V O L U M E 25, NO. 5, M A Y 1 9 5 3 therefore not due in a significant degree to the wave-length difference but is largely, if not solely, due to microabsorption effects as dealt with by Brindley ( 8 ) ,Wilchinsky ( 9 ) and others. However, the. effect on the practical usefulness of the technique is small by currently accepted standards of acouracy for x-ray diffraction analysis. APPLICATION O F DIFFRACTION-ABSORPTION EQUATION IN ANALYSIS
The application of the diffraction-absorption equation in analysis of powder crystallites rests upon determining the value of C for the crystallite under study, the particular instrument and tube target used, and the sample preparation technique employed; the variation in C value determines the analytical accuracy attainable. It is necessary, as an initial step, to determine the diffraction correction factor. This may be done by preparing a series of weight fractions of the test crystallite in association with one or more crystdlit,es having tho same tatakabsorption coefficient 8;s the test material. The C value may then be found by determining the diffraction and absorption ratios for a series of weight fractions of the test crystallite associated with materials having mass absorption coefficient.sover a wide range. The use of the same specimen, t.arget, and tube voltage for both diffraction and absorption messuremenbs is subject to certain limitations. Diffraction may be influenced by the recognized factors discussed by Brosky (a), Kay (6),and Klug et al. (7). Ahsorption limitations already discussed in this paper may relate to the effect of associated crystallites in a mixture on conformity of transmitted intensities to the range of linearity of the Geiger tube. In these instances, change in sample thickness, or reduction of tube voltage, or use of another target to yield shorter wave-length radiation is indicated. Absorption discontinuities become important when two wave lengths are employed
143
but the range of absorption coefficients met in practice is significantly less than the range used experimentally in this labaratory and a quartz associate giving rise to absorption discontinuity difficultieshas yet to be identified in any sample received here for quarts analysis. Similar Considerations are involved for the analytical problems typical of the ceramic and refractory, paint, cement, fertilizer, rubber and plastic industries, and the use of two wave lengths, if necessary, should present no problem in these cases. It is reasonable to expect that some analytical applications will involve compounds containing elements invalidating the diffraction-absorption q u a t i o n due to absorption discontinuities when two wave lengths are used. This may occur, in particular, in the metallurgicsl field, where materials commonly contain elements such as nickel, copper, and zinc. In these cases use of a single wave length for absorption and diffraction measurements is indicated. A short wave length mould be suitable if superposition is not 8. factor and 8. long wave length if the absorption rango to be covered is limited. LITERATURE CITED
P.. ANAL.CHEM.,20, 856 (1949). (2) Brindley, G. W., Phil. Mag., 36, 347 (1945). (3) Brosky, S., Pitfsbuvgb Tesling Lab. .??em, 12, 2 (1945). (4) Compton, A. H., and Allison, S. K.. "X-Ray in Theory and Eaperiment," p. 802, New York, D. Van Nostrand Co.. 1947. (5) Internationale Tabellen eur Bestimmund von Kristallstrukturen. Birmingham, Kynooh Press, 1952. (6) Kay, K., Am. Ind. Hug. Assoc. Quarl.. 11, 185 (1950). (7) Kluc, H. P., Alexander, L., and Kummer. E.. J . Ind. Hug. Tozieol.. 30, 166 (1948). ( 8 ) Parish, W., Science, 110, 368 (1949). (9) Wilchinsky. Z. W.. Acta Cndsf., 4, I (1951). (1) Alexander. L.. and Klug, H.
R l c n v n n for ieview October 24. 19i2. Accepted Jannsry 21. 1963. Presented before the Pittsburgh Conferonce on Analytiozl Chelnirtry arxd Applied SDceti.o5eow. M a d 1832.
of auxite
Samples
A n X-Ray Diffraction Method RORSON H. BLACK Aluminium Laboratories Ltd., Amido, Quebec, Canado
VV.
HEN a new bauxite deposit is being evaluated thousands of drill hole samples must be examined. To analyze them chemically, even by short-cut methods, has always been slow and expensive. A spectrographic method was described by Churchill and Russell (7), but x-ray diffraction appeared more promising for our purpose beoituse it gives information on the compounds present, rather than only on the elements or chemical groups. A number of workers have described applications of the Geiger-counter x-ray spectrometer t o quantitative analysis of two-component mixtures, or to the determination of one camponent (usually quartz) in various matrices (4,6, 8, 10-12). A survey of the literature has not, however, revealed any method whereby quantitative x-ray diffraction analysis of multioomponent mixtures, has been accomplished on a mass-production basis. EQUIPMENT
Figure 1 shows the North American Philips' Geiger-counter spectrometer (Model 12021) and auxiliary equipment. At the far left is a strip-chart recorder used in producing patterns for qualitative analysis. The same cabinet houses a Sorensen 2-kva. regulator to stabilize the power supply for this and the other units, as well as a time switch to turn on the power early each morning for warm-up purposes. The spectrometor proper has been de-
l c1
Figure 1.
Xorelco Spectrometer and Auxiliary Equipment
scribed in the literature (9). An iron-tarpet x-my tube and a
.~~~ ~
~
ceived and p r h t the i n t e g k e d intensities as numbers on a paper tape. The box is insulated t o muffle the sound of the mechanism.
ANALYTICAL CHEMISTRY
144
The Norelw spectrometer has been applied to the analysis of bauxite in order to reduce the wst of wet ehemieal methods and to provide additional information. Diffracted intensities are wnverted to mineral percentages via calibration curves based on chemically analyzed samples. Gibbsite, boehmite, kaolinite, quartz, and total iron minerals are determined; equivalent percentages of water, silicon dioxide, ferric oxide, and aluminum oxide are calculated therefrom. Comparisons with chemical values indicate a standard deviation of about 1 to 2% ahsolute. Routine samples require about 10 man-minutes each. Increased information on the mineral eomposition and a fivefold reduction in labor relative to chemical methods have resulted. The x-ray diffraction analysis of other materials may be facilitated by a printing-type recorder, a calculating hoard which compensates for drifts in x-ray intensity and (in part) for matrix changes, and punched card equipment for computing and listing results.
eter and illustrates the shielding which was added as a precautionary measure. One shield (made of '/winch lead) turns with the specimen, a second moves with the Geiger counter, and a third is attached to the original cover plate, which is lowered except when changing samples. Apertures are provided so that the specimen can be ''seen" only by the x-ray source and Geigercounter slit. Tests made with film placed around the unit and left for about 10 days showed this type of shieldmg to be entirely effective. BASIS FOR T H E METHOD
. . , ~ ~ ~ ~ ~ ~ ~ ~ duced to a manseeable ;ate hy x scale-of-64 circuitrare countedin t h e recording unit. After 30 seconds the t , i m r r c u t s off t h e pulses; the r e c o r d e r mints its count and reBcts to zero, and a light flashes on. The o n e r a t o r moves the goniometer arm to the next angle and again pushes the button to r e p e a t t h e process. During t h e seven counting periods of 30 seconds each, he has ample time t n prepare another speoimen. A zero is printed to mark the completion of each s a m p l e . E v e r y 90 minutes, the hematite peak intensity is recorded for four preparations of a standard bauxite sample; twice Figure 2. Specimen IIolder, Showing Added Protective the average value is a Shields convenient figure for ~~
~~
~
~
~~~~~~~
The bauxites of greatest interest from a eommerci~lviewpoint are composed chiefly of the minerals listed in Table I. Figure 3 illustrates the strip-chart record obtained for one particular speoimen of bauxite. The less prominent reflections &re lost in the background, but the principal peaks for kaolinite, boehmite, gihhsite, goethite, quartz, and hematite are discernible. Anatase is not normally determined, since it O C O U ~ Sas a nearly constant constituent and is subject to interference from kaolinite. The relative heights of the peaks and of the general background level are related to the proportions of the various minerals present. and this relation forms the basis of the analytical method. It is, however, unnecessary to record the entire pattern in routine quantitative analysis; the intensities a t the six angles marked, and one background intensity (messured a t a point between the boehmite and gihhsite peaks) me all that is required. The exact angular settings are determined in advance; Table I1 lists these, with the corresponding interplanar spacings. PROCEDURE
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Each bauxite sample is received as a powder, ground to -100 mesh. A representative specimen is obtained from the 20 g r a m
Table I. Principal Minerals in Bauxite Name Gibbsite
composition
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Boehmite Kaolinite Hematite Goethite n..
