remote sensing

X-Ray Powder Diffraction

This handout provides background on

the use and theory of X-ray powder

diffraction. Examples of applications of

this method to geologic studies are

provided.

Introduction

Rocks, sediments, and precipitates are

examples of geologic materials that are

composed of minerals. Numerous

analytical techniques are used to

characterize these materials. One of

these methods, X-ray powder diffraction

(XRD), is an instrumental technique that

is used to identify minerals, as well as

other crystalline materials. In many

geologic investigations, XRD

complements other mineralogical

methods, including optical light

microscopy, electron microprobe

microscopy, and scanning electron

microscopy. XRD provides the

researcher with a fast and reliable tool for

routine mineral identification. XRD is

particularly useful for identifying finegrained

minerals and mixtures or

intergrowths of minerals that may not

lend themselves to analysis by other

techniques. XRD can provide additional

information beyond basic identification.

If the sample is a mixture, XRD data can

be analyzed to determine the proportion

of the different minerals present. Other

information obtained can include the

degree of crystallinity of the mineral(s)

present, possible deviations of the

minerals from their ideal compositions

(presence of element substitutions and

solid solutions), the structural state of the

minerals (which can be used to deduce

temperatures and (or) pressures of

formation), and the degree of hydration

for minerals that contain water in their

structure. Some mineralogical samples

analyzed by XRD are too fine grained to

be identified by optical light microscopy.

XRD does not, however, provide the

quantitative compositional data obtained

by the electron microprobe or me textural

and qualitative compositional data

obtained by the scanning electron

microscope.

Theory and Methodology

The three-dimensional structure of

nonamorphous materials, such as

minerals, is defined by regular, repeating

planes of atoms that form a crystal

lattice. When a focused X-ray beam

interacts with these planes of atoms, part

of the beam is transmitted, part is

absorbed by the sample, part is refracted

and scattered, and part is diffracted.

Diffraction of an X-ray beam by a

crystalline solid is analogous to

diffraction of light by droplets of water,

producing the familiar rainbow. X-rays

are diffracted by each mineral

differently, depending on what atoms

make up the crystal lattice and how these

atoms are arranged.

In X-ray powder diffractometry, X-rays

are generated within a sealed tube that is

under vacuum. A current is applied that

heats a filament within the tube, the

higher the current the greater the number

of electrons emitted from the filament.

This generation of electrons is analogous

to the production of electrons in a

television picture tube. A high voltage,

typically 15-60 kilovolts, is applied

within the tube. This high voltage

accelerates the electrons, which then hit a

target, commonly made of copper. When

these electrons hit the target, X-rays are

produced. The wavelength of these Xrays

is characteristic of that target. These

X-rays are collimated and directed onto

the sample, which has been ground to a

fine powder (typically to produce particle

sizes of less than 10 microns). A

detector detects the X-ray signal; the

signal is then processed either by a

microprocessor or electronically,

converting the signal to a count rate.

Changing the angle between the X-ray

source, the sample, and the detector at a

controlled rate between preset limits is an

X-ray scan (figs. 1 and 2).

When an X-ray beam hits a sample and

is diffracted, we can measure the

distances between the planes of the

atoms mat constitute the sample by

applying Bragg's Law. Bragg's Law is

Figure 1. Simplified sketch of one

possible configuration of the X-ray

source (X-ray tube), the X-ray detector,

and the sample during an X-ray scan. In

this configuration, the X-ray tube and the

detector both move through the angle

theta (q), and the sample remains

stationary.

n l = 2 d sin q, where the integer n is the

order of the diffracted beam, 1 is the

wavelength of the incident X-ray beam, d

is the distance between adjacent planes

of atoms (the d-spacings), and q is the

angle of incidence of the X-ray beam.

Since we know l and we can measure q,

we can calculate the d-spacings. The

geometry of an XRD unit is designed to

accommodate this measurement (fig. 1).

The characteristic set of d-spacings

generated in a typical X-ray scan

provides a unique "fingerprint" of the

mineral or minerals present in the

sample. When properly interpreted, by

comparison with standard reference

patterns and measurements, this

"fingerprint" allows for identification of

the material.

Applications

XRD has a wide range of applications

in geology, material science,

environmental science, chemistry,

forensic science, and the pharmaceutical

industry, among others. At the U.S.

Geological Survey, researchers use XRD

to characterize geologic materials from a

wide variety of settings; a few examples

follow.

Mineral-Environmental Studies

In studies of areas affected by acid

mine drainage, the identification of

secondary minerals and fine-grained

precipitates is a critical element. Acid is

generated when iron sulfide minerals,

such as pyrite, weather. Elements

derived from the alteration of the sulfide

minerals can form secondary minerals or

go into solution. Elements that go into

solution may form mineral precipitates as

conditions (temperature, acidity, solution

composition) change. Accurate

mineralogical characterization of the

precipitates and secondary minerals,

together with hydrogeochemical data,

helps us to better understand the

solubility, transport, and storage of

metals.

Ore Genesis Studies

Minerals form under specific ranges of

temperature and pressure. Mineralogical

identification of ore minerals and

associated minerals, including finegrained

hydrothermal alteration minerals,

provides evidence used to deduce the

conditions under which ore deposits

formed and the conditions under which,

in many cases, they were subsequently

altered.

Predictive Stratigraphic

Analysis

Mineralogical characteristics of

paleosols (ancient buried soil horizons)

and underclays (the fine-grained detrital

material lying immediately beneath a

coal bed) have been instrumental in

correlating coal zones from the

Appalachian basin into the Western

Interior basin. They have been the key to

quantifying the paleolatitudinal climate

gradient in North America during the late

Middle Pennsylvanian.

Remote-Sensing Studies

Mineralogical analysis by XRD is used

in conjunction with remotely sensed data

in several research investigations. XRD

is used to identify the minerals

composing clay-rich, hydrothermally

altered rocks that occur on several

Cascade volcanoes. Such rocks are

believed to play an important role in the

generation of large landslides and

mudflows. XRD is used to analyze

saline minerals, including borates.

Many saline hydrate minerals produce

diagnostic spectral bands, and spectral

data provide a basis for mineral

exploration using remote-sensing data.

Figure 2. Example of an X-ray powder diffractogram produced during an X-ray scan.

The peaks represent positions where the X-ray beam has been diffracted by the

crystal lattice. The set of d-spacings (the distance between adjacent planes of atoms),

which represent the unique “fingerprint” of the mineral, can easily be calculated from

the 2-theta (2q) values shown. The use of degrees 2-theta in depicting X-ray powder

diffraction scans is a matter of convention and can easily be related back to the

geometry of the instrument, shown in figure 1. The angle and the d-spacings are

related by Bragg’s Law, as described in the text.

Analysis of airborne imaging

spectrometer data can directly map

mineral occurrences by detecting

diagnostic spectral absorption bands, the

shape and position of which are

determined by individual mineral

structures. A detailed knowledge of

sample mineralogy, provided at least in

part by XRD, is required to understand

the observed spectral absorption features.

Genesis of Coal Beds

XRD is one of the primary tools used

to evaluate the lateral and vertical

variations in mineral matter and major,

minor, and trace elements in coal beds.

These data are used to help determine the

impact of geologic and geochemical

processes on coal bed formation in order

to understand and predict both inorganic

and organic variability within and among

mineable coal beds.

Mineral-Resource

Assessments

Mineralogical characterization

provides part of the data required to

determine the particular kind of mineral

deposits encountered in mineral-resource

assessment studies. XRD allows us to

identify fine-grained mixtures of

minerals found in associated gangue and

alteration assemblages, which cannot be

resolved by other methods.

-Prepared by Marta J.K. Flohr

For more information, please contact:

Frank T. Dulong

U.S. Geological Survey, MS 956

Reston, VA 20192

Telephone: (703) 648-6416

E-mail: fdulong@usgs.gov

John C. Jackson

U.S. Geological Survey, MS 954

Reston, VA 20192

Telephone: (703) 648-6321

E-mail: jjackson@usgs.gov

U.S. Department of the Interior

U.S. Geological Survey May 1997