays are generated by a cathode ray tube, filtered to produce monochromatic radiation, collimated to concentrate, and directed toward the sample. The interaction of the incident rays with the sample produces constructive interference (and a diffracted ray) when conditions satisfy Bragg's Law (nλ=2d sin θ).

Bragg’s law can easily be derived by considering the conditions necessary to make the phases of the beams coincide when the incident angle = reflecting angle .

The second incident beam b continues to the next layer where it is scattered by atom C

The second beam must travel the extra distance BC+CD if the two beams a & b are to continue travelling adjacent and parallel.

This extra distance must be an integral (n) multiple of the wavelength for the phases of the two beams to be the same.

• The space between diffracting planes of atoms determines peak positions.

• The peak intensity is determined by what atoms are in the diffracting plane.

Schematic representation of XRD by regularly spaced planes of atoms in a crystal. Theta (θ) is the angle that the beam makes with the atomic planes; 2θ is the angle that the diffracted beam deviates from the primary beam; d is the distance between equivalent atomic planes in the crystal (d-spacing); and λ is wavelength of the radiation. Note that when DE + EF = nλ, where n is an integer, the diffracted beams from each plane of atoms would be in phase, leading to constructive interference which accounts for XRD. In effect, when that condition is met, an XRD peak is observed. The Bragg equation can be used to calculate d-spacing from the 2θ angle at which the diffraction peak occurs.

background radiation

strong intensity = prominent crystal plane

nλ = 2dsinθ

(1)(1.54) = 2dsin(15.5 degrees)

1.54 = 2d(0.267)

d = 2.88 angstroms

d-spacing Intensity

2.88 100

2.18 46

1.81 31

1.94 25

2.10 20

1.75 15

2.33 10

2.01 10

1.66 5

1.71 5

• Production of X-Rays (X-ray Tube: the source of X Rays)

• The goniometer: the platform that holds and moves the sample, optics, detector, and/or tube

• Collimator• Monochromator

Filter Crystal monochromator

• Detector (count the number of X Rays scattered by the sample)

Photographic methods Counter methods

Instrumentation

25

Identification of Clay Minerals

Fig. 4–6. Sequences of X-ray diffraction patterns for soil clays (2.0–0.2 μm) scanned after specified cation-saturation (Mg and K), glycerol (Gly) solvation, and heat treatments.

Peaks are labeled by d-spacings (Å). (A)The clay from the Orangeburg series (Ap horizon, Georgia) shows a 14-Å peak that is minimally affected by cation saturation and shifts and broadens with heat treatment to a peak at 12 Å. This behavior typifies hydroxy-interlayered vermiculite as it occurs in the coastal plain of the southeastern USA. There is a small peak that persists at 14 Å at 500°C, which is due to a small amount of chlorite. Also present are kaolinite (7.18 and 3.57 Å), gibbsite (4.85 Å), and quartz (4.26 and 3.34 Å). Note that peaks for gibbsite and kaolinite disappear at 300 and 500°C, respectively, due to dehydroxylation.

(B) Clay from the Sharkey soil (Ap, Louisiana) shows peaks at 18 and 14 Å that have mainly shifted to 10 Å at 300°C. Note the increase in intensity of the 10-, 5- (second-order), and 3.34-Å (third-order) peaks with heat. This behavior suggests that both smectite and vermiculite are present.

A small peak (not labeled) intermediate between 14 and 10 Å may be attributable to some resistance to collapse of these expansible phyllosilicates. The small 14-Å peak at 500°C indicates that some of the 14-Å peak is attributable to chlorite.

Also present are mica (illite) (10, 5, and 3.34 Å), kaolinite (7.18 and 3.57 Å), and quartz (4.26 and 3.34 Å).

Note that at 25°C the 10-Å peak is completely attributable to mica, but is enhanced by the collapse of expansible phyllosilicates with increasing temperature.

Also, quartz, mica, smectite, and vermiculite all contribute to the 3.34-Å peak at 500°C, whereas only mica and quartz contribute to it at 25°C.