AlllDE 771flDl

Certification of SRM 1879

Respirabie Cristobaiite As A

Quantitative X-Ray Diffraction SRM

Madeline A. Kuchinski, Camden R. Hubbard and Carl Robbins

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Ceramics Division

Gaithersburg, MD 20899 -

April 1988

-QC

100

^

WmENT of COMMERCE

. U 5 6

JReau of standards

#88-3742

1988

C.2

stimulating America's Progress

1913-1988
i

(
Research Information Center

^National Bureau of Standards

Gaithersburg, Maryland 20899

NBSIR 88-3742

1 u ^

CERTIFICATION OF SRM 1879

RESPIRABLE CRISTOBALITE AS A

QUANTITATIVE X-RAY DIFFRACTION SRM

Madeline A. Kuchinski, Camden R. Hubbard and Carl Robbins

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Ceramics Division

Gaithersburg, MD 20899

April 1 988

U.S. DEPARTMENT OF COMMERCE, C. William Verity, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

NBS INTERNAL REPORT

CERTIFICATION OF SRM 1879 RESPIRABLE CRISTOBALITE

AS A QUANTITATIVE X-RAY DIFFRACTION SRM

Madeline A. Kuchinski

Camden R. Hubbard

Carl Robbins

National Bureau of Standards

Institute for Materials Science & Engineering

Ceramics Division

Gaithersburg, MD 20899

ABSTRACT

Well characterized, highly crystalline standard materials are

essential for accurate quantitative x-ray powder diffraction work.

Synthetic cristobalite powder was certified as a Standard Reference

Material for quantitative x-ray diffraction analysis of cristobalite

content. Qualitative analyses of the bulk powder by Guinier de Wolff

camera and focusing diffractometer indicated only crystalline

a - cristobalite. Elemental analysis revealed only trace impurities; the

largest being aluminum at 1000 ppm. Quantitative determination of thecrystallinity of the material was performed. A quantitative technique

which employs a modified method of additions to directly analyze for low

amorphous content in crystalline matrices was developed and tested. For

this method, known amounts of amorphous material are added to the starting

powder ("spiking"). The method uses the ratio of a measure of the

intensity of the amorphoiis phase to the intensity of a reference line from

a crystalline phase. A critical step of the method is to correctly

establish the background intensity. A completely crystalline material of

similar scattering power was used to establish background intensity. An

amorphous phase close in composition to the amorphous phase existing in the

analyte was used for spiking. The certified crystalline purity of the

cristobalite SRM 1879 was 98.0 weight percent crystalline cristobalite.

The estimated standard deviation was 1.0 weight percent arising from a standard deviation of 0.5 weight percent due to random and inhomogeneity

errors plus an estimated likely systematic error of 0.5 weight percent.
TABLE OF CONTENTS

Page

ABSTRACT 1 LIST OF TABLES 3

LIST OF FIGURES 5 INTRODUCTION 6

LITERATURE REVIEW 7

THEORY OF METHOD OF ADDITIONS 9

EXPERIMENTAL CONSIDERATIONS 10

MATERIALS AND EXPEELLMENTAL PROCEDURE 12

RESULTS 14

DISCUSSION: Tests for Homogeneity 28

DISCUSSION: Analysis of Amorphous Content 39

CONCLUSION 58

REFERENCES 60

2
LIST OF TABLES

Page

Table 1. Chemical Analysis of As-Received Cristobalite by 15

D.C. Arc Emission Spectroscopy

Table 2. Equivalent Spherical Diameters of Cristobalite 17

Measured Using An X-Ray Sedigraph Technique

Table 3. Internal Standard Peak Positions used to Determine 20

the Polynomial Correction for the Cristobalite Reflections

Corresponding to Sample 211 from Table 6, Run 2

Table 4. Observed, Corrected and Calculated 29 and D- spacing 21

Values from Least Squares Refinement of Data for Lattice Parameter

Determination Corresponding to Sample 211 from Table 6, Run 2

Table 5. Lattice Parameters Measured from 16 Randomly 22

Selected Bottles, Procedure as outlined in Powder Diffraction

with W and FP as two theta calibration standards

Table 6. Lattice Parameters Test A (B#211) - Sample Mounted 23

Once, Removed & Reinserted for Three Runs in Diffractometer

Table 7. Lattice Parameters Test B (B#211) - Sample Mounted 23

Once, Three Consecutive X-Ray Runs Without Removing Sample

Table 8. Lattice Parameters. Test C (B#211) - Sample Remounted 23

Between Each of Three X-Ray Analyses

Table 9. Lattice Parameters Test D (B#211) - Three Mixtures, 23

Same Bottle, One X-Ray Analysis Each

Table 10. Relative Intensity Data - Cristobalite 24

Table 11. Reference Intensity Ratio - <RIRcris Cr o ^ 26

2 3 measured using experimentally determined values

for that bottle

Table 12. Reference Intensity Ratio - <RIR^j.^g 0 > 27

2 3 measured using the average l^®i values input from Table 10

Table 13. Amorphous Content in Cristobalite Determined by 29

Modified Method of Additions, Using Ground Quartz for

Background Approximation

Table 14. Amorphous Weight Fraction and RIR Constant of 30

Acid Washed Samples 126 and 187

Table 15. Relative Intensity Data - Test for Preferred 34

Orientation

3
Page

Table 16. Input Data for P0WD12 - Calculated Patterns of 37

Cristobalite and Cr203

Table 17. Relative Intensity Data from Table 10 Converted to 38 = 100 Scale

Table 18. Relative Intensity Data from RIR Measurement 40

Converted to I®®^(j^q 2 ) = 100 Scale

Table 19. Estimated Errors for Analysis of Sample 126 43

Table 20. Results of the Amorphous Content Determination 45

for Samples 92 and 304 using Three and Four Spiked Samples

Table 21. Amorphous Content of Cristobalite Determined 47

by the Modified Method of Additions, Using Silicon

(SRM 640a) for the Background Approximation

Table 22. Results of the Method of Additions for Determination 49

of the Amorphous Silica Content in Cristobalite Samples 126

and 304 Using Modified Background Intensity

Table 23. Results of the Method of Additions for Determination 50

of the Amorphous Silica Content in Cristobalite Samples 126

and 304 Using Modified Background Intensity

Table 24. Results of. the Modified Method of Additions for 56

Determination of the Amorphous Silica Content in Cristobalite

Samples 211 and 304 using Redefined Area 3

Table 25. Results of the Modified Method of Additions for 57

Determination of the Amorphous Silica Content in Cristobalite

Sample 304, Using Silica Gel as Diluent, Four Spiked Samples

and Redefined Area 3

4
LIST OF FIGURES

Page

Figure 1 . Amorphous Regions and Reference Lines used in 8

Glass Phase Quantitative Analysis of Quartz.

Figure 2. X-Ray Diffraction Pattern of Cristobaiite from 16

0 to 90 Degrees Two-Theta.

Figure 3. X-Ray Diffraction Pattern of Cristobaiite Sample 18

10 Mixed with Tungsten for High Angle Calibration and

Fluorophlogopite for Low Angie Calibration.

Figure 4. Lattice Constant a (in A) versus Weight Percent 31

Amorphous for 8 Samples of Cristobaiite. Error bars

indicate 2a . Figure 5. Lattice Constant c (in A) versus Weight Percent 32

Amorphous for 8 Samples of Cristobaiite. Error bars

indicate 2a.

Figure 6. Scanning Electron Micrographs of Cristobaiite 35

Sample 211 at Magnifications of 2000X and 5000X.

Figure 7. Weight Percent Amorphous versus RIR Constant for 41

8 Samples of Cristobaiite. Error bars indicate 2a.

Figure 8. versus Grams of Analyte Added per Gram 44

of Original Material for Amorphous Content Determination

in Cristobaiite Sample 126.

Figure 9. Amorphous Regions and Reference Lines used in 52

Amorphous Phase Quantitative Analysis of Cristobaiite.

Figure 10. Measured Intensities of Amorphous Areas and 53

Reference Lines for Sample 211.

Figure 11. X-Ray Diffraction Pattern of Cristobaiite Sample 55

211 from 15 to 35 Degrees Two-Theta.

5
INTRODUCTION

Respirable dusts in the work place have long presented health hazards

to industrial workers. X-Ray diffraction methods have been widely adopted

to quantify the amounts of crystalline phases in the dusts. These methods,

however, require crystalline standards. Recently the National Institute of

Occupational Safety and Health (NIOSH) published regulations for testing of

respirable dusts which require standards of quartz and its polymorphs^

The Standard Reference Material 1878, Respirable Quartz, was certified as a quantitative x-ray powder diffraction standard to serve as a standard for

testing of respirable silica as well as providing a standard material for

analysis of crystalline quartz in ceramic materials. A modified method of

additions was chosen to directly determine the amorphous content of the

quartz SRM and, by difference, the crystallinity.

In quantitative x-ray diffraction the most common method for

crystalline phase analysis is the internal standard method. The premise is

that a material whose crystallinity is "known" is used as a standard;

uncertainty in the crystallinity of the standard results in a corresponding

uncertainty in the analysis. Thus a cristobalite powder, certified as to

crystallinity is needed as an internal standard for accurate quantitative

x-ray diffraction analysis. Here cristobalite may be used as a qualitative

standard material as well as a SRM for quantitative analysis.

Respirable cristobalite is the second in the series of Si02 standards

intended for use as a calibration standard for quantitative analysis by x- ray diffraction methods. Cristobalite was chosen- as a standard reference

material (SRM 1879) for quantitative x-ray powder diffraction for several

reasons. In particular, cristobalite is required as a health and safety

standard for analysis of respirable cristobalite by XRD methods according

to NIOSH Analytical P & CAM 259^ . This (and equivalent methods) is also

useful for quantitative analysis of cristobalite in silica-containing

ceramics such as refractory brick and glass ceramics. Due to its

anisotropic coefficient of thermal expansion, modest concentrations of

cristobalite can cause failure due to thermal cycling induced fracture.

The need for a sensitive technique for the direct determination of

amorphous content arises because of the importance of glassy phases in

ceramics . Glassy grain boundaries in ceramic compacts may arise from

amorphous content in the starting materials. Many ceramic reinforcements,

fiber and whisker alike, may contain glassy phases which influence their

intrinsic properties and interactions in composite materials. The

properties and structural performance of glass -ceramics and partially

devitrified glasses are controlled by the composition and amount of

amorphous phase.

Cristobalite also presents a second "model" system for testing of the

modified method of additions technique developed during certification of

SRM 1878, Respirable Quartz, for the direct determination of amorphous

silica in crystalline matrices. This synthetic cristobalite is useful as a model because it is single phase, highly crystalline, and contains only

amorphous silica. However, "pure" cristobalite in either single crystal or polycrystalline form is not as common in nature as is quartz. Therefore,

establishing the background intensity for cristobalite becomes a greater

problem than for quartz. Cristobalite is a natural progression in

6
complexity from the quartz model. Thus in certifying cristobalite as a quantitative x-ray diffraction SRM, the modified method of additions for

direct analysis of amorphous content was tested.

This report is concerned with the extension and further validation of

the modified method of additions adopted for the quartz certification. All

the data collected for the synthetic cristobalite certified in this study,

will be presented. These include qualitative analysis for phase

homogeneity and sample purity, quantitative analysis for direct

determination of the amorphous silica content in cristobalite and methods

to evaluate the accuracy of the modified additions method.

LITERATURE SEARCH

Since the early 1950' s, polymer scientists have developed and tested

absolute and relative methods for determining the crystallinity of

polymers. The advent of automated x-ray diffraction and computer

refinement of data have spurred ceramists to adopt and tailor these methods

to complex ceramic systems. Until recently, amorphous content has been

determined indirectly by known quantitative x-ray diffraction techniques.

Chung^ and Knudsen^ identified crystalline phases and quantified these

phases utilizing an internal standard method. Mass balance and subtraction

from 100% yields the weight percent amorphous. Large uncertainties in the

weight percent amorphous are common problems of this approach. Relative

methods for rating the crystallinity of polymers were adapted by Ohlberg

and Strickler^ to a glass-ceramic system, however the need for an absolute

measurement technique still remained. Cervinka and D\xsil^ determined the

amorphous content in partially devitrified glasses by measuring the

intensity in a two-theta region where the intensity was due solely to the

amorphous component and background scattering. The background intensity

was estimated by the measured background component of a mixture of

crystalline oxides in the same proportion as used for preparation of the

parent glass. Three samples varying in degree of crystallinity were

prepared from the same melt and compared to the parent glass. The degree

of crystallinity was determined for each step within the interval of

measurement. Torii® et al. , devised a two-point background method for

determining the amorphous content in natural zeolites containing iron. No

exact mention is made of how they corrected for the diffuse scattering

contribution to the background under the amorphous intensity. However the

authors used a mixture of the crystalline phases present in the rocks as a standard and monitored the influence of amorphous additions as well as ferric oxide additions for two points associated with background

scattering. It is assumed that this mixture was used to establish the

background scattering component of the measured intensities at the two

values chosen for the analysis.

In highly crystalline samples, the uncertainty of analysis of each

crystalline phase by the internal standard method leads to a large

uncertainty in the content of the amorphous phase as determined by

difference. In the certification of SRM 1878, Respirable Quartz, Hubbard^

explored two methods to directly analyze for the amorphous content. The

first consisted of a least squares fit of reference patterns to the

observed pattern. This technique, just as the internal standard methods,

requires a reference material that is 100 percent crystalline, a goal that

7
INTENSITY

23%Amorphous Si02

77%a-Si02 (quartz)

Figure 1 . Amorphous Regions and Reference Lines used in Glass Phase

Quantitative Analysis of Quartz.

8
is effectively impossible, to achieve. The second method, which was adopted

for the certification, was a modified method of additions. In this

approach, the required reference material is a 100 percent amorphous phase,

a requirement easily met in practice. Three amorphous areas and two Bragg

diffraction reference lines were chosen for the quartz analysis as shown in

Figure 1 . The contribution to the observed intensity by low concentrations

of the amorphous phase, will be low and corrections for background

scattering will be important. Background scattering arises from several

sources: scattering from slits, scattering from the air, and incoherent

scattering from the sample. Incoherent scattering is a function of atomic

number and scattering angle. Thus, to obtain an approximation to the total

background, a material of similar elemental composition is required.

Single crystal quartz ground to approximately 10 microns was used to

establish the background intensity for the amorphous regions.

Recently, Nakamura et al.®, studied the method of additions for direct

determination of amorphous content in quartz. For high initial

concentrations of amorphous material the halo can be identified and matched

by carefully choosing an amorphous analyte material. Nakamura found the

results obtained with an external standard analysis compared well with

those obtained with the additions method. In cases of low concentration of

amorphous phase the matching of amorphoxis analyte is not possible as the

halo intensity is quite low.

The need for a rapid, direct measurement of amorphous content in

ceramic materials led to the evaluation of the present crystalline

quantitative techniques. The standard additions method was subsequently

modified for direct amorphous determination.

THEORY OF METHOD OF ADDITIONS

The intensity of diffraction line i of phase a from a flat brickette

is given by the equation®

(1) Ii« = Ki, \/[p^ (m/p)„]

where is the weight fraction of phase a, is the density of phase a,

(p/p)^ is the mass absorption coefficient of the mixture and is a constant determined by the crystal structure, diffraction line and set of

experimental conditions . Let ^ represent a second phase in the same

sample, the ratio of intensities of the two phases is given by

(2) Ii« p^

Pa

By definition X^ is the original number of grams of phase a per gram

of sample, and X^ is the grams of phase ^ per gram of sample. If we add

grams of phase a per gram of sample, then the new concentration X'^ and X'^ are given by

(3) X'„ = (X„ + Y^)/(l + Y,,) and

9
(4j X'^ = X^/(l + Y„).

Since the ratio of the intensity of line i from phase a to line j of

phase ^ is given by equation (2) then after adding Y„ grams of phase a per

gram of original sample the ratio of intensities is given by

(5) ^ _

Kjg (X, -H Y,)

Pa ^0

which can be simplified to

(6) = K(X„ + Y„)

where K is the slope of a plot of versus Y„ with Y^ in units of

grams of analyte added per gram of sample. Extrapolation to the negative

X- intercept gives , which is the original amount of phase a in the

sample. Multiple measures of the initial weight fraction are possible!

through inclusion of several independent measures of intensity (i=l,2,3...)

and (j=l,2,3. .

. )

EXPEEOMENTAL CONSIDERATIONS

Acceptance testing of the respirable cristobalite powder required

x-ray powder diffraction analyses to assure the qualitative purity of the

the powder. Certification required: proof of homogeneity and measurement

of crystallinity. Quantitative phase analyses were used to prove that

there was little bottle- to-bottle variation. Once the material was

bottled, the 500 bottles were randomly sampled and tested to ensure

homogeneity.

One excellent indicator of possible sample inhomogeneity is

measurement and comparison of lattice parameters. X-ray measurement of

cell constants are often sensitive to variations of several parts in one

hundred thousand. If these variations in lattice parameters were observed,

they would indicate variations in chemical composition within the host

crystal lattice. A less sensitive x-ray technique is the measurement of

relative intensities, which is only accurate to a few percent changes in

chemical composition of the crystal lattice. Another check on homogeneity

is particle size analysis. Relative intensities were also compared to

calculated values to check for an overall variation in the material and the

possibility of preferred orientation.

\ To test for phase homogeneity and to certify the crystallinity of the

sample two methods of quantitative x-ray analysis were performed.

Measurement of the reference intensity ratio (RIR) of the cristobalite

relative to Cr203 (SRM 674) provides a measure of relative crystallinity

with a sensitivity of approximately 5 weight percent. Comparison to the

calculated RIR should give an overall indication of crystallinity. The

theory of the reference intensity ratio has been well documented in the

literature^ ’ ^ ° ^

^

The modified method of additions gave a direct measure

of the amorphous content of the cristobalite. As a check on both methods,

a hot phosphoric acid wash of the cristobalite specimen was used to

10
prefe^rentially remove amorphous material from the surface . Both

quantitative analyses were performed again to check the validity and

sensitivity of the techniques.

In order to implement the method of additions, three data collection

regions were identified as areas of the amorphous halo not interfering with

Bragg diffraction peaks of cristobaiite. Area 1 lies in the two -theta

interval from 16.0 to 19.5 degrees, area 2 lies between 23.5 and 24.5

degrees two-theta and area 3 is in the two-theta region from 26.0 to 27.5

degrees, ). Three reference lines of cristobaiite, the (101) and

a (102) reflections and the (200,112) doublet were selected for the analysis.

The integration range for these reflections were 20.0 to 23.4 degrees twotheta, 30.6 to 32.3 degrees two-theta and 35.1 through 37.2 degrees twotheta, respectively. The reflections were chosen such that their

intensities were high enough to minimize counting errors and they were

close in position to the amorphous areas of interest. These selections

enable multiple measurements (9 per sample) of the amorphous content (3

areas x 3 reference lines) . The total counting time for integrating each

reference line and area was minimized utilizing a fixed error mode. The

counting error specified was 1% with a maxi mum counting time of 10 minutes.

If the total counting time to achieve a 1% counting error exceeds the

preset T^^^x minutes, then the minimum error achievable within T^^^

obtained to avoid excessively long count times integrating weak peaks. All

peaks/areas had counting errors less than 0.05% and maximum counting times

never exceeded TmaxIn theory the method of additions requires measurement of the ratio

lijg/Ij^, equation (6). Ij^^^ is the measured intensity of the amorphous

phase corrected for background and Ij^ is the background corrected

intensity of a reference line from the crystalline cristobaiite. Hence,

the first step in applying the method of additions for this analysis

requires estimation of the instr\imental background using a highly

crystalline material with the same chemical composition as the analyte and

with a particle size less than 10 microns. Quartz powder ground from a single crystal and passed through a 400 mesh screen, was used to estimate

the background. Next, a material close in composition to the amorphous

phase present was chosen. Generally the position of the amorphous halo of

the analyte to the diluent should be matched. In this case the position

and shape of the amorphous halo of the amorphous material present in the

cristobaiite could not be determined. A fused quartz was chosen as a representative material. The technique involves measurement of the

original material and comparison of that to various "spiked” samples. The

ratio of the amorphous intensity to the reference line intensity above

background was calculated for several reference lines and amorphous areas

for ail test samples. Linear regression analysis was performed on the data

and extrapolated to the negative x- intercept. This value is the weight

percent amorphous phase present in the starting material.

n
MATERIALS AND EXPEELEMENTAL PROCEDURE

Cristobalite was prepared for NBS by the Trans Tech Company using

Berkeley 5 micron MIN-U-SIL(R)* . A two kilogram sample was heated at

1600®C for eight hours. The sample was then air quenched, treated with 6N

HCl and jet-milled. The +325 mesh fraction was then removed by sieving.

Chemical analysis of the as -received cristobalite was performed by

d.c. arc emission spectroscopy with samples compared to a commercially

prepared SiOg -based reference material. Preliminary x-ray diffraction

analysis was carried out using a vertical x-ray diffractometer equipped

with a diffracted beam graphite crystal monochromator, Cu Ka radiation from

a Cu long fine- focus tube operated at 40kv and 35ma. A powder pattern was

collected at 1/8" 20 per minute from 5" to 90" 20. Further qualitative

analysis was conducted on a Guinier - de Wolff camera.

The material was mixed thoroughly and bottled in 5 gram amounts . A

total of 500 bottles were prepared. Sixteen bottles were randomly selected

for material homogeneity and crystalline content testing.

Subsequent x-ray measurements for qualitative and quantitative

analysis were carried out on vertical goniometers with Cu long fine- focus

x-ray tubes operated at 40 kv and 35 ma, equipped with diffracted beam,

graphite crystal monochromators and scintillation counters . The

diffraction unit designated AM-1 utilizes a flat specimen geometry and

samples were mounted according to the d-spacing and intensity sample

mounting methods described by McMurdie^^ et al. Measurements made on AM-1

were performed with a 0.1 degree receiving slit. The unit designated AM

was equipped with a sample spinner, and measurements were made with a 0.2

degree receiving slit. Intensity data were collected on both units. All

data collection was automated using NBS*AUT0^ ^

The particle size distribution was determined using a commercially

available x-ray sedigraph and particle dispersion liquids. Analyses were

made at 34 "C with a liquid of density 0.748 g/cc and a viscosity of 1.34

centipoise. A 90 to 1.38 micron range was scanned for each sample. Nine

samples were randomly selected for analysis and these were analyzed in

duplicate or triplicate. Samples were dispersed ultrasonicaily for three

minutes and magnetically stirred for five minutes prior to analysis.

The density of the cristobalite powder was determined with a helium

autopycnometer. The sample was placed in a sealable container and dried at

100"C in a vacuum oven for three hours. The container with sample was then

removed from the oven and placed in a desiccator. The sample was allowed

to cool to room temperature and sealed before weighing. Six determinations

of density were made on one sample of cristobalite.

For accurate lattice parameter measurement the cristobalite was mixed

with tungsten for high angle calibration and fluorophiogopite (SRM 675) for

Commercial equipment or materials are identified in this paper to

adequately describe the experimental procedure and do not imply

endorsement by the National Bureau of Standards.

12
low angle calibration. The choice of internal standard depends upon the

need for low and/or high angle calibration and unobstructed reflections.

Mixtures were prepared such that the intensities of the strongest

reflection from each phase were within a factor of two. Approximately one gram mixtures were thoroughly combined using a small porcelain mortar and

pestle. Samples from 16 bottles were analyzed. Digital data was

collected from 5“ to 90“ 20, with a 0.01“ 20 step width for either 7 or 15

hours. All data was processed with the series of programs JCPDS-NBS Powder

Pattern^ ^ and lattice constants were refined by JCPDS-NBS*LSQ85 derived

from the program of Appleman and Evans^^.

Relative intensities were also measured for each test sample. To

reduce the possibility of preferred orientation each sample was mixed with

approximately equal portions of amorphous fused quartz. A side-drifted

mount was also used to further promote randomization of particles. As a check on instrumental variation, samples were also run on AM-2 using a sample spinner, which improves particle statistics. The intensities of ail

reflections between 15* and 50“ 20 were collected and relative integrated

intensities were determined by NBS’<K5UANT84^0 . For comparison, relative

intensities were calculated using P0WD12^® and space group, lattice

constants, atomic positions and thermal parameters of cristobaiite^ ^ ^ ®

.

To check for preferred orientation, samples from bottles 211 and 282

were packed into a front loading sample holder. Preferred orientation was

induced by pressing on the front surface with a smooth microscope slide. A

sample from bottle 282 was also prepared in a manner analogous to that used

by clay scientists^® to orient plate- like clay particles. The sample was

mixed with water, dropped on a glass slide, and dried overnight. This

method causes plate- like particles such as clays to orient face up thus

inducing preferred orientation of the sample. Relative intensities were

measured from these samples. Optical and scanning electron microscopy were

utilized to check particle shape.

Two quantitative x-ray analyses were performed: (1) the reference

intensity ratio (RIR) of cristobalite was measured as an indication of

crystallinity; and, (2) a direct determination of the amorphous content of

the cristobalite using a modified method of additions. RIR's were measured

from mixtures of 30 percent cristobalite by weight with 70 percent chromiiam

oxide by weight. The (101), (111), (102), (113), and (212) reflections of

cristobalite and the (104) and (006) reflections of Cr203 were chosen for

this analysis because they were free from interference with other

diffraction peaks. Samples were mixed and blended with a small mortar and

pestle. Both diffraction units were employed as a check on instrximental

variation, utilizing the appropriate mounting methods. The RIR's of all

sixteen samples of cristobalite were obtained from data processed by

program RUNFIL of NBS'^UANT84

.

The direct determination of the amorphous silica content in

cristobalite by the modified method of additions requires measurement of

the undiluted samples and at least one diluted or spiked sample. For the

purposes of certification three or more spiked samples were prepared to

permit error analysis. Only AM-2 was chosen for this analysis as it was optimized for intensity measurements. The spiking material chosen was fused quartz and three spiked mixtures (6, 11, and 24 percent by weight

13
fused quartz) were prepared for samples from eight bottles of cristobalite

A fourth spiked mixture containing 30 percent by weight fused quartz was

prepared for two samples of cristobalite. Spiked mixtures substituting

silica gel for the fused quartz were prepared for one sample.

Approximately one gram mixtures of cristobalite with diluent were combined

in a small porcelain mortar and pestle and mounted in the holder of the

sample spinner and intensities were measured according to the preceding

conditions for AM-2, All x-ray data was processed with program AMORPH of

NBS*QUANT84.

As a check on this method, a sample of cristobalite was also acid

washed with hot orthophosphoric acid in an attempt to enhance the

dissolution of amorphous relative to crystalline cristobalite. The

procedure outlined by Stephen Altree-Williams^ ° et al. was followed. The

sample was collected by vacuum filtration with a nuclepore 0.22 /xm filter

and rinsed several times with distilled water. The residue was dried at

105“C for approximately 5 hours, cooled in a desiccator and measured

following the conditions outlined above.

RESULTS

The results of the preliminary chemical analysis of the lot of

cristobalite appear in Table 1 . The cristobalite was compared to a commercially prepared Si02 reference material and impurity concentrations

are reported in parts per million (ppm) . The results of the Guinier - de

Wolff analysis of the cristobalite powder showed no phase impurities. The

slowly collected powder diffraction pattern of the cristobalite is shown in

Figure 2.

The results of the x-ray sedigraph analysis of the particle size

distribution of the samples of cristobalite are found in Table 2. On

average 97 percent by mass of the cristobalite particles were finer than 10

/xm, 50 percent by mass of the particles were finer than 3.3 /xm, and eight

percent by mass were finer than 2 /xm. The particle size distributions of

each sample of cristobalite show little variability from sample to sample.

Any small variability is within the error of measurement given the tendency

of the cristobalite to agglomerate. The results of the helium

autopycnometer density measurement gave an experimental density of 2.344

g/cc ± 0.004 g/cc as compared to the calculated density of 2.317 g/cc.

Sixteen randomly sampled bottles of cristobalite were first tested for

homogeneity of crystalline cristobalite. Digital data were processed with

the analysis package JCPDS-NBS POWDER PATTERN, a series of programs which

reads the digital data and determines peak positions by the second

derivative technique as described by McMurdie^^ et al. Sample peaks which

are partially or completely hidden by standard peaks were read from strip

chart recordings of the sample diffraction pattern (Figure 2) , then

PLOT. PATTERN was used to plot the powder pattern on a scale of one degree

per inch and each pattern was visually inspected (Figure 3) . A polynomial

correction curve was then calculated by the program POWDER. CALIBR. An

instrument 29 calibration correction was applied to both sample and

internal standard 29 positions. Next, an internal standard calibration

curve was derived from the observed and instrument corrected internal

standard peak positions and their theoretical positions. The resulting

14
Table 1. Chemical Analysis of As -Received Cristobalite by D.C. Arc

Emission Spectroscopy

SAMPLE

ELEMENT 1 2

Ag <10 ppm <10

A1 1000 1000

B <100 <100

Ba 10 10

Be <10 <10

Bi <10 <10

Ca 200 200

Cr 10 10

Cu 3 5

Fe 500 500

K <100 <100

Li <10 <10

Mg 100 100

Mn <10 <10

Mo <10 <10

Na 200 200

Ni 10 10

Pb <100 <100

Rb <100 <100

Si XXXX XXXX

Sn <50 <50

Sr <50 <50

Ti 100 100

V <10 <10

Zn <100 <100

Zr 40 40

15
Intensity

(A.U.)

Intensity

(A.U.)

Figure 2. X-Ray Diffraction Pattern of Cristobalite from 0 to 90

Degrees Two-Theta. J 5
Table 2. Equivalent Spherical Diameters of Cristobalite Measured Using An

X-Ray Sedigraph Technique

BOTTLE

MASS % FINER THAN

EQUIVALENT SPHERICAL DIAMETER (Microns)

50% BY MASS

FINER THAN

NUMB£!R 10 9 8 7 6 5 4 3 2 (Microns)

50 A 95 93 92 89 85 78 64 39 8 3.3

B 97 96 95 93 89 83 70 46 12 3.1

69 A 95 95 93 91 87 80 66 42 10 3.3

B 98 96 95 93 90 84 71 48 14 3.1

92 A 96 95 94 91 87 81 65 41 7 3.3

B 98 96 95 93 88 83 68 44 10 3.1

104 A 97 95 94 92 87 79 60 32 4 3.5

B 97 95 94 91 87 78 60 34 6 3.4

154 A 96 95 93 91 86 80 65 39 8 3.3

B 97 95 94 92 87 80 66 40 8 3.3

165 A 97 96 93 91 86 77 62 32 4 3.5

B 96 95 94 91 88 79 64 34 5 3.5

C 97 95 93 90 87 79 62 36 5 3.5

211 A 98 97 96 93 88 82 68 44 9 3.3

B 99 98 96 93 88 82 67 42 9 3.2

227 A 95 93 93 90 86 79 64 41 8 3.3

B 97 96 94 92 88 82 69 44 11 3.2

304 A 95 93 93 91 87 80 65 40 8 3.3

B 96 93 93 91 88 81 66 44 10 3.2

17
Intensity

CPS

Intensity

CPS

Intensity

CPS

Two-Theta Degrees

Two-Theta Degrees

Key:

W = Tungsten

FP= Fluorophlogopite

(SRM 675)

Figure 3. X-Ray Diffraction Pattern of Cristobalite Sample 10 Mixed

with Tungsten for High Angle Calibration and Fluorophlogopite

for Low Angle Calibration.

18
polynomial correction was applied to all peaks of the pattern. The data

reported in Table 3 was used by POWDER . CALIBR to generate the polynomial

correction for a sample from bottle 211, corresponding to Table 6, Run 2.

An editing program, POWDER . EDTPKS was used to flag reflections for use in

the least squares refinement. Generally, Cu Ka2 reflections as well as partially overlapped sample reflections were excluded from the refinement.

EDTPKS was also used to enter space group notation and preliminary lattice

constants which were used for initial indexing and refinement. The least

squares program JCPDS-NBS*LSQ82 was used to assign hki's and refine lattice

constants. The ceil refinement was based upon corrected 20 values and only

those values which were unambiguously indexed. This program minimized the

value X(®coRR " ®calc)^- information reported in Table 4 was used for

the least squares refinement of data for the lattice parameter

determination corresponding to sample 211 from Table 6, Run 2. The refined

lattice parameters were measured for all 16 samples and results appear in

Table 5. The average lattice parameters and standard deviations were

<A>=4. 9732(7) and <0=6.9226(10).

To examine the precision of the lattice parameter measurements one

bottle was chosen to be repeatedly tested. Test 1 consisted of mounting

the sample and collecting three x-ray diffraction patterns, the only

variation being that the sample holder was removed from the diffractometer

after each measurement and reinserted for each new measurement. For test 2

the sample was mounted and inserted in the diffractometer, AM-1. Data for

three consecutive runs was collected without disturbing it in any way

between measurements. Test 3 involved remounting the sample for each of

three measurements. Test 4 required that two new mixtures be prepared from

the test bottle and that three completely different sample's were measured.

The results of these tests are shown in Tables 6-9.

The relative intensities of samples from all 16 bottles were measured.

All reflections less than 50“ 29 were collected. AUTO requested all

information req-uired for the specified analysis at the time of data

collection. In order to meas-ure the relative intensities the peak regions

were defined. From a strip chart recording a low angle background value

and a high angle background value were selected such that the tails of the

peak were not contributing intensity at these two values. AUTO also

requested an approximate peak position, generally the top of the peak as read off the strip chart recording. For the fixed error mode, AUTO

requested the desired or tolerable error in the final net intensity (1%)

and the maximum counting time (10 minutes) . The digital data was converted

into a counts per second scale, and the net background corre-cted intensity

was computed^ ° . The relative intensities were computed from the equation

(7) . =1- /I

where ^ ^ is the intensity of line i and ^ is the intensity of the

strongest line for phase a. The digital data from the relative intensity

measurements were plotted area by area to visually inspect any changes in

profile shape or background that could cause changes in measured

intensities and to ensure that the correct areas were being measured. No

changes were observed. The results appear in Table 10.

19
Table 3. Internal Standard Peak Positions used to Determine the Polynomial

Correction for the Cristobalite Reflections Corresponding to Sample 211

from Table 6, Run 2

STANDARD

TYPE

20Corr 20Thaox 20Cal

c

ERROR

20

FP 8.871 8.834 8.853 8.851 .002

FP 17.773 17.753 17.759 17.762 -.003

FP 26.778 26.772 26.774 26.776 -.002

W 40.241 40.252 40.262 40.256 .006

W 58.215 58.243 58.251 58.254 -.003

W 86.993 86.985 86.996 86.996 .000

20
Table 4. Observed, Corrected and Calculated 20 and D- spacing Values from

Least Squares Refinement of Data for Lattice Parameter Determination

Corresponding to Sample 211 from Table 6, Run 2.

HKL 2®Obs 2®Corr 20Calc

-nCor r Obs

101 21.986* 21.979* 21.989* 4.0408A

110 25.343 25.340 25.305 3.5120

111 28.449 28.449 28.444 3.1348

102 31.456 31,460 31.460 2.8413

200 36.063 36.073 36.073 2.4879

112 36.373 36.383 36,391 2.4674

201 38.484 38,497 38.434 2.3366

211 42 . 642 42.661 42.663 2.1177

202 44.816 44.837 44.844 2.0198

113 47.039 47.063 47.066 1.9293

212 48.579 48.605 48.620 1.8717

220 51.926 51.957 51.962 1.7585

004 52.849 52.881 52.861 1.7300

203 54.133 54.167 54.183 1.6919

104 56.214 56.250 56.236 1.6341

301 57.053 57.090 57.082 1.6120

213 57.457 57.495 57.505 1.6016

310 58.611 58.650 58.652 1.5728

222 .. 58.830 58.870 58.860 1.5674

311 60.267 60.308 60.299 1.5335

302 61.991 62.034 62,021 1.4949

312 65.048 65.095 65.091 1.4318

204 65.633 65.680 65.679 1.4204

223 66.813 66.862 66.840 1,3982

214 68.611 68.662 68.662 1.3658

321 69.352 69.403 69.419 1.3531

303 69.748 69.800 69.799 1.3463

105 70.491 70.543 70.556 1.3340

313 72.636 72.690 72.702 1.2998

322 73.868 73.923 73.904 1.2811

224 77.238 77«.296 77.295 1.2334

401 77.963 78.021 78.018 1.2237

410 79.313 79.372 79.374 1.2063

323 81.112 81.172 81.175 1.1840

215 81.831 81.892 81.896 1.1754

314 82.779 82.840 82.878 1.1643

20c ale is based upon the refined cell.

21
Table 5. Lattice Parameters Measured from 16 Randomly Selected Bottles,

Procedure as outlined in Powder Diffraction with W and FP as two theta

calibration standards.

BOTTLE LATTICE PARAMETERS*

NUMBER a a c a

10 4.973014 .000212 6.922584 .000481

23 4.971610 .000302 6.921444 .000736

50 4.974654 .000241 6.920568 .000635

69 4.973358 .000205 6.922301 .000441

92 4.973999 .000209 6.922479 .000469

104 4.973862 .000283 6.923643 .000714

126 4.973345 .000247 6.923554 .000530

154 4.972303 .000278 6.921087 .000648

165 4.972804 .000256 6.922750 .000566

187 4.972641 .000263 6.922160 .000640

211 4.973455 .000362 6.923040 .000687

227 4.973526 .000255 6.923748 .000544

253 4.972392 .000265 6.924079 .000658

274 4.973977 .000243 6.922927 .000535

282 4.973427 .000191 6.922072 .000416

304 4.973213 .000227 6.923531 .000532

ALL** 4.97322 6.92262

SD*** 0.00073 0.00098

Tetragonal symmetry, with Space Group P4j^2^2; a and c are lattice

parameters in A. a is the estimated standard deviation from

least-squares cell refinement of 36 calibrated reflections.

n

ALL = 1/n • ^ a^^ i=l

n

SD ={X(a^

i=l

<a>)2/(n -

22
Table 6. Lattice Parameters Test A (B#211) - Sample Moimted Once, Removed

& Reinserted for Three Runs in Diffractometer

a a c a

Run 1 4.973575 .000223 6.922071 .000457

Run 2 4.973466 .000211 6.922291 .000461

Run 3 4.973709 .000220 6.923043 .000485

ALL 4,97358

SD 0.00010 0.00042

Table 7. Lattice Parameters Test B (B#211) - Sample Mounted Once, Three

Consecutive X-Ray Runs Without Removing Sample

a a c a

Run 1 4.974218 .000277 6.923722 .000589

Run 2 4.973122 .000257 6.924218 .000517

Run 3 4.973134 .000252 6.922786 .000546

ALL 4.97349 • 6.92358

SD 0.00051 0.00059

Table 8. Lattice Parameters Test C (B#211) - Sample Remounted Between Each

of Three X-Ray Analyses

a a c a

Run 1 4.973657 .000210 6.923682 .000442

Run 2 4.973597 .000226 6.922207 .000489

Run 3 4.973159 .000314 6.923353 .000593

ALL 4.97347 6.92308

SD 0.00022 0.00063

Table 9. Lattice- Parameters Test D (B#211) - Three Mixtures, Same Bottle,'

One X-Ray Analysis Each

a a c a

Run 1 4.973446 .000224 6.922810 .000480

Run 2 4.972922 .000294 6.923252 .000595

Run 3 4.973657 .000210 6.923682 . 000442

ALL 4.97334 6.92325

SD 0.00031 0.00036

23
Table 10. Relative Intensity Data * Cristobalite

BOTTLE

NUMBER 101 110 111

HKL

102 200,112 201 211 202 113 212

AM

10 100 ,.636 8.837 10.714 17.176 .198 2.714 2.464 4.989 4.941

10 100 .543 8.564 10.547 17.249 .175 2.756 2.503 4.983 4.968

23 100 .539 8.598 10.722 17.225 .152 2.748 2.475 5.008 4.961

50 100 .507 8.572 10.445 17.066 .135 2.680 2.504 4.892 4.872

69 100 .493 8.750 10.926 17.636 .165 2.785 2.526 5.046 5.020

92 100 .489 8.605 10.699 17.227 .222 2.731 2.485 5.036 4.957

104 100 .538 8.715 10.768 17.290 .134 2.700 2.528 5.019 5.060

126 100 .411 8.634 10.796 17.686 .184 2.814 2.485 5.107 5.043

154 100 .491 8.513 10.563 17.259 .185 2.669 2.480 4.931 4.877

165 100 .527 8.576 10.814 17.439 .173 2.751 2.514 4.977 5.017

187 100 .620 8.916 10.940 17.840 .190 2.814 2.560 5.174 5.119

211 100 .572 8.654 10.764 17.325 .136 2.707 2.492 5.003 4.974

ALL

AM100 0 .50 8.7 10.7 17.4 0 .20 2.7 2.50 5.0 5.0

SD --- .06 .1 .1 .2 .03 .1 .03 .1 .1

AM-1

211 100 .554 8.598 11.163 18.909 .119 2.975 2.665 5.324 5.664

227 100 .536 8.679 10.746 17.401 .171 2.745 2.505 5.021 4.992

253 100 .558 8.895 10.758 17.721 .205 3.059 2.611 5.001 5.218

274 100 .473 8.809 10.743 17.611 .291 3.085 2.602 5.124 5.394

282 100 .565 8.670 10.780 17.772 .267 2.873 2.658 4.990 5.384

304 100 .491 8.484 10.798 17.896 .186 2.954 2.493 4.955 5.217

ALL

AM-1 100 0 .50 8.7 10.8 17.9 0 .2 2.9 2.6 5.1 5.3

SD — .04 .1 .1 .5 .1 .1 .1 .1 .2

ALL

SD

100 0.50

--- .1

8.7

.1

10.8

.2

17.5

.4

0.20

.04

2.8

.1

2.5

.1

5.0

.1

5.1

.2

Calc

jRel 100* 1 9 12 15, 4 0 3 3 6 6

iRel 100** 1 9 11 14, 4 0 3 3 5 5

Pattern calculated based upon Si"^^ , 0"^ ions.

Pattern calculated based upon neutral Si, 0 atoms.

24
Two tests were performed to check the reproducibility of the measured

relative intensities. In the first test, the relative intensities of the

sample from bottle 10 were collected twice without disturbing the sample in

any way between measurements. Variability in the relative intensities

ranged from 2 percent for the (102) reflection to 3 percent for the (111)

reflection, which lies within the expected reproducibility of this

measurement. In the second test, the relative intensities of sample number

211 were measured on AM-1. Then the same sample was remounted and the

relative intensities were measiired using AM-2. The results indicated

greater variability in the measured relative intensities, with a 10 percent

variation in intensity for the (200,112) doublet. The results of these two

tests appear in Table 10.

The reference intensity ratio was measiired for each sample. Chromium

oxide was chosen as the reference material and mixed in a 70/30 ratio by

weight with cristobalite . As in the relative intensity measurement, AUTO

requested all information for the specified analysis at the time of data

collection, creating a runfile which specified the high and low background

angles as well as the approximate peak position for all peaks of interest.

Cristobalite peaks were labeled as the unknown, while Cr203 peaks were

labeled as internal standard peaks. Each peak was counted utilizing the

fixed error mode with one percent tolerable error in the final net

intensity and 10 minutes maximum counting time. AUTO also requested the

weight fraction of standard added to 1.0 grams of unknown. Then AUTO used

this runfile to collect x-ray information for all samples for the analysis.

Measurements were made initially on AM-1 and verified on AMt2. All

intensity data was processed with Q85RNF from NBS*QUANT84. RIR's were

calculated using the equation

(8) li. X,

* = hs X,

where Ij^^ is the intensity of line i of phase a and 1^ g is the intensity of

line j of the standard phase, l^®^ is the relative intensity of the line of

the respective phase, Xg is the weight fraction of standard phase, X, is

the weight fraction of the sample phase and RIR^ g is the reference

intensity ratio of phase a to the standard phase. Q85RNF offers two

methods to determine the RIR. One method uses the actual relative

intensity as measured to determine the RIR. The second method uses input

relative intensity values (in this case the average relative intensity

reported in Table 10, from all 16 bottles) to calculate the RIR. Both

methods were employed and the results appear in Tables 11 and 12,

respectively.

As a final test for homogeneity and to establish the percent amorphous

content eight of the sixteen samples were tested using the method of

additions. AUTO again requested a runfile which designated the amorphous

intervals to be scanned as well as the high and low background angles and

approximate peak positions for the reference lines. The minimum acceptable

error in net integrated intensities was one percent with a maximum counting

time of 10 minutes. For each sample, AUTO requested the weight fraction of

spiking material added to 1.0 grams of starting powder. This runfile was then used to collect the data for all the samples (from each of eight

25
Table 11. Reference Intensity Ratio • <RIR^^i, cr o ^ measured using

2 3 experimentally determined values for that bottle

AM-1 AM-2

BOTTLE BOTTLE

NUMBER RIR RMS* NUMBER RIR RMS

10 3.0194 .0177 10 2.9327 .0081

50 3.3789 .0485 69 3.0594 .0201

69 3.4079 .0396 104 3.2814 .0322

92 2.9848 .0361 104 3.3096 .0418

126 3.0521 .0269 126 3.1567 .0061

154 3.0306 .0400 187 2.9131 .0100

165 3.0667 .0181 187 3.0700 .0154

211 2.8431 .0325 211 2.7930 .0127

211 2.8296 .0767 282 3.2483 .0344

227 2.9943 .0278 304 3.5294 .0101

253 2.7902 .0142

274 2.9853 .0582.

282 3.4164 .0744

304 3.3107 .0652

ALL 3..10

SD** 0..21

RMS = ^

n

S [(Xi-<X>)2/a2 ] 1/2

i=l i

(n-l) S dV)

i=l i

n = number of measurements

The RMS estimate is weighted by the estimated standard

deviation of the observation based on counting statistics

which were kept low compared to the expected variations.

SD is the standard deviation from the average value (ALL)

26
Table 12. Reference Intensity Ratio - <RlRcris,Cr o ^ measured using the

average values input from Table 10 ^ ^

AM-1

BOTTLE RIR RMS

NUMBER

10 3.1160 .0381

50 3.5289 .0645

69 3.5197 .0595

92 3.1615 .0594

126 3.1372 .0456

154 3.0531 .0436

165 3.1050 .0408

211 2.9154 .0745

211 3.0096 .0785

227 3.0318 .0600

253 2.7914 .0364

274 2.9540 .0513

282 3.5409 .0840

304 3.5278 .0793

ALL 3.16

SD 0.24

AM-2

BOTTLE

NUMBER

RIR RMS

10 3.0039 .0336

69 3.2395 .0562

126 3.1846 .0342

187 2.9736 .0351

187 3.0607 .0315

211 2.8021 .0216

282 3 . 3449 .0681

304 3.5879 .0210

27
bottles) . Quartz powder ground from a single optical grade crystal to less

than 40 fjm was used to determine the background intensity under the

amorphous areas. First a very slow x-ray powder pattern of quartz (1/8® 2©

per minute) was collected and examined. Areas less than 40“ 29 which did

not include peak or tail intensity were collected and read into a least

square fit algorithm. In this case the best fit through the data was a straight line although the program was equipped to fit non-linear functions

to the data. The program fitted the best curve through the data and

produced as an output file a continuous backgroimd curve. The intensities

measured at the 20 values of the simulated quartz background corresponding

to the 20 values of the designated amorphous regions in cristobalite were

used to establish the instrumental background under the amorphous regions

by Q85AMR (part of the NBS*QUANT84 package) . The results from the eight

samples, each spiked three times (Table 13) show that the percent amorphous

content is between 0 and 3 percent.

Two samples of cristobalite, 187 and 126 were washed in hot

orthophosphoric acid. After this acid treatment the RIR's and weight

percent amorphous were remeasured. These results appear in Table 14.

DISCUSSION: Tests for Homogeneity

The chemical analysis of the as-received cristobalite is indicative of

a relatively pure cristobalite. Inhomogeneities or impurities in great

enough quantity would be detected as added, unidentified reflections in the

cristobalite pattern by x-ray phase analysis . Lattice imperfections or chemical inhomogeneities would be detected as changes or variability in

lattice constants and to a lesser degree as changes in relative

intensities

.

The Guinier de Wolff camera is often used for trace phase detection.

Analysis of the Guinier camera film as well as the x-ray diffractometer

trace (Figure 2) did not reveal any reflections not accounted for by

cristobalite. Thus, the cristobalite was characterized as "x-ray pure" and

ready for the second phase of analysis.

Tables 5-9 report the lattice parameters, a value a (the estimated

standard deviation from least squares refinement of 35 or 36 reflections)

,

and SD (the standard deviation of measured lattice parameters from the

average) . The lattice parameter studies indicate some probability of

bottle to bottle variability based on the larger standard deviation (SD)

value for the 16 bottles (Table 5) in comparison with the SD value observed

in measurement reproducibility (Tables 6-9). The possible variability,

however, is quite small and should have no observable effect on the

certified percent crystalline content. No systematic correlation between

lattice parameter and analyzed weight percent amorphous exists, as shown in

Figures 4 and 5.

The results of the relative intensity measurements (Table 10) show

that the average values of each reflection less than 50“ 20 are very

reproducible and in very good agreement with calculated integrated relative

intensity values. It should be noted here that the calculated cristobalite

patterns from data reported by Dollase^^ and by Peacor^® are essentially

identical. A much larger difference in the calculated pattern results by

28
Table 13. Amorphous Content in Cristobalite Determined by Modified Method

of Additions, Using Ground Quartz for Background Approximation

BOTTLE REFERENCE WEIGHT PERCENT AMORPHOUS

NUMBER LINE AREAl AREA2 AREA3 ALL RMS

10 ALL 0.90 1.60 2.49 1.66 0.69

69 ALL 0.66 1.44 2.20 1.43 0.67

92 ALL 2.35 2.37 3.49 2.74 0.57

126 ALL 0.26 1.32 1.72 1.10 0.65

165 ALL 2.15 1.93 3.59 2.56 0.78

187 ALL 1.44 2.07 2.88 2.13 0.63

211 ALL 1.98 1.90 3.48 2.45 0.77

304 ALL 1.29 1.80 2.67 1.92 0.61

ALL ALL 1.38 1.80 2.82 2.00

SD ALL 0.70 0.32 0.63 0,54

29
Table 14. Amorphous Weight Fraction and RIR Constant of Acid Washed

Samples 126 and 187

BOTTLE REFERENCE WEIGHT PERCENT AMORPHOUS

NUMBER LINE AREAl AREA2 AREA3 ALL RMS

126 101 0.46 1.47 1.54

102 0.46 1.46 1.54

200 0.44 1.44 1.52

ALL 0.45 1.46 1.54 1.15 0.52

187 101 1.99 2.06 3.59

102 1.89 1.96 3.46 ' 200 1.93 2.00 3.51 2.48 0.77

BOTTLE

NUMBER

RTR RMS

126 2.91 0.02

187 2.81 0.01

30
O O

o

in -

inrMCD to

a: •

f , 1 tv ^ (M t—

LlJ a S in- CH rg CH

cnQ_ rg • LJ

C_)

t—

cc

1 1 1 0.00 0.25 0.50 0.75 l.OO 1.25 1.50 1.75 WT. V. RMORPHOUS

2.00 2.25 2.50 2.75 3.00

Figure 4. Lattice Constant a (in A) versus Weight Percent Amorphous

for 8 Samples of Cristobaiite . Error bars indicate 2a.

31
Figure 5, Lattice Constant c (in A) versus Weight Percent Amorphous

for 8 Samples of Cristobalite . Error bars indicate la.

32
changing the ionic nature of the cristobalite atoms, Table 10. Relative

intensities were obtained for the (110), (201), (211), and (202)

reflections, however the values for these four very weak peaks were not

certified because the error in the net integrated intensity due to counting

statistics was greater that the acceptable 1% error. No instrumental

variability in measured relative intensities is evident based on the <RI>

and SD values reported in Table 10. That is, all values seem to be within

measurement reproducibility.

Preferred orientation also causes variability in measured intensities,

which in turn will affect q\iantitative x-ray diffraction analysis. It is

relatively simple to determine whether or not preferred orientation is

having an effect on intensity measiorement . Samples from bottles 211 and

282 were mounted by methods which attempt to orient the sample, and

relative intensities were measured. The results of Table 15 show that by

simply packing the pure, undiluted sample into the cavity of the sample

holder and pressing to a smooth surface produces measured relative

intensities which are comparable to those obtained by methods minimizing

preferred orientation. The sample which was mixed with distilled water and

dried on a glass slide shows much greater variability. Closer examination

of this data shows that this variability is due to a transparency affect;

the sample is too thin and the incident beam penetrates through the sample

to the glass slide. As 20 increases, the penetration of the x-rays

increases and there should be a decrease in intensity with increasing 29, as is the case with this data.

Optical and scanning electron microscopy analysis of the cristobalite

powder showed irregularly shaped particles which tended to agglomerate

(Figure 6) . However agglomeration tended toward spherical shapes which is

what methods such as spray drying utilize to reduce preferred

orientation^^. From the x-ray analyses and the microscopy examination of

shape, preferred orientation was not expected to contribute uncertainty to

the quantitative analysis. No further techniques to minimize preferred

orientation were implemented based upon this observation. Since all

bottles were judged to be effectively identical and since quantitative

analysis depends upon the integrated intensities of specified reflections,

the possible bottle to bottle variability detected by lattice parameter

measurements should not affect quantitative analyses appreciably.

The combination of the qualitative x-ray results, particle size

analysis and density determinations with preliminary x-ray diffraction

results and chemical analysis indicate that the lot of cristobalite is

homogeneoxis , to the extent that no significant inhomogeneities exist

relative to the precision of the quantitative analyses of this material.

The experimentally determined RIR in Tables 11 and 12, was greater

than the calculated value. The calculated value was obtained from the

^cris/^corundum ^alue calculated by P0WD12 divided by the reported

value reported for the Cr203 of the intensity set SRM 674. Unlike the

calculated value, Icris/^Cor> which is free from the effects of extinction,

preferred orientation and microabsorption, the value reported for

SRM 674 - Cr203 was an experimentally determined constant, 2.1. The latter

may or may not be free from the effects of extinction and microabsorption

and thus constitutes a probable sotirce of error. Thus reliable crystal

33
Table 15. Relative Intensity Data - Test for Preferred Orientation

BOTTLE

NUMBER 101 110 111 102

HKL

200,112 201 211 202 113 212

282* 100 .59 8.83 11.22 18.51 .19 2.82 2.71 5.27 5.58

282** 100 .55 8.11 9.99 15.55 .16 2.10 2.05 3.93 3.96

211* 100 .63 8.68 11.3 18.88 .18 2.82 2.70 5.38 5.63

ALL*** 100 .50 8.7 10.8 17.5 .20 2.8 2.5 5.0 5.1

Indicates a "packed" mount as described in text.

*

ir it it

Indicates a glass slide mount.

ALL is the average certified relative intensity from Table 10.
Figure 6. Scanning Electron Micrographs of Cristobalite

Sample 211 at Magnifications of 2000X and 5000X.

35
structure data^^*^^ was obtained and used to calculate the I/I^or value for

Cr203 . Based upon the input data of Table 16, P0WD12 calculated an

value of 2.2; very close to the reported experimental value of 2.1 ± .05.

The discrepancy between the experimentally determined lcria/^Cr 203

"calculated" value may be due to the effect of extinction, microabsorption

or preferred orientation or a combination of these effects . Thus , the

calculated RIR could not be used to determine the amount of amorphous

silica present in the cristobalite.

Secondary extinction is an effect caused by strongly scattering, but

imperfect crystallites. During diffraction crystallites oriented properly

diffract x-rays at the Bragg angle. Those crystallites near the surface

reduce the effective intensity of the incident beam for crystallites deeper

in the sample. The diffracted beam may likewise be reduced in intensity by

secondary scattering as it leaves the sample. The combination of these

effects acts to reduce the recorded intensity of reflections (acts to

increase the apparent absorption coefficient of the material) . This effect

is much more pronoxmced for strong reflections than for weak ones®

.

Pronounced extinction effects have been shown to affect quantitative

analysis by as much as 50 percent^

Because of the possible influence of extinction on intensity

measurements we sought to determine if secondary extinction was a problem

with the cristobalite powder. Checking for secondary extinction requires

monitoring the intensity of the most intense peak, the (101) reflection of

cristobalite in this case, relative to a smaller peak such as the (102)

peak and comparing it to the calculated relative intensity of the (101)

reflection converted to the same scale. All experimental values were

compared to the calculated values because calculated values reflect ideal

(extinction - free) conditions. If ail the sample conditions were

identical, then the extinction effect would be the same for all bottles.

However, the orientation and size of crystallites could not be held

constant from sample to sample so if extinction was present it would vary

from sample to sample.

First, the integrated intensity of the (101) reflections from the

relative intensity measurements were monitored; that is, the intensity of

the (101) reflection relative to the (102) reflection was calculated, Table

17. The data in Table 17 represents conversion of the data in Table 10 to

the new relative intensity scale. The resultant variability in the

intensity of the (101) reflection relative to the (102) reflection ranges

from 914.2 for sample 187 to 953.9 for sample 227 (Table 17), which lies

within the standard deviation of the measurement. The average relative

intensity of the (101) reflection relative to the (102) reflection is 933

which is approximately three percent greater than the calculated value of

909 and 11 percent greater than the calculated value of 833. There is

little discrepancy between the average experimental and calculated values.

If extinction was affecting the intensity measurements then there would be

a much greater variability in the intensity of the (101) peak than of a weaker peak such as the (102) reflection. Reproducibility of the relative

intensity measurements for sample 10 (Tables 10 and 17) indicate that the

(101) peak and the (102) peak show approximately equal variations in

intensity. Note that this includes any mounting and instrumental

variabilities already present.

36
Table 16. Input Data for P0WD12 - Calculated Patterns of Cristobalite and

Ct2 O3 CRISTOBALITE DATA

REFERENCE (Dollase, 1965)^® (Peacor, 1973)^^

CRYSTAL SYSTEM Tetragonal Tetragonal

SPACE GROUP P4i2i2 P4i2i2

LATTICE PARAMETERS a = b = 4.978 a = b = 4.978

c = 6.948 c = 6.948

a = ^ = 7 = 90.0® a = = 7 = 90.0“

ATOMIC POSITIONS Si Si

X = y = 0 . 30004 X = y = 0.3002

z = 0.0 z = 0.0

0 0

X = 0.23976 X = 0.2394

y

= 0.10324 y= 0.1049

z = 0.17844 z = 0.1785

VALUE OF ISOTROPIC Si Si

TEMPERATURE FACTOR P(iso) = 0.83 0(iso) = 0.76

0 0

;3(iso) = 1.55 0(±so) = 1.43

CHROMIUM OXIDE DATA

REFERENCES (Finger and Hazen, 1980)^^ ; (McMurdie, et al., 1987)^^

CRYSTAL SYSTEM Trigonal

SPACE GROUP R3C

LATTICE PARAMETERS a = b = 4.95876

c = 13.5942

a ^ 0 = 90.0“

7 = 120 . 0

“

ATOMIC POSITIONS Cr

X = y = 0

z = 0.34766

0

X = 0.3051

y = 0.0

z = 0.25

VALUE OF ISOTROPIC Cr

TEMPERATURE FACTOR ^(iso) = 0.25

0

^(iso) = 0.28

37
Table 17. Relative Intensity Data from Table 10 Converted to I^®^d 02 ) 100 Scale

BOTTLE NUMBER

AM

101

HKL

102

10 933.4 100.0

10 948.3 100.0

23 932.7 100.0

50 957.6 100.0

69 915.4 100.0

92 934.8 100.0

104 928.8 100.0

126 926.5 100.0

154 946.9 100.0

165 924.9 100.0

187 914.2 100.0

211 929.1 100.0

AM-1

227 953.9 . 100.0

253 929.6- 100.0

274 931.2 100.0

282 927.6 100.0

304 926.2 100.0

ALL 933.0 100.0

SD 11.8

CALCULATED

jR 0 1 * 833 100

jRal** 909 100

Calculated based upon Si'*’^ , 0"^ ions.

Calculated based upon neutral Si, 0 atoms.

38
The data in Table 18 represent conversion of the raw data used to

calculate the RIR in Table 11 to the new relative intensity scale. The

resultant variability in the intensity of the (101) reflection relative to

the (102) reflection ranges from 811.7 for sample 282 to 983.2 for sample

253. This also includes any variability introduced by different mounting

methods and different instruments. The average relative intensity of the

(101) reflection is 924.8, which is only two percent greater than the

calculated value of 909 and ten percent greater than the average value of

833. This variability lies well within the variability in relative

intensity measurements when different mounting methods and instruments were

employed. There seems to be no extinction effect in the cristobalite

The particle size distribution data of Table 2 lend further support to

the assumption that secondary extinction is not having an effect on the

intensities of the cristobalite reflections. Because the total diffracted

intensity from crystallites depends on its volume, secondary extinction

doe-s not generally occur for fine powders with particle sizes less than 10

/xm. Inspection of the particle size distribution of cristobalite in Table 2

shows that less than 5 percent by mass of the particles are larger than 10

/xm and even that 5 percent is suspect due to the high degree of

agglomeration that occurs with the cristobalite.

The experimental RIR should vary with the crystallinity of a sample

and therefore could be used as a check on the direct determination of

amorphous content by the modified method of additions technique

Comparison of the RIR data in Table 17 with the amorphous spike data in

Table 13 shows that the RIR constant is relatively insensitive to small

fluctuations in percent amorphous. The relationship plotted in Figure 7

shows that all the data lies within the relative error of both

measxirements . In general, the percent amorphous data indicates a low

variability in amorphous content.

DISCUSSION: Analysis of Amorphous Content

The effects of extinction, microabsorption and preferred orientation

have a profound effect on measured integrated intensities and are very hard

to isolate. Methods such as the internal standard method which rely upon

calibrating standards for quantitative analysis are often affected because

it is hard to hold the effects of extinction, crystallite size and

absorption constant for calibrating materials and the actual sample. The

possibility that these effects will occur is eliminated when using the

method of additions because this type of analysis does not require

calibration standards. Thus quantitative techniques for the x-ray

diffraction determination of the amorphous content in cristobalite which

rely upon the internal standard method for analysis while ideally possible

are difficult to implement in practice.

The method of additions for quantitative determination of the

amorphous weight fraction in cristobalite is inherently a more reliable

technique than the internal standard method for this analysis. The method

can be modified by choosing to add an amorphous phase to the cristobalite

rather than a crystalline phase such that it is a direct technique for

determining the amorphous weight fraction. Secondly, changes in the

39
Table 18. Relative Intensity Data from RIR Measurement Converted to

I^*^(i 02 ) = 100 Scale

BOTTLE NUMBER CRISTOBALITE HKL

AM-1 101 102

10 967.8 100.0

920.0 100.0

50 923.2 100.0

928.0 100.0

69 954.4 100.0

940.6 100.0

92 933.5 100.0

912.4 100.0

126 940.3 100.0

926.6 100.0

154 917.6 100.0

896.6 100.0

165 950.4 100.0

954.2 100.0

211 900.0 100.0

906.8 100.0

211 889.1 100.0

852.8 100.0

227 902.1 . 100.0

944.3 100.0

253 983.2 100.0

944.4 100.0

274 944.9 100.0

977.2 100.0

282 887.2 100.0

928.4 100.0

304 930.6 100.0

906.6 100.0

ALL 924.8 100.0

SD 36.7

CALCULATED

jR 9 1 833 100

jR a 1 909 100

BOTTLE NUMBER CRISTOBALITE HKL

AM-2 101 102

10 941.3 100.0

950.5 100.0

69 917.5 100.0

942.9 100.0

126 976.0 100.0

957.8 100.0

187 957.3 100.0

976.0 100.0

187 936.7 100.0

947.1 100.0

211 913.7 100.0

874.2 100.0

282 811.7 100.0

826.8 100.0

304 914.9 100.0

883.5 100.0

40
• AM-1

O AM-2

O

Figure 7 . Weight Percent Amorphous versus RIR Constant for 8 Samples

of Cristobalite . Error bars indicate 2a.

41
effects of microabsorption are diminished because the absorptive powers of

cristobaiite and fused silica are approximately equal.

The modified additions method requires measurement of the background

corrected integrated intensities of the amorphous areas, and the

reference lines, for an undiluted sample and samples diluted with known

amounts of amorphous material. The use of three amorphous areas and three

reference lines results in nine determinations of amorphous content per

sample of cristobaiite. The use of three diluted mixtures per sample

permits error analysis of the linear regression. The data in Table 19 is a complete analysis of the amorphous content determination for sample 126.

The amorphous content determined for each area/line combination as well as the slope and y

- intercept of the regression line are reported in this

table. Figure 8 is a plot of versus grams of amorphous spike added

per gram of cristobaiite which shows the linear fit of the data for each

amorphous area and reference line combination for cristobaiite sample 126.

No significant deviations from linearity are observed; thus the

experimental evidence strongly supports the validity of this modified

method of additions and corrections for background.

The measured amorphous content (Table 13) varied between 1.10 weight

percent for sample 126 and 2.74 weight percent for sample 92. This

prompted further exploration of the data. Is there a bottle to bottle

variation in amorphous content predicted by the values above or do these

values lie within the experimental error, actual level of detection, of the

techniques? The answers to these questions were pursued as follows. For

samples 92 and 304 a fourth spike was prepared in the ratio 30 weight

percent fused quartz to 70 weight percent cristobaiite, and the x-ray

intensities were measured as described earlier in the procedure. The x-ray

data from the undiluted plus three spiked samples were processed by Q85AMR

and the weight percent amorphous was determined. Next the data from the

original, undiluted sample plus four diluted samples were processed. The

results of this analysis for samples 92 and 304 appear in Table 20.

For sample 92, the average weight percent amorphous as determined with

three spiked samples is 1.98 while the average weight percent amorphous as determined with four spiked samples is 2.74, a difference of 0.76 weight

percent. For sample 304, the average weight percent amorphous varies from

1.52 from measurement of three spiked samples to 1.92 from measurement with

four spiked samples, a difference of 0.40 weight percent amorphous. The

standard deviation from the average of the measurement of amorphous content

for sample 92 is 0.9 weight percent while the standard deviation of the

measurement of amorphous content for sample 304 is 0.3 weight percent.

The two values of amorphous content are determined from the same data

so they are influenced by the same errors in mixing and mounting. What

changes is the number of data points used by the regression analysis to

determine the best line through the data and to extrapolate to the x - intercept, which is the weight fraction amorphous in the original material.

Measurement of the undiluted sample and one diluted sample is enough for

the linear regression analysis to establish a line and extrapolate to the

value of amorphous content in the original material. However, the accuracy

of the method should increase with the number of diluted samples. Thus,

comparison of the two values of amorphous content for the same bottle is

42
Table 19. Estimated Errors for Analysis of Sample 126

AMORPHOUS REFERENCE SLOPE INTERCEPT CORRELATION WEIGHT LEAST

AREA LINE PERCENT SQUARES

ESD*

Area 1 101 0.2819 0.0009 1.000 0.32 0.19

Area 2 101 0.0898 0.0012 1.000 1.39 0.12

Area 3 101 0.0866 0.0016 1.000 1.79 0.17

Area 1 102 2.6253 0.0062 0.999 0.23 0.25

Area 2 102 0.8364 0.0107 1.000 1.28 0.07

Area 3 102 0.8068 0.0135 1.000 1.68 0.21

Area 1 200 1.5501 0.0037 1.000 0.24 0.21

Area 2 200 0.4938 0.0063 1.000 1.28 0.06

Area 3 200 0.4763 0.0080 1.000 1.68 0.19

Area 1 ALL 0.26 0.05

Area 2 ALL 1.32 0.06

Area 3 ALL 1.72 0.07

ALL ALL 1.10

RMS 0.65

Least squares ESD is derived from the variances based on the

counting statistics. The a reflects both internal consistency

and random errors based purely on counting statistics and the

propagation of these errors through various computations

.

43
a

Figure 8. versus Grams of Analyte Added per Gram of Original Material for Amorphous Content Determination in Cristobaiite Sample

126.

^

44
Table 20. Results of the Amorphous Content Determination for Samples 92

and 304 using Three and Four Spiked Samples

BOTTLE

NUMBER

NUMBER

SPIKES

REFEEIENCE

LINE AREA

92 3 ALL 1.29

92 4 ALL 2.35

304 3 ALL 0.85

304 4 ALL 1.29

WEIGHT PERCENT AMORPHOUS

AREA 2 AREA 3 ALL RMS

1.91 2.75 1.98 0.64

2.37 3.49 2.74 0.57

1.41 2.32 1.52 0.64

1.80 2.67 1.92 0.61

45
truly a measure of the accuracy of the modified additions method. The data

of Table 20 seems to indicate that at this level of detection (0-5 weight

percent amorphous content) the method can be applied with 1.0 weight

percent accuracy in the determination, that is, the actual amorphous

content for sample 92 is 2.74 ± 1.0 weight percent. This standard

deviation of 1,0 weight percent is a combination of 0.5 weight percent from

random errors plus 0.5 weight percent as an estimation of systematic errors

in the average value

.

The modified method of additions is not without its own limitations

.

In order to accurately determine the integrated intensity due to the

amorphous component, an accurate measure of background intensity below the

halo is required. Quartz powder ground from a single optical quality

crystal to less than 40 microns was used to establish the background

intensity under the halo as described earlier in the results.

In an effort to understand the affect of instrumental background

correction on the amorphous content measurement several tests were

performed. The first test consisted of trying different materials to

establish the background intensity due, in part, to incoherent scattering

from the cristobalite powder, as a single crystal of cristobalite was not

available. Several materials, silicon powder (SRM 640a), a single crystal

quartz plate, and ground quartz from an optical quality single crystal,

were examined.

A well characterized material SRM 640a, Silicon Powder, was used in

the background approximation. This material was suggested because it is

one of the elemental constituents of cristobalite and amorphous silica.

The weight percent amorphous content of the cristobalite samples 10, 69,

187 and 187 (after acid treatment) as determined using Si (SRM 640a) for

the background approximation are presented in Table 21. The measured

amorphous content was approximately double that found using quartz powder

background data. The increase in measured amorphous content based upon a background simulated by silicon (SRM 640a) suggests that the scattering

power of silicon powder is not sufficiently close to the scattering power

of cristobalite. This difference in the measured amorphous content as determined with the ground quartz background approximation and the silicon

background approximation is explained by the dependence of Compton modified

scattering on atomic number. Compton modified or incoherent scattering is a component of the diffuse background which arises from the sample itself.

Compton modified scattering becomes more intense with decreasing atomic

number^ so the incoherent scattering from cristobalite or quartz Si02 would

be greater than the incoherent scattering from silicon. The amorphous

intensity corrected for background by silicon appears greater than the

amorphous intensity corrected for background by quartz, as indicated by the

data in Tables 13 and 21. It appears that the influence of Compton

modified scattering on the background intensity measured for cristobalite

is great enough that silicon cannot be used to accurately model the

cristobalite background.

Data from the single crystal quartz plate was collected on AM-1 and

could be used for the background approximation for the amorphous areas in

cristobalite. It was felt that the degree of perfection of the quartz

plate could not adequately approximate the imperfections and possible

46
Table 21. Amorphous Content of Cristobalite Determined by the Modified

Method of Additions, Using Silicon (SRM 640a) for the Background

Approximation

BOTTLE REFERENCE WEIGHT PERCENT AMORPHOUS

NUMBER LINE AREA 1 AREA 2 AREA 3 ALL RMS

10 ALL 4.11 3.74 2.29 3.38 0.84

69 ALL 3.55 3.48 4.91 3.98 0.70

126 ALL 3.27 3.42 4.50 3.73 0.59

187 ALL 4.82 4.33 5.85 5.00 0.69

47
defects in the cristobalite crystals. Diffuse scattering from the sample

itself can be broken into an incoherent component and a coherent component.

The coherent component is dependent upon temperature -diffuse scattering and

diffuse scattering due to imperfections in the crystals. Any kind of

randomness or strain present in the crystals will cause coherent scattering

which contributes to diffuse backgrounds^ . The quartz plate showed a lower

background intensity than the cristobalite. The larger background for the

cristobalite powder was attributed to amorphous silica present in the

cristobalite as well as the greater degree of perfection of the quartz

plate.

Thus, a quartz powder ground from a single crystal of optical

quality was investigated as an alternate source. Ideally the particle size

of powders to be used for quantitative x-ray analysis should be less than

10 /im, in order to minimize particle effects such as extinction. However,

extensive grinding of materials has been shown to introduce amorphous

surface^® which would add to the measured background of the quartz powder.

This would introduce error in the weight percent amorphous content

measurement of the cristobalite powder so a compromise was achieved.

Single crystal quartz was ground such that 95 percent by mass of the

particle were finer than 40 /xm, 60 percent by mass less than 20 /xm, and 20

percent by mass less than 5 /xm. Based upon the investigation of various

materials to approximate the background scattering from cristobalite, the

quartz powder ground to less than 40 /xm was chosen as the best model of the

incoherent scattering power of cristobalite.

The data of Table 13 seem to indicate that a small 20 dependent

systematic error in estimating the background is present. In an effort to

pinpoint this systematic error it is important to understand the assumption

used to correct for background. It was asstimed that quartz ground from an

optical grade single crystal would have the same incoherent scattering

power as highly crystalline cristobalite because they have the same

chemistry. The background beneath the amorphous areas was approximated by

the background measured from the quartz powder. Thus a second

investigation was undertaken to determine the sensitivity of the technique

to changes in the measured background. The measured background, as determined from the diffraction pattern of ground quartz, was modified in

the following ways. First the background approximation for each of the

three selected amorphous regions was increased by the same factor. The

counts per second of the measured background for ail areas were increased

by 2 , 3, 5, 10 and 20 percent and also decreased by 20 percent. These

various "backgrounds" were then used with the spike data files for samples

126 and 304, and the weight percent amorphous was determined for each

sample using each modified background. This output appears in Table 22.

Next, the intensity of the background in area 2 remained as measured and

the background was modified to slope up or down through area 2. This was

accomplished by decreasing the counts per second in area 1 by 2 , 3, 5 or 10

percent while at the same time increasing the counts per second of area 3

by 2, 3, 5, or 10 percent respectively. To make the background slope

downward through area 2, the counts per second of area 1 were increased by

2, 3, 5, or 10 percent while the counts per second of area 3 were decreased

by 2 , 3, 5, or 10 percent, respectively. The amorphous content was then

determined for samples 126 and 304 using these various background

approximations, and is reported in Table 23.

48
Table 22. Results of the Method of Additions for Determination of the

Amorphous Silica Content in Cristobalite Samples 126 and 304 Using Modified

Background IntensityBOTTLE BACKGROUND REFERENCE WEIGHT PERCENT AMORPHOUS RMS

NUMBER NUMBER* LINE AREA 1 AREA 2 AREA 3 ALL ALL

126 1 ALL -0.20 1.01 1.29 0.70 0.69

2 ALL -0.44 0.85 1.07 0.49 0.71

3 ALL -0.94 0.52 0.61 0.06 0.76

4 ALL -2.03 -0.19 -0.38 -0.87 0.87

5 ALL -4.60 -1.85 -2.73 -3.06 1.21

6 ALL 4.59 4.23 5.70 4.84 0.67

304 1 ALL 0.71 1.44 2.17 1.44 0.64

2 ALL 0.41 1.26 1.91 1.19 0.65

3 ALL -0.22 0.88 1.36 0.67 0.71

4 ALL -1.60 0.06 0.17 -0.45 0.86

5 ALL -4.90 -1.87 -2.67 -3.15 1.36

6 ALL 6.61 5.10 7.33 6.35 0.99

Background numbers 1, 2, 3, 4, 5 and 6 correspond to increasing

the background intensity by 2 , 3, 5, 10, and 20 percent and

decreasing the background intensity by 20 percent, respectively.

49
Table 23. Results of the Method of Additions for Determination of the

Amorphous Silica Content in Cristobalite Samples 126 and 304 Using Modified

Background Intensity

BOTTLE BACKGROUND REFERENCE WEIGHT PERCENT AMORPHOUS RMS

NUMBER NUMBER* LINE AREA 1 AREA 2 AREA 3 ALL ALL

126 1 ALL -0.20 1.32 2.12 1.08 1.02

2 ALL -0.44 1.32 2.32 1.06 1.21

3 ALL 1.33 1.32 0.61 1.09 0.36

4 ALL 2.45 1.32 -0.38 1.13 1.'24

5 ALL -0.94 1.32 2.69 1.02 1.59

6 ALL -2.03 1.32 3.73 1.01 2.50

304 1 ALL 0.71 1.80 3.15 1.89 1.06

2 ALL 0.41 1.80 3.38 1.86 1.29

3 ALL 2.62 1.80 1.36 1.92 0.56

4 ALL 4.01 1.80 0.17 1.99 1.67

5 ALL -0.22 1.80 3.83 1.80 1.76

6 ALL -1.60 1.80 5.05 1.75 2.88

Background numbers 1,2,3 and 4 correspond to sloping the

background upward through area 2 by decreasing the intensity of

area 1 by 2,3,5 or 10 percent while simultaneously increasing the

intensity of area 3 by 2,3,5 or 10 percent, respectively.

Background niombers 5 and 6 correspond to sloping the background

downward through area 2 by increasing the intensity of area 1 by

5 or 10 percent while decreasing the intensity of area 3 by 5 or

10 percent.

50
An increase of the entire background by 10 or 20 percent results in a negative determination of the amorphous content for both samples. This is

indicative of a backgrotind approximation which is greater than the actual

background of the cristobalite sample. An increase of the entire

background by 2, 3 and 5 percent respectively results in a decrease in the

amount of amorphous content with increasing background intensity, as expected. A decrease of the entire background intensity by 20 percent

results in an amorphous content as great as four times the amount reported

in Table 13. The regression analysis is still able to fit straight lines

through this data with very high correlation factors. What changes is the

slope of the lines and the x - intercepts. Thus it is important to note

that simply judging the linearity of the regression analysis gives no

indication of how well the background intensity has been corrected.

However, these results help to determine the sensitivity of the technique

to changes in background as the amorphous content changes appropriately

depending upon which background is used. We conclude that the method of

additions is sensitive to changes in background intensity as small as 2

percent

.

The results of Table 23 seem to indicate that the trend which is

present in the data of Table 13 is unaffected by changes in slope of the

background intensity. By changing the slope of the background intensity

the area-by-area determination of amorphous content remains sensitive to

the background intensity however the overall determination lies within the

expected accuracy of the technique. Changing the slope of the background

does change the amorphous content determined for each area , but the change

in area 1 is the same magnitude but of opposite sign of the change in area

3 so in effect the change in area 1 is offset by the change in area 3 and

the resultant weight percent amorphous lies within the expected error of

measurement. The data indicate that the amorphous determination is

sensitive to changes in slope of the background. However, by averaging the

determination from each area to get a resultant measurement of amorphous

content the effects are averaged out and the final weight percent amorphous

remains insensitive to the change. For changes in background intensity of

2 percent the average change in amorphous content was 0.23 weight percent.

We take this (or two times this) as the level of residual systematic errors

due to background.

One suggested explanation of the trend evident in the data of Table 13

is that the areas chosen as regions of the amorphous halo were not free

from interference with tails of the Bragg peaks. Specifically, the

increased amorphous intensity of area 3 may be due to the additional

intensity of the tails of the (110) and (111) reflections. The regions of

amorphous intensity were selected carefully from the strip chart pattern of

a 50:50 mixture of cristobalite with fused qiiartz collected on AM-1, Figure

9. However, a small change in the alignment of the instrument or the fact

that AM- 2 was used for the actual amorphous determination may have been

enough to shift the peaks slightly as well as the region of the tail into

the designated amorphous region. Although plots of the data for sample

211, Figure 10, clearly indicate that the reference peaks were not shifted

into the amorphous areas, the fact that the (110) and (111) peaks were not

collected as reference lines does not allow adequate information as to

whether or not the tails of these peaks interfered with area 3 . Thus data

51
Figure

9.

Amorphous

Regions

and

Reference

Lines

used

in

Amorphous

Phase

Quantitative

Analysis

of

Cristobali

te

.
Intensity

CPS

Intensity

CPS

Intensity

CPS

Two-Theta Degrees

Two-Theta Degrees

30.6 30.8 31.0 31.2 31.4 31.6 31.8 32.0 32^ Two-Theta Degrees

Two-Theta Degrees

Two-Theta Degrees

Two-Theta Degrees

Figure 10. Measured Intensities of Amorphous Areas and Reference Lines

for Sample 211.

53
for sample 211 was recollected on AM-1 in an effort to isolate the (110)

and (111) regions of the cristobalite pattern including the tail region.

AM-1 was operated at 40 kv and 40 ma, and data was collected from 15 to 35“

20 with a step width of 0.01“ 20 with a count time of 2 seconds. Two

reference lines were collected so that shifts in peak position could be

noted and all data normalized to that value. The data from the two

measurements were compared visually (Figures 9, 10, and 11) and on a couints

per second scale to determine if the tails of the (110) and (111)

reflections were contributing to the integrated intensity of amorphous area

3 and if so how much of an effect did this contribution have on the

amorphous content determination.

While the tail of the (110) reflection did not contribute any

intensity to area 3, it was noted that the tail of reflection (111) did

extend into area 3. In order to estimate the effect of this overlap on the

amorphous content determination, area 3 was redefined as the interval from

26.0 to 27.035 degrees two-theta and the x-ray data collection files for*

samples 211 and 304 were modified to reflect this change. The modified

files were processed by Q85AMR and the results appear in Table 24. It is

evident from these results that intensity from the tail of the (111)

reflection added to the intensity measured in amorphous area 3. By

narrowing the region of collection the measured amorphous content

determined from area 3 decreases as expected. The estimated contribution

of the uncorrected intensity of area 3 on the amorphous content

determination is 0.24 weight percent, which is the standard deviation of

the two average amorphous content values for sample 304 as reported in

Tables 13 and 24. The estimated contribution of the uncorrected intensity

of area 3 on the amorphous content determination for sample 211 is 0.29

weight percent. The systematic trend identified in the results of Table 13

is the result of the incorrect assumption that the intensity of area 3 is

due solely to amorphous scattering corrected for background.

Nakamura et al.®, observed that different amorphous silicas do show

slight shifts in the position of the diffraction halo. An effort should be

made to match the position of the halo of the diluent to the position of

the halo in the original material before implementation of the modified

additions method. Unfortunately there is not enough amorphous silica

present in the cristobalite to identify the position of the halo. Fused

quartz was chosen as the most likely approximation of any amorphous silica

present in the cristobalite.

In order to evaluate the effect of the diluent on the amorphous

content determination, the spiking analysis was performed using another

sample from bottle 304 and silica gel as the amorphous diluent. To

properly isolate the effect of diluent on the amorphous content

determination area 3 was redefined such that it was free from overlap with

the tail of the crystalline (111) reflection. The results of the spiking

analysis of sample 304 with silica gel as amorphous diluent appear in Table

25 . Although the average amorphous determination is very close to the

value for sample 304 reported in Table 24 (within the expected uncertainty

of the measurement) the area-by-area determinations of amorphous weight

fraction show a systematic trend. The data in Tables 24 and 25 suggest

that the structure of the fused quartz diluent is a better approximation of
(SdG) XilSNHlNI

COLU

UUan

CDUJa

55

Figure

11.

X-Ray

Diffraction

Pattern

of

Cristobalite

Sample

211

from

15

to

35

Degrees

Two-Theta,

(CuK^^,).
Table 24. Results of the Modified Method of Additions for Determination of

the Amorphous Silica Content in Cristobalite Samples 211 and 304 using

Redefined Area 3

AMORPHOUS REFERENCE SLOPE INTERCEPT CORRELATION WEIGHT LEAST

AREA LINE PERCENT SQUARES

ESD

SAMPLE 211

Area 1 101 0.2442 0.0048 0.998 1.97 0.44

Area 2 101 0.0852 0.0016 0.999 1.90 0.24

Area 3 101 0.0565 0.0010 0.999 1.72 0.26

Area 1 102 2.2410 0 . 0447 0.998 2.00 0.47

Area 2 102 0.7821 0.0150 0.999 1.92 0.25

Area 3 102 0.5189 0.0090 0.999 1.74 0.27

Area 1 200,112 1.3341 0.0261 0.998 1.96 0.45

Area 2 200,112 0.4656 0.0088 0.999 1.88 0.25

Area 3 200,112 0.3089 0.0053 0.999 1.71 0.27

RMS

Area 1 ALL 1.98 0.02

Area 2 ALL 1.90 0.02

Area 2 ALL 1.73 0.02

ALL ALL 1.87 0.11

SAMPLE 304

Area 1 101 0.2280 0.0031 0.999 1.36 0.45

Area 2 101 0.0797 0.0015 0.999 1.87 0.39

Area 3 101 0.0526 0.0007 0.999 1.29 0.28

Area 1 102 2.1007 0.0271 0.998 1.29 0.49

Area 2 102 0.7344 0.0132 0.999 1.79 0.42

Area 3 102 0.4843 0.0059 0.999 1.23 0.31

Area 1 200,112 1.2439 0.0152 0.999 1.22 0.41

Area 2 200,112 0,4349 0.0075 0.999 1.72 0.35

Area 3 200,112 0.2868 0.0033 1.000 1.16 0.25

RMS

Area 1 ALL 1.29 0.07

Area 2 ALL 1.80 0.07

Area 3 ALL 1.23 0.07

ALL ALL 1.44 0.28

56
Table 25. Results of the Modified Method of Additions for Determination of

the Amorphous Silica Content in Cristobalite Sample 304, Using Silica Gel

as Diluent, Four Spiked Samples and Redefined Area 3

AMORPHOUS REFEEIENCE SLOPE INTERCEPT CORRELATION WEIGHT LEAST

AREA LINE PERCENT SQUARES

ESD

Area 1 101 0.1893 0.0038 0.998 2.01 0.53

Area 2 101 0.0980 0.0017 0.999 1.74 0.39

Area 3 101 0.0758 0.0009 0.999 1.22 0.38

Area 1 102 1.7741 0.0313 0.998 1.76 0.59

Area 2 102 0.9180 0.0138 0.999 1.50 0.36

Area 3 102 0.7099 0.0071 0.999 1.00 0.35

Area 1 200,112 1.0481 0.0188 0.998 1.80 0.59

Area 2 200,112 0.5423 0.0083 0.999 1.53 0.34

Area 3 200,112 0.4194 0 . 0043 0.999 1.03 0.33

RMS

Area 1 ALL 1.86 0.13

Area 2 ALL 1.59 0.13

Area 3 ALL 1.08 0.12

ALL ALL 1.51 0.36

57
the structure of the amorphous silica present in the cristobaiite than the

silica gel.

Another method employed to check the modified additions method was

acid washing of the cristobaiite powder as outlined by Altree-Williams et

al^° . Two samples of cristobaiite were washed with hot phosphoric acid in

an attempt to dissolve any amorphous silica present in the powder. Samples

126 and 187 were treated in this way, then the RIR constant and amorphous

content were remeasured using the methods outlined previously. The results

of Table 14 show that the RIR's and weight percent amorphous content

actually increased for the acid washed samples. Careful inspection of the

data for both bottles indicates that the values of RIR and weight percent

amorphous content of the acid washed samples lie within the predicted

uncertainty of the measurements. The change in RIR is less likely to

indicate a change in the actual amorphous content of the sample because of

the factors previously mentioned which limit its accuracy. For sample 126

the reported weight percent amorphous in Table 13 is 1.10 weight percent

amorphous and the reported weight percent amorphous of the acid washed

samples as reported in Table 14 is 1.15 weight percent; a difference of

0.05 weight percent amorphous. The expected precision of this technique at

this level of amorphous content is about 0.50 weight percent amorphous.

The results indicate that the acid wash did not dissolve any amorphous

silica. It is plausible that the amorphous material is located at or near

grain boundaries , instead of at free surfaces which would result from

grinding of the material.

CONCLDSION

The results of the chemical analysis of the lot of cristobaiite, shown

in Table 1, show a minimum of trace impurities. By x-ray standards the

cristobaiite was "phase pure" and suitable for quantitative analysis of the

amorphous silica content.

Based upon the particle size analysis (Table 2) , all bottles appear to

have a uniform particle size distribution. From the lattice parameter

studies, there is some probability of bottle to bottle variability as indicated by the larger SD value for the 16 bottles (Table 5) compared with

the SD value observed for measurement reproducibility (Tables 6-9) . The

possible variability, however, is quite small and should have no observable

effect on the certified percent crystalline content. This is confirmed in

the I^®^ results reported in Table 10; no bottle is judged to be

significantly different than any other.

Fundamentally, the RIR constant is an indicator of crystallinity.

However, limited choice of standard material as well as the effects of

extinction and absorption on the RIR measurement, prevented the use of the

RIR constant to calculate the amorphous content of the cristobaiite. A

change in amorphous content as measured by the method of additions should

correspond to a change in RIR. At this level of detection (0-5 weight

percent amorphous) the RIR constant was relatively insensitive to small

changes in the weight percent amorphous present in the cristobaiite.

The method of additions was modified to allow direct determination of

the amorphous silica content present in synthetic cristobaiite powder. The

58
modification was accomplished by using fused quartz as the diluent and

measuring the background corrected integrated intensity of amorphous

scattering.

The sensitivity of the technique to changes in the approximate

background intensity was evaluated. The method of additions was sensitive

to variations in background intensity as small as 2 percent.

Three areas of the amorphous halo and three cristobalite reference

lines not interfered with by other Bragg diffraction peaks were chosen to

give nine independent measures of the amorphous silica content. Three

sources of error were identified as possible causes of the systematic trend

apparent in Table 13. First, the condition that all areas be free of

interference with Bragg diffraction peaks was not completely met. The tail

of the (111) reflection added intensity to the measured amorphous intensity

of area 3. This caused the determination of amorphous content using area 3

to be consistently higher than that determined with the other areas

.

Another source of error arises from the inability to detect the exact

position of the amorphous halo in the undiluted cristobalite sample. The

mismatch in position of the amorphous halo between the diluent and the

amorphous silica present in the cristobalite contributes to this error. The data indicate that the fused quartz chosen for this analysis was the

best approximation of the amorphous silica present in the cristobalite as compared to the silica gel. Finally, the effect of using ground quartz

powder to simulate the background intensity under the amorphous halo in

cristobalite was examined as a source of error in the measurement. The

estimated contribution of these sources of error to uncertainty in the

measurement was 0.5 weight percent amorphous content.

The average weight percent amorphous silica content in the

cristobalite as determined with this method was 2 weight percent with a standard deviation of 1.0 weight percent. This standard deviation was

based upon an estimated 0.5 weight percent contribution due to random

errors plus a 0.5 weight percent estimation of the systematic errors in the

average value.

59
REFERENCES

1. "Free Silica (Quartz, Cristobaiite , Tridymite) in Airborne Dust,”

NIOSH Manual of Analytical Methods . Volume 5, pp. 259-1-259-13, may be

ordered from the Superintendent of Documents, U.S. Government Printing

Office, Washington, D.C. 20402 or the National Information Service,

Springfield, VA 22161.

2. Frank H. Chung, "Quantitative Interpretation of X-Ray Diffraction

Patterns of Mixtures .1. Matrix- Flushing Method for Quantitative

Multicomponent Analysis," J . Annl . Crvst

.

. 7, 519-525, 1974.

3. Torben Knudsen, "X-Ray Methods for the Determination of the Amounts of

Glass Phase in Mullite Refractory Grog," X-Rav Spectrometry . 5, 197-199,

1976.

4. S.M. Ohlberg and D.W. Strickier, "Determination of Percent

Crystallinity of Partly Devitrified Glass by X-ray Diffraction," Journal of

the American Ceramic Society . 45, 170-171, 1962.

5. L. Cervinka and J. Dusil, "Determination of Crystallinity in

Crystallized Glasses by X-Ray Diffraction," J. Non-Crvst. Solids . 21, 125-

136, 1976.

6. Kazuo Torii, Masami Hotta and Makotta Asaka, "Quantitative Estimation

of Mordenite and Clinoptiloiite in Sedimentary Rocks (I)," J. Japan. Assoc.

Min. Petr. Econ. Geol, . 74, 251-264, 1979.

7. National Bureau of Standards Certificate of Analysis, Standard

Reference Material 1878, Respirable Alpha Quartz (Quantitative X-Ray Powder

Diffraction Standard)

.

8. Toshihiro Nakamura, Katsumi Sameshima, Kiyoyuki Okunaga, Yoshitaka

Sugiura and Jun Sato, "Determination of Amorphous Phase in Quartz Powder

by X-Ray Powder Diffractometry and Effect of Amorphous Phase on Relative

Intensity Ratio of Quartz," submitted to Powder Diffraction .

9. Harold P. Klug and Leroy E. Alexander, X-Rav Diffraction Procedures for

Polvcrvstalline and Amorphous Materials . John Wiley and Sons, New York,

1974, pp. 239-241, 366, 541-543.

10. Robert L. Snyder and Camden R. Hubbard, "NBS^QUANT 84: A System for

Quantitative Analysis by Automated X-Ray Powder Diffraction, " available

from authors.

11. Camden R. Hubbard and Robert L. Snyder, "RIR - Measurement and Use in

Quantitative X-Ray Diffraction," submitted to Powder Diffraction . 1987.

12. Howard F. McMurdie, Marlene C. Morris, Eloise H. Evans, Boris

Paretzkin, Winnie Wong-Ng and Camden R. Hubbard, "Methods of Producing

Standard X-Ray Diffraction Patterns," Powder Diffraction . 1, No. 1, 40-43,

March 1986.

60
13. R.L. Snyder, C.R. Hubbard and N.C. Panagiotopoulos , "A Second

Generation Automated Powder Diffraction Control System," Advances in X-RAv

Analysis . 25, 245-260, 1982.

14. N.P. Pyrros and C.R. Hubbard, "Powder- Pattern: A System of Programs

for Processing and Interpreting Powder Diffraction Data," Advances in X-Rav

Analysis . 26, 63, 1983.

15. D.E. Appleman and H.T. Evans, Jr., Report #PB216188, U.S. Dept, of

Commerce, National Technical Information Service, 5285 Port Royal Rd.

,

Springfield, VA 22151.

16. Deane K. Smith, Monte C. Nichols, and Michael E. Zolensky, "POWDlO: A

Fortran IV Program for Calculating X-ray Powder Diffraction Patterns - Version 10," available from authors.

17. W.A. Dollase, "Reinvestigation of the Structure of Low Cristobalite ,

Zeitschrift fur Kristallogranhie . 121, 369 - 377, 1965.

18. Donald R. Peacor, "High-Temperature Single- Crystal Study of the

Cristobalite Inversion," Zeitschrift fur Kristallogranhie . 138, 274 - 298,

1973.

19. Private communication, Lois Johnson, Rutgers - The State University of

NJ , Dept, of Ceramics, Piscataway, NJ.

20. Stephen Altree-Williams , John G. Byrnes and Bernard Jordan, "Amorphous

Surface and Quantitative X-Ray Powder Diffractometry , " Analyst . 106, 69 - 75, 1981.

21. James P. Cline and Robert L. Snyder, "The Effects of Extinction on XRay Powder Diffraction Intensities," Advances in X-Rav Analysis . 30, 447-

456, 1987.

22. Larry W. Finger and Robert M. Hazen, "Crystal Structure and Isothermal

Compression of Fe203 , Cr203 , and V2 O3 to 50 kbars , " J. Annl. Phvs

.

. 51(10),

5362, 1980.

23. Howard F. McMurdie, Marlene C. Morris, Eloise H. Evans, Boris

Paretzkin, Winnie Wong-Ng and Yuming Zhang, "Standard X-Ray Diffraction

Powder Patterns from the JCPDS Research Associateship , " Powder Diffraction . 2(1), 45, 1987.

24. B.D. Cullity, Elements of X-Rav Diffraction . Addison-Weslev Publishing

Company, Inc., 1978.

61
NBS»TT4A (REV. 2««C) U.«. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET fSee instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 88-3742

4.

TITLE AND SUBTITLE

Publication 0«|ce

APRIL 1988

CERTIFICATION OF SRM 1879 RESPIRABLE CRISTOBALITE AS A QUANTITATIVE X-RAY

DIFFRACTION SRM

5.

AUTHOR(S)

Madeline A. Kuchinski and Camden R. Hubbard

6.

PERFORMING ORGANIZATION (If joint or other than NBS. see instructions) 7, Contract/Grant No. NATIONAL BUREAU OF STANDARDS -

U.S. DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, State, ZIP)

. Type of Report & Period Covered

NBS

10.

SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

Well characterized, highly crystalline standard materials are essential for

accurate quantitative x-ray powder diffraction work. S3mthetic cristobalite powdep

was certified as a Standard Reference Material for quantitative x-ray diffraction

analysis of cristobalite content. Qualitative analyses by Guinier de Wolff camera and

focusing diffractometer of the bulk powder indicated only crystalline a.- cristobalite.

Elemental analysis revealed only trace impurities; the largest being A1 at 1000 ppm.

Quantitative determination of the crystallinity of the material was performed. A

quantitative technique which employs a modified method of additions to directly analyze

for low amorphous content in crystalline matrices was developed and tested. Known

amounts of amorphous material are added to the starting powder ("spiking"). The method

uses the ratio of a measure of the intensity of the amorphous phase to the intensity

of a reference line from a crystalline phase. A critical step of the method is to

correctly establish the background intensity. A completely crystalline material of

similar scattering power was used to establish background intensity. An amorphous

phase close in composition to the amorphous phase existing in the analyte was used

for spiking. The certified crystalline purity of the cristobalite SRM 1879 was 98.0

weight percent crystalline cristobalite. The estimated standard deviation was 1.0

weight percent arising from a standard deviation of 0.5 weight percent due to random

and inhomogeneitv errors plus an estimated likely systematic error of 0.5 weight percent .

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

amorphous content; cristobalite; crystallinity;

quantitative analysis; x-ray powder diffraction

13. AVAILABILITY

Standard reference material;

14. NO. OF

PRINTED PAGES

I yI Unlimited

I I For Official Distribution. Do Not Release to NTIS

I I Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

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jX~| Order From National Technical Information Service (NTIS), Springfield, VA. 22161 $13.95

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