ASTM E3294-23

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E3294 − 23 An American National Standard
Standard Guide for
Forensic Analysis of Geological Materials by Powder X-Ray
Diffraction
This standard is issued under the fixed designation E3294; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2. Referenced Documents
1.1 This guide covers techniques and procedures for the use 2.1 ASTM Standards:
of powder X-ray diffraction (XRD) in the forensic analysis of C1365 Test Method for Determination of the Proportion of
geological materials (to include soils, rocks, sediments, and Phases in Portland Cement and Portland-Cement Clinker
materials derived from them such as concrete), to enable Using X-Ray Powder Diffraction Analysis
non-consumptive identification of solid crystalline materials D934 Practices for Identification of Crystalline Compounds
present as single components or multi-component mixtures. in Water-Formed Deposits By X-Ray Diffraction
E620 Practice for Reporting Opinions of Scientific or Tech-
1.2 This guide makes recommendations for the preparation
nical Experts
of geological materials for powder XRD analysis with adapta-
E1492 Practice for Receiving, Documenting, Storing, and
tions for samples of limited quantity, instrumental configura-
Retrieving Evidence in a Forensic Science Laboratory
tion to generate high-quality XRD data, identification of
E2917 Practice for Forensic Science Practitioner Training,
crystalline materials by comparison to published diffraction
Continuing Education, and Professional Development
data, and forensic comparison of XRD patterns from two or
Programs
more samples of geological materials to support criminal
E3272 Guide for Collection of Soils and Other Geological
investigations.
Evidence for Criminal Forensic Applications
1.3 Units—The values stated in SI units are to be regarded
2.2 ISO Standard:
as standard. Other units are avoided, in general, but there is a
ISO/IEC 17025:2017 General Requirements for the Compe-
long-standing tradition of expressing X-ray wavelengths and
tence of Testing and Calibration Laboratories
lattice spacing in units of Ångströms (Å). One Ångström =
–10
10 meter (m) = 0.1 nanometer (nm).
3. Terminology
1.4 This standard is intended for use by competent forensic
3.1 Definitions of Terms Specific to This Standard:
science practitioners with the requisite formal education,
3.1.1 Bragg equation or Bragg’s law, n—describes the
discipline-specific training (see Practice E2917), and demon-
physical phenomenon of X-ray scattering from a crystallo-
strated proficiency to perform forensic casework.
graphic three-dimensional lattice plane as nλ=2dsinθ, in which
1.5 This standard does not purport to address all of the
n is any integer, λ is the wavelength of the X-ray, d is the
safety concerns, if any, associated with its use. It is the crystal plane separation, also known as d-spacing, and θ is the
responsibility of the user of this standard to establish appro-
angle between the crystal plane and the diffracted beam, also
priate safety, health, and environmental practices and deter- known as the Bragg Angle.
mine the applicability of regulatory limitations prior to use.
3.1.2 crystal, n—a homogeneous, solid body of a chemical
1.6 This international standard was developed in accor-
element or compound, having a regularly repeating atomic
dance with internationally recognized principles on standard-
arrangement that can be outwardly expressed by plane faces
ization established in the Decision on Principles for the
(adapted from Ref (1)).
Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
1 3
This guide is under the jurisdiction of ASTM Committee E30 on Forensic Available from International Organization for Standardization (ISO), ISO
Sciences and is the direct responsibility of Subcommittee E30.01 on Criminalistics. Central Secretariat, Chemin de Blandonnet 8, CP 401, 1214 Vernier, Geneva,
Current edition approved Nov. 1, 2023. Published November 2023. Originally Switzerland, https://www.iso.org.
approved in 2022. Last previous edition approved in 2022 as E3294 – 22. DOI: The boldface numbers in parentheses refer to the list of references at the end of
10.1520/E3294-23. this standard.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E3294 − 23
3.1.3 crystal lattice, n—the three-dimensional regularly re- 3.1.12 Rietveld refinement, n—a procedure for carrying out
peating set of points that represent the translational periodicity a crystal-structure refinement using X-ray or neutron powder
of a crystal structure. diffraction data, in which an entire powder pattern is simulated
3.1.3.1 Discussion—Each lattice point has identical sur- for a trial structure(s) and matched against the observed
powder pattern; atomic parameters and other variables are
roundings. Lattice is the abstract pattern used to describe the
internal geometric structure of crystals. Lattice and structure modified to achieve an acceptable fit between the calculated
and observed powder patterns (adapted from Ref (1)).
are not synonymous, as structure refers to the real mineral
material (adapted from Ref (1)).
3.1.13 unit cell, n—the smallest group of atoms of a crystal
lattice that has the overall symmetry of a crystal of that
3.1.4 crystalline, adj—having a crystal structure or a regular
substance, and from which the entire lattice can be built up by
arrangement of atoms in a crystal lattice.
repetition in three dimensions.
3.1.5 d-spacing, n—in diffraction of X-rays by a crystal, the
3.1.14 X-ray diffraction (XRD), n—the diffraction of a beam
distance or separation between successive and identical parallel
of X-rays, usually by the three-dimensional periodic array of
planes in the crystal lattice; d-spacing is expressed as d in the
atoms in a crystal that has periodic repeat distances (lattice
Bragg equation (adapted from Ref (1)).
dimensions) of the same order of magnitude as the wavelength
3.1.6 diffractometer, n—an instrument that records either
of the X-rays (1).
powder or single-crystal X-ray diffraction patterns.
3.1.15 X-ray diffraction pattern or diffractogram, n—the
3.1.7 known sample, n—known samples of geological ma-
characteristic interference pattern obtained when X-rays are
terial are intentionally collected, typically from crime scene or
diffracted by a crystalline substance; the geometry of the
alternate locations, for comparison to a questioned sample.
pattern is a function of the repeat distances (lattice dimensions)
3.1.7.1 Discussion—Geological materials are typically more
of the periodic array of atoms in the crystals; the intensities of
heterogeneous than manufactured materials, so a greater num-
the diffracted beams give information about the atomic
ber of known samples of geological material are needed to
arrangement, and unit-cell dimensions (adapted from Ref (1)).
represent the range of variation (see Guide E3272). Reference
3.1.16 X-ray powder diffraction, n—diffraction of a beam of
sample and control sample are synonyms of known sample.
X-rays by planes of atoms in a powdered crystalline sample;
3.1.8 mineral, n—a naturally occurring inorganic element or
the powders are prepared so they ideally represent all possible
compound having an orderly internal structure and character-
crystal orientations to the X-ray beam (adapted from Ref (1)).
istic chemical composition, crystal form(s), and physical
properties, or an element or chemical compound that is
4. Summary of Guide
crystalline and that has formed as a result of geological or
pedogenic (soil-formed) processes (adapted from Ref (1)). 4.1 Powder X-ray diffraction produces results related to the
crystal structure(s) of one or more crystalline components of
3.1.8.1 Discussion—Artificial and biogenic crystalline ma-
the material being analyzed that can allow phase identification.
terials are not minerals but can occur in geological materials
(for example, cement powder, lime, lye, biogenic calcite,
4.2 This guide recommends specific techniques and proce-
biogenic hydroxyapatite, bricks) and can be detected by XRD.
dures for XRD analysis of geological materials in forensic
3.1.9 phase, n—a part of a chemical system that is casework, including XRD analysis of minimally modified
homogeneous, physically distinct and at least hypothetically
materials, small quantities of material (a common limitation in
separable, and which has single or continuously variable forensic casework), and in situ XRD of material adhering to a
chemical and mechanical properties (adapted from Ref (1)). substrate.
3.1.10 provenance, n—a place of origin; specifically, the
4.3 XRD patterns are compared to reference databases as
area from which the constituent materials of a sedimentary
means of identifying the crystalline constituents of a sample.
rock or facies are derived (adapted from Ref (1)).
4.4 XRD can be used to determine the crystal structure of a
3.1.10.1 Discussion—In the context of forensic provenance
material, but this is not described in this guide.
analysis, geological material is analyzed and interpreted to
4.5 XRD patterns from various samples are compared to
estimate or limit the geographic or environmental conditions of
each other for forensic comparison and provenance purposes.
the source of this material to provide an investigative lead. For
example, soil on a shovel can be examined to aid in the search
5. Significance and Use
for a clandestine grave, typically by comparison of observa-
tions to reference data. Geographic attribution is an alternative
5.1 The overarching goals of the forensic analysis of geo-
term for provenance.
logical materials include (A) identification of an unknown
3.1.11 questioned sample, n—geological evidence of un- material (see 11.3), (B) analysis of soils, sediments, or rocks to
known origin, or a questioned sample, typically consists of restrict their possible geographic origins as part of a prov-
debris adhering to an evidentiary object (for example, tire, enance analysis (see 11.4), and (C) comparison of two or more
wheel well, garment, shoe, digging tool); exogenous soil left at samples to assess if they could have originated from the same
a crime scene (transferred from a shoe/tire, or adhering to a source or to exclude a common source based on observation of
re-buried body/object); or debris recovered from within a body exclusionary differences (see 11.5). XRD is only one analytical
(nasal, stomach, or lung contents). method that can be applied to the evidentiary samples in
E3294 − 23
service of these distinct goals. Guidance for the analysis of 6.2.2 A McCrone Micronizing Mill (9), or other mills;
forensic geological materials can be found in Refs (2-4). 6.2.3 Filtration apparatus (10).
5.2 Within the analytical scheme of geological materials, 6.3 Sample Holders:
XRD analysis is used to: identify the crystalline components 6.3.1 Bulk sample holders can be used when sample volume
within a sample; identify the crystalline components separated is sufficient to fill them.
from a mixture, typically clay-sized material (see 8.8), or a 6.3.2 Sample holders with minimal contribution to the
selected particle class for which additional analysis is needed diffraction pattern are recommended for many applications.
(see 8.11); or compare two or more samples based on the Two “zero background” sample holder substrates are:
identified crystalline phases or diffraction patterns (see 11.5). 6.3.2.1 A silicon crystal cut parallel to the 510 plane;
5.2.1 Non-destructive XRD analysis can be performed in 6.3.2.2 A quartz crystal cut 6° from the 0001 plane.
situ on geological material adhering to a substrate (see 8.12.3). 6.3.3 Filters or other substrates can be used, but they could
5.2.2 The most common forensic applications of XRD to contribute to the background signal. In interpretation of dif-
geological materials are (A) identification or confirmation of a fraction patterns, this background should be considered.
selected phase or fraction of a sample (see 8.12), (B) identifi-
6.4 Reference Materials:
cation of minerals in the clay-sized fractions of soils (see 8.8),
6.4.1 Standard Reference Materials—Standard reference
and (C) identification of the phases of the hydrated cement
materials, commonly silicon, corundum, or microcrystalline
component of concrete or mortar.
quartz, are available from NIST (National Institute of Stan-
5.3 This guide is intended to be used with other methods of dards and Technology) or some instrument manufacturers.
analysis (for example, polarized light microscopy, scanning Standard reference materials are used to document peak
electron microscopy, palynology) within a more comprehen- position, resolution/peak width, and intensity of the diffraction
sive analytical scheme for the forensic analysis or comparison peaks.
of geological materials. 6.4.2 Mineral reference materials.
5.3.1 Comprehensive criteria for forensic comparisons of 6.4.3 Crystalline materials for “internal calibration” or ref-
geological material integrating multiple analytical methods and erence material mixed into the analyte.
provenance estimations (see 11.4) are not included and are 6.4.3.1 Common crystalline materials that can be used
beyond the scope of this guide. include zincite (zinc oxide), halite (sodium chloride), diamond,
silver, corundum, or another crystalline substance that is absent
6. Apparatus and Materials
from the sample and that does not, or minimally, interferes with
sample diffraction peaks, as demonstrated by prior sample
6.1 Powder X-Ray Diffractometer:
analysis or prior knowledge of the sample.
6.1.1 Powder X-ray diffractometers are commonly config-
ured with a 2θ or θ - θ geometry.
6.5 Powder XRD Reference Data—Reference diffraction
6.1.2 Alternative instrumentation configurations permit si-
data for known materials are available as: powder XRD
multaneous collection of diffracted beams at multiple angles
patterns that are digital or graphical data representing the
(stationary position sensitive detectors), or transmission pow-
intensity of the diffracted X-ray beam versus degrees 2θ (for a
der XRD. specified X-ray wavelength); tables of diffraction peaks listing
6.1.3 The X-ray tubes in XRD most commonly have copper
the degrees 2θ or the d-spacing (Å), the relative intensity of
targets, generating a K wavelength (λ) of 0.15418 nm these peaks, and the crystallographic plane (defined by h, and
α1,2
(1.5418 Å).
k, and l) causing the diffraction; and modelled diffraction
6.1.3.1 X-ray tubes with cobalt targets (Co K of patterns derived from crystal structure data. Several commer-
α1,2λ
0.17902 nm or 1.7902 Å) reduce fluorescence in XRD of
cial and freely available sources are listed below; however, this
iron-bearing materials (5).
list is not inclusive. The source of reference data should be
6.1.4 Parallel Beam Optics, if available, could be beneficial
considered when used to identify phases within the XRD
for: samples that are not flat (see 8.12.1); in situ analysis,
pattern of an unknown sample.
(8.12.3), or micro-XRD (8.12.4), but can reduce the diffraction
6.5.1 Commercial Sources of Powder XRD Patterns and
signal (6). Adaptations to achieve parallel beam optics include:
Structural Data:
6.1.4.1 Göbel mirrors,
6.5.1.1 International Centre for Diffraction Data (ICDD,
6.1.4.2 Polycapillary collimators (7),
PDF or Powder Diffraction File) (11).
6.5.1.2 NIST Inorganic Crystal Structure Database (ICSD)
NOTE 1—The parallel beam adaptations need to be matched to the
(12).
specific applications.
6.5.2 Free Sources of XRD Patterns and Structural Data of
6.1.5 Spinning Sample Stage, if available, could be benefi-
Geological Material:
cial for samples of limited quantity.
6.1.6 Adjustable XYZ Tri-axial Goniometer Head, if
The McCrone Micronizing Mill (McCrone Microscopes and Accessories, 850
available, could be beneficial for samples of limited quantity.
Pasquinelli Drive, Westmont, IL 60559) is the only suitable commercial product for
the optimal particle size reduction of geological materials for quantitative powder
6.2 Sample Milling and Preparation Equipment—The fol-
XRD (8) known to the committee at this time. If you are aware of alternative
lowing materials can be helpful in sample preparation.
suppliers, please provide this information to ASTM International Headquarters.
6.2.1 Mortar and pestle composed of agate, alumina, or
Your comments will receive careful consideration at a meeting of the responsible
ceramic; technical committee, which you may attend.
E3294 − 23
6.5.2.1 The RRUFF Project (13) integrating the American fractionation, the XRD patterns for comparison should be
Mineralogist Crystal Structure Database. derived from the same particle size fraction.
6.5.2.2 The Crystallography Open Database or COD (14)
8.3.2 Other physical or chemical treatments should be
integrating the American Mineralogist Crystal Structure Data- similar for direct comparison of XRD patterns (for example,
base.
drying, crystallite orientation, glycolation, selective dissolution
6.5.3 Sources of Powder XRD Tables of Geological and (28), grinding, type of sample holder).
Related Materials:
8.4 Sample Quantity—Diffraction data suitable for phase
6.5.3.1 Table 5.18 of Brown and Brindley (15).
identification can be acquired from very thin samples (see
6.5.3.2 Common soil minerals in Table 4-1 of Ref (16).
8.12.2), but a powder thickness of at least ~100 μm is optimal
6.5.3.3 Common minerals in clay-sized material (17).
(16, 29).
6.5.3.4 Clay mineral-specific diffraction data (16, 18-21).
8.4.1 If samples are thinner than ~100 μm and if the sample
6.5.3.5 Cement- and concrete-specific data (Ref (22), see
holder is not a low background material (see 6.3.2), contribu-
Test Method C1365).
tion from the sample holder should be considered in the
6.6 Powder XRD Analysis Software for Phase Identifica- interpretation of the diffraction pattern.
tion:
8.4.2 The minimum sample required for acquisition of XRD
6.6.1 Diffractometer manufacturers typically provide data suitable for phase identification will vary by sample
instrument-specific, high-quality peak detection and phase composition, but several milligrams on a zero background
identification software. sample holder can be sufficient (2).
6.6.2 Alternatively, there are additional commercial (for
8.5 Sample Particle Size—An optimal sample for powder
example, Jade (23)) and no cost software packages (for
XRD data acquisition consists of fine particles, typically less
example, CrystalSleuth (13), ReX (24), or GSAS II (25); see
than 10-20 μm diameter (8, 16, 29).
review in Ref (26)).
8.5.1 Reduction of Particle Size—Samples of limited quan-
tity are often ground using a mortar and pestle (16, 29).
7. Hazards
8.5.1.1 Grinding in a liquid medium such as acetone or an
7.1 X-rays are a hazardous source of ionizing radiation and alcohol is recommended. Grinding in water, while possible,
should be contained within the safety shielding of a commer- will destroy potentially soluble minerals (for example, salts
cial diffractometer whenever the X-ray tube is energized. The such as halite or gypsum). Certain minerals can be altered by
X-ray source should be registered with the appropriate juris- aggressive grinding (29, 10).
dictions.
8.5.1.2 Particle reduction mills (such as a McCrone Micron-
izing Mill) produce an ideal narrow particle range of the right
7.2 The X-ray tube requires high voltages that present a risk
size for powder XRD (8) but are not recommended for samples
of electrocution if instrument safety mechanisms are over-
of limited quantity.
ridden.
NOTE 3—To preserve the grain morphology and to permit subsequent
grain selection, it can be prudent NOT to grind samples of very limited
8. Sample Preparation
quantity. See 8.12.1.
8.1 Recommend sample preparation methods are described
NOTE 4—Contributions from the micronizing media should be consid-
below for acquisition of high-quality XRD data, but useful ered when interpreting powder diffraction profiles.
XRD data can be produced with minimal sample modification
8.6 Sample Placement on Sample Holder—Powders should
(for example, in situ analysis (8.12.3), or from samples of
be placed into a clean sample holder, with the powder surface
limited quantity (8.12.1, 8.12.2, 8.12.4).
flush with the top of the sample holder to align with the
focusing circle of the diffractometer.
8.2 Sub-sampling of Particle Assemblages—Representative
sub-sampling of particulates can be achieved with a sample 8.6.1 Samples too small to fill a sample holder can be placed
splitter, or cone and quartering, both of which can be imprac- directly on a low background substrate.
tical for samples of limited quantity (27).
8.6.2 The powder should ideally cover the entire area
8.2.1 An alternative method of representative sub-sampling irradiated by the X-ray beam, which will depend on the
that is appropriate for small quantities of powder, is first instrument configuration and the 2θ scan range, but is not
mixing the particles, moistening to cause particle adhesion, required with a zero-background sample holder.
then scooping of one or more sub-samples.
8.7 Random Particle Orientation—With the exception of
samples intentionally prepared with a preferred orientation (see
NOTE 2—Systematic sub-sampling of particulates is crucial for quan-
titative XRD methods (see 10.6) to minimize bias in both particle size and
8.8.3), powders should be prepared to confer random orienta-
modal abundance of particle type.
tion of crystals.
8.3 Sample Treatments for Comparisons—Sample treat- 8.7.1 Avoid pressing too hard on the powder surface to
ments should be similar when comparing diffraction patterns of maintain random orientation of the crystals.
geological materials (questioned-to-known, questioned-to- 8.7.2 Clay minerals, common in soils, are particularly
questioned, or known-to-known) (see 11.5).
susceptible to preferred orientation (9, 10, 16, 21).
8.3.1 If there is a distinct particle size difference between 8.7.3 Use of a side-loading sample holder can minimize
samples, and if there is sufficient material to conduct size these effects (9, 10).
E3294 − 23
8.8 Clay Mineral Analysis Methods—XRD analysis is one treatments can irreversibly alter the evidence, but solvation is
of the principal means of differentiating minerals in the reversible and can be informative.
clay-sized fraction (<2 μm diameter) of soils and sedimentary
8.9 Standard Addition for Internal Calibration—Due to the
rocks as these particles are too small to be analyzed by optical
limitations often encountered with evidentiary samples, it is
microscopy. Clay minerals are phyllosilicates commonly found
not always possible to achieve the ideal sample height,
in the clay-sized particle range. Because clay minerals have
particularly for in situ XRD. Because sample height is critical
significant effects on soil chemical and physical properties,
for both phase identification and comparison (see 10.7.3), the
specific sample preparation protocols have been developed to
use of an internal standard represents one way to ensure
functionally differentiate among these minerals by XRD analy-
accurate knowledge of peak position (° 2θ). Alternatively, the
sis.
presence of another independently established phase (such as
8.8.1 Dispersion of Samples—Dispersion of minerals in an
quartz) can also be used to serve as an internal peak position
aqueous solution is useful both for separation of size fractions
calibrant.
to segregate clay sized material for XRD (see 8.8.2), and for
8.10 Standard Addition for Quantification—For quantitative
removal of fine materials coating sand and silt grains prior to
XRD analysis (see 10.6), a common approach is to add a
grain mount preparation for analysis by light microscopy.
specified weight percent (~10 to 20 weight %) of an internal
8.8.1.1 Commonly a dispersant/surfactant is added in trace
standard (see 6.4.3) to the sample, usually corundum, or zincite
amounts to aid dispersion. Sonication in an ultrasonic bath or
(31).
with an ultrasonic probe aids in dispersion.
8.8.1.2 The presence of carbonates or gypsum can interfere 8.10.1 Standards should be absent from the sample and
should lack preferred orientation.
with dispersion but standard methods to remove these minerals
are not recommended for samples of limited quantity.
8.10.2 This approach is not recommended for samples of
limited quantity.
NOTE 5—Dispersion in water will destroy potentially soluble minerals
(for example, salts such as halite).
8.11 Segregation and Concentration of a Sample Compo-
8.8.2 Separation of Clay-sized Fraction—To separate the nent:
clay-sized fraction (<2 μm), the sample is typically dispersed in 8.11.1 The segregation and concentration of a component
water (see 8.8.1) and allowed to settle a known distance for a
can be done using a range of methods including hand picking
known length of time (either under gravity or in a centrifuge) particles, density separation, magnetic separation, or selective
in accordance with Stokes’ Settling Law (19, 21).
dissolution (29, 28, 32).
8.8.3 Oriented Samples—To analyze clay minerals by XRD,
8.11.2 Physical concentration of a component of a mixture
many methods are designed to intentionally create samples in
will aid in its identification when it is otherwise present below
which the platy clay minerals are oriented parallel to the
the XRD detection limit (see 10.7.4) in a bulk sample.
diffraction focal plane, selecting for the 00l lattice spacing.
8.11.3 Segregation of the hydrated cement component of
This can be achieved by:
concrete prior to XRD enhances the signal of the cement
8.8.3.1 Allowing a clay mineral suspension to sediment out
phases that are useful in cement comparisons
and dry on the sample holder,
8.11.4 Hydrated cement can show zoning around aggregate
8.8.3.2 Filtering it through a membrane filter and transfer-
grains and near the concrete surface; thus, sampling and
ring the sediment film on the filter to the sample holder,
analysis from representative zones of the evidentiary samples
8.8.3.3 Generating an XRD pattern of the material in situ on
is recommended.
the filter, or
8.12 Adaptations of XRD Methods to Evidentiary Samples
8.8.3.4 Smearing a dense clay paste across the sample
of Limited Quantity—Evidentiary geological materials are
holder.
commonly limited in quantity or may be adhered to other items
8.8.3.5 Diffraction patterns derived from oriented samples
in which removal could lead to loss of evidence. Several
should not be compared to standard powder XRD reference
optional adaptations of sample preparation and XRD methods
data by search-match methods due to the suppression of all
can be applied based on the sample characteristics, and
peaks except 00l.
availability of equipment (8.12.1 – 8.12.4).
8.8.4 Clay Treatments—The clay-sized fraction (<2 μm
8.12.1 Unground Scant Sample—To preserve grain mor-
diameter) of soils or other geological material can be subjected
phology and to permit subsequent grain selection, XRD pat-
to treatments prior to sequential XRD analysis aiding differ-
terns can be acquired from unground particles of geological
entiation among clay mineral varieties (10, 18, 19, 21, 30;
material.
section 7A1 in Ref (20); Table 4.2 in Ref (16)). Common
8.12.1.1 A collection of “large” unground grains (fine sand
treatments include:
2+ + +
to coarse silt sized) can be placed directly on a sample holder.
8.8.4.1 Saturation with Mg , K , or Li ;
8.8.4.2 Solvation with glycerin, ethylene glycol, or forma- 8.12.1.2 A preparation of scant material consisting of large
(over 25 μm), or uneven-sized particles will have an uneven
lin; or
surface and will likely have few diffracting crystals (29).
8.8.4.3 Heating to specific temperatures, typically 300 °C,
400 °C, 500 °C, or 550 °C. 8.12.1.3 Longer data acquisition times and a rotating sample
8.8.4.4 For samples of limited quantity, application of cation stage are recommended for scant samples to acquire sufficient
saturation or heat treatment is not recommended because these signal for peak detection and phase identification.
E3294 − 23
8.12.1.4 The resulting diffraction peaks from unground 8.12.4.1 Better results for μXRD require a focused, colli-
samples could be displaced (see 10.7.3) and peaks could mated X-ray beam (see 6.1.4).
broaden due to the uneven sample height (9).
8.12.4.2 Use of an adjustable XYZ tri-axial goniometer
head (6.1.6) can assist mitigating focal plane offset, and the
8.12.1.5 Use of parallel beam optics can mitigate the arti-
presence of fewer diffraction domains in samples of limited
facts of an uneven sample surface (6).
quantity.
8.12.2 Ground Scant Sample—A small subsample can be
8.12.4.3 Sample preparations for μXRD include: miniatur-
ground in a mortar in a solvent and pipetted onto the center of
ized sample holders, enclosure within a glass capillary, or
a zero background sample holder, minimizing the amount of
attachment to a filament extended to the focal plane.
morphologically modified material.
8.12.4.4 Use of a sample rotational mechanism, either a
8.12.2.1 Scant samples often require increased data collec-
rotational stage or a spinning needle, will improve detection of
tion time to acquire sufficient signal for peak detection and
diffraction peaks (36).
phase identification.
8.12.4.5 μXRD methods can be applied to individual par-
8.12.2.2 The minimum sample quantity required for XRD
ticles or to powders.
will vary between geological materials.
8.12.3 In Situ XRD Pattern Collection—In certain types of
9. XRD Data Acquisition
evidence, geological materials are present in trace amounts,
9.1 Range of 2θ Angles or d-Spacing:
either as intact fragments or as a collection of particles, on an
object such as a bullet, fabric, or shoe outsole. When removal 9.1.1 Soils, Sediments, and Rocks—The important diffrac-
tion peaks for soil, sediment and rocks typically occur for a
of the geological material from its substrate could lead to loss
or modification of evidence, it is beneficial to create an in situ d-spacing range of 2.9 nm to 0.13 nm; 3° to 70° 2θ for Cu-Kα
X-rays, 29 Å to 1.3 Å (31).
XRD pattern by mounting the object in or near to the focal
surface of the X-ray diffractometer (examples are described in
NOTE 7—The lowest angles of 2θ (largest d-spacings) in this range are
6, 32-34).
useful for the analysis of clay minerals, particularly mixed-layer clay
minerals. Some diffractometers will not permit scanning below 5° 2θ.
8.12.3.1 Acquiring in situ XRD patterns is non-
consumptive, leaving the sample available for other analytical
9.1.2 Cement—The important diffraction peaks of cement
methods (for example, chemical analysis, DNA extractions; or
typically occur between 0.763 nm to 0.175 nm; 11.5° to 52.2°
minimally manipulated bullet for toolmark examination).
2θ for Cu-Kα X-rays, 7.63 Å to 1.75 Å (see Test Method
8.12.3.2 When a clean area of the substrate is present and
C1365).
compatible with the instrument geometry, collection of a
9.2 Factors Affecting Diffraction Intensity—Several factors
background diffraction pattern from an area of the substrate
affect the intensity of resultant diffraction peaks, including:
without the geological material can enable identification of
9.2.1 Intensity of incident X-rays on the sample is affected
possible diffraction peaks originating from the interaction of
by the X-ray tube’s age and tuning, the type of the X-ray tube,
the substrate with the X-ray beam. For example, kaolinite is
and the focusing slits used.
added in the production of many rubber materials (tires,
footwear), and when identified in the substrate should be NOTE 8—Use of high energy X-ray sources at synchrotron facilities
allows the XRD-based detection of low abundance and poorly crystalline
considered in the in situ XRD analysis of geological material
materials in forensic geological materials (37), but these facilities are not
adhering to such items.
easily accessible to most forensic examiners.
8.12.3.3 If heterogeneous sample deposition is suspected,
9.2.2 Scan rate of the diffractometer (typically expressed in
collection of multiple diffraction patterns from the in situ
° 2θ per second, or count time per step increment).
sample can capture the intra-sample variability.
9.2.3 Sensitivity of the X-ray detector.
8.12.3.4 Diffraction peak position shifts due to the irregular
9.2.4 Quantity and crystallinity of the sample.
height of the in situ sample should be taken into consideration
(Sample displacement, see 10.7.3).
NOTE 9—Certain minerals inherently produce strong diffraction peaks
(for example, quartz). For samples with very little material, poor
8.12.3.5 Displacement artifacts of in situ samples can be
crystallinity, or with very small crystal sizes, use of slower scan rates
mitigated with the use of parallel beam optics (see 6.1.4).
could enable detection of pertinent XRD peaks.
8.12.3.6 To account for possible peak displacement with in
9.3 XRD Data Quality Assurance Practices:
situ XRD analyses, samples can be reanalyzed after the
9.3.1 XRD methods should be validated prior to use in
addition of a crystalline standard, known to be absent from the
casework.
samples, to allow for a displacement correction.
NOTE 10—ISO/IEC 17025-2017 provides guidance on criteria to be
NOTE 6—These standards should amount to less than ~10 weight %
evaluated during method validation.
applied by uniformly sprinkling over the sample(s). Example standards
include zincite (zinc oxide), halite (sodium chloride), diamond, corundum,
9.3.2 Periodically, an XRD pattern should be collected from
or another crystalline substance that is clearly absent in the XRD scan
a standard reference material (for example, silicon, corundum,
made prior to its addition.
or microcrystalline quartz; see 6.4.1), under specified condi-
8.12.4 μXRD—Standard powder XRD methods can be tions; a reasonable frequency for this check is every month of
adapted for the analysis of small samples (micro-XRD) (see 6, use, when an instrument is serviced, and when the instrument
32-35) performance is suspect.
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NOTE 11—Regular analysis of a standard reference material can confirm
10.3.2.3 When considering multiple candidate minerals,
the proper alignment of the instrument by observation of the correct
take into consideration prior knowledge of the mineralogy
diffraction peak position (typically an offset < 0.05 ° 2θ), sufficient peak
derived from other methods if available.
intensity, and sufficient resolution of peaks. A significant decline in peak
10.3.2.4 The most intense peak that is not attributed to the
intensity could indicate aging of the X-ray source.
first identified mineral is assessed with a similar approach to
that used for the largest peak, but some of the peaks could be
10. Interpretation of XRD Data for Mineral/Phase
obscured by the XRD peaks of the previously identified
Identification
mineral.
10.1 XRD patterns can be used to identify a single phase or
10.3.2.5 Attribution of all remaining diffraction peaks pro-
the major crystalline materials in a poly-phase sample. Quan-
ceeds systematically.
tification of the components of a mixture, while possible, is
10.4 Identification of Crystalline Components Using Auto-
rarely used on casework samples (see 10.6).
mated Search and Peak Match Software:
10.2 Peak Detection—Diffraction peak positions should be
10.4.1 The phase identification software will provide a
identified and, optionally, the background can be subtracted
ranked-list of the best matches to the diffraction peaks in a
from the diffraction pattern.
sample, based on their reference diffraction data.
NOTE 12—In general, the relative peak sizes should correspond to the 10.4.2 These tentative identifications should be verified by a
reference diffraction data, but factors like preferred orientation (see
combination of visual alignment with reference diffraction
10.7.2) and the overlap of peaks from other crystalline components can
patterns, prior knowledge of the sample (for example, elemen-
cause the relative peak sizes in the observed pattern to deviate from the
tal composition information, polarized light microscopy
reference patterns. In addition, preferred orientation artifacts can be
results), and include explanations for omissions or overlap of
present in the reference patterns.
the diffraction peaks.
10.2.1 It is recommended that users rely on default proce-
10.4.3 Following the tentative identification of minerals, the
dures within software packages for peak detection, baseline
relative peak sizes (peak area counts, ideally) of the candidate
subtraction, and peak position determination for consistency
minerals should be assessed to ensure that all major peaks (that
between samples.
is, >30 % of the maximum peak for the mineral) are either
NOTE 13—If a possible peak is near the detection threshold, the analyst present or obscured by the peak of another mineral.
can collect a new diffraction pattern with a slower scan speed/longer count
10.4.4 Many software systems enable differential diffraction
time to improve the signal to noise ratio.
analysis, which permits subtraction of XRD peaks from pro-
10.2.1.1 Peak detection thresholds should meet or exceed
visionally identified minerals to facilitate identification of the
three times signal to noise. remaining minerals generating the lower intensity peaks.
10.2.2 Clay minerals typically have small crystallite (dif-
10.5 Criteria for Phase Identification—There are no univer-
fraction domain) sizes that result in broad XRD peaks (38);
sally accepted criteria for phase identification by powder XRD,
these broad XRD peaks can be undetected by default software
but the following two criteria are suggested:
peak detection settings and can require manual peak identifi-
10.5.1 All of the peaks in the reference diffraction pattern
cation.
greater than 30 % of the maximum peak size should be present.
10.3 Identification of Crystalline Components Without Aid 10.5.2 At least five diffraction peaks, for phases that have at
of Software—Use of XRD analysis software is preferred (see
least five characteristic diffraction lines, should be observed
10.4), but methods for the identification of minerals without the (39) (see Test Method C1365 and Practice D934).
aid of analysis software are briefly described here.
NOTE 14—In mixtures, some phases exhibit overlapping peaks; the
10.3.1 Determination of Diffraction Peak Positions and
relative intensities of these peaks should be evaluated prior to their
Relative Intensities: assignment to a specific phase. These criteria can be relaxed by augmen-
tation from independent analytical data, for example, by microscopy,
10.3.1.1 After conversion of peak positions from ° 2θ to
selective dissolution, or clay solvation.
d-spacing using Bragg’s Law, the peak positions / d-spacings
10.5.3 Provisional Phase Identification—If fewer than five
and relative peak intensities are compared to published refer-
peaks (39) (see Test Method C1365 and Practice D934) are
ence tables (see 6.5).
detected, without other supporting analytical data, the phase
10.3.1.2 Tables of diffraction peaks to aid in mineral iden-
identification should be noted as provisional.
tification are typically ordered based on the d-spacing of the
most intense peak (Hanawalt Tables) or listing each entry’s top
NOTE 15—For consistency within this guide, the term “provisional” is
four most intense peaks (Fink Tables). These two approaches
used to denote an uncertain phase identification, but individual laborato-
are often applied together. ries may use alternative language for similar conditions (for example,
uncertain, tentative, preliminary, unverified).
10.3.2 Manual Identification of the Minerals by XRD:
10.3.2.1 A tentative identification of the mineral causing the
largest peak is assessed by consulting reference data, with
confirmation that, at minimum, all peaks with intensities Five diffraction peaks are insufficient to determine unit cell dimensions of
crystals of low symmetry, like those in the triclinic crystal system (40). However, in
greater than 30 % of the highest peak are present.
this guide, XRD data are not being used to determine crystallographic parameters,
10.3.2.2 Any additional peaks caused by this mineral pres-
but rather the diffraction peaks and relative intensities are being applied as
ent in the diffraction pattern should be noted. characteristic markers of the crystal, irrespective of the crystal symmetry.
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10.5.3.1 A provisional phase identification can be probative 10.7.1.2 Many mineral structures permit elemental substi-
in many cases. tutions (solid solution) that can have minimal effect on the
crystal structure. Thus, an absolute mineral species identifica-
10.5.3.2 The phase identification can be confirmed by the
concentration of the phase (see 8.11) and reanalysis by XRD to tion by XRD alone for these phases could be impossible, but
categorization to a class of minerals with similar structures
enable enhancement of the diffraction peaks or by the appli-
cation of alternative methods like optical microscopy, energy could be appropriate. For example, with XRD data alone, the
interpretation could be a trioctahedral mica, but the specific
dispersive X-ray spectrometry, or Raman spectroscopy.
mica could be indeterminate (for example, biotite, or mineral
10.5.3.3 Confirmation can also be made by the disappear-
with similar structure).
ance of a peak upon selective dissolution, for example, removal
10.7.1.3 XRD can differentiate polymorphs (minerals of the
of halite with water or calcite with acids.
same composition with distinct crystal structures, for instance,
10.6 Quantification of Major Crystalline Components:
quartz and cristobalite are both silicon oxides) whereas el-
10.6.1 Quantification of minerals within geological materi-
emental analysis techniques such as SEM-EDS cannot.
als by XRD is performed infrequently because (1) geological
Alternatively, independent observations (for example, polar-
materials in casework often have the potential for particle size
ized light microscopy, scanning electron microscopy-energy
bias (for example, differential attachment or loss between
dispersive spectroscopy, Raman spectroscopy) can provide the
contact materials and soil or sediment) that can change the
needed supplemental information to definitively identify which
relative proportion of minerals between the evidence and its
of several minerals with similar lattice configurations (iso-
source, (2) the limited quantity of evidentiary material can
morphs) is present in the sample.
prevent the optimal sample preparation necessary for proper
10.7.2 Preferential Orientation Artifacts—Introduction of
quantification (33) and may not be representative of the modal
powders containing crystals with one or more dominant crystal
abundance of its source.
growth or cleavage planes into a sample holder will often lead
10.6.2 Quantitative XRD methods are often performed on
to preferential alignment of the dominant crystal face with the
powders to which an internal standard material is added to
XRD focal plane. The result of biased orientation is that
improve quantitation and verify instrument alignment and
crystallographic planes that are preferentially aligned parallel
sample position (see 6.4.3).
to the diffractometer focal plane have enhanced diffraction
10.6.3 When using quantitative XRD analysis for compari-
intensity relative to the XRD peaks corresponding to other
sons of geological materials, collection of diffraction patterns
crystallographic planes (9).
from multiple sub-samples (see 8.2) is highly recommended to
10.7.3 Displacement (Parafocusing Violation)—If the
better represent intra-sample v
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