des Monnaies frappees sous L’Empire Romain, communement appelees Medailles Imperiales,” 2nd ed., Rolfin and Feuardent, Paris, 1880. (3) Darken, L. S., Gurry, R. W., “Physical Chemistry of Metals,” p. 444, McGraw-Hill, Sew York, 1953. (4) Gruebner, H: A . , “A Catalog of the Roman Coins in the British Museum,” London, 1910. (5) Hall, E. T., Archaeomeky 3, 29-35 (1960). ( 6 ) Zhid., 4, 62-6 (1961).
(7) Hall, E. T., Endeavour 18, 83-7 (1959). (8) Hall, E. T., “Recent Advances in Conservation,” pp. 29-32, Butterworths, London, 1961. (9) Hall, E. T., Roberts, G., Archaeomelry 5 , 28-32 (1962). (10) Hornblower, A. P., Ihid., pp. 108-12. ( 1 1 ) Jost, W., “Diffusion in Solids, Liquids, Gases,” pp. 234-5, Academic Press, New York, 1952. (12) Kilday, B. A,, Michaelis, R. E., A p p l . Spectry. 16, 136-8 (1962).
(13) Liebhafsky, H. A., Winslow, E. H., Pfeiffer, H., ANAL. CHEM. 3, 2240R (1960). (14) Seaby, H. A., “Roman Coins and Their 1 alues,” B. A . Seaby, Ltd., London, 1954. (15) Sydenham, E. A , , “The Coinage of
the Romm Republic,” Spink, London, 1952.
RECEIVED for review January 20, 1964. Accepted March 25, 1964.
Qua nti ta tive Mineral Ana lysis of Kaolin-Bearing Rocks by X-Ray Diffraction ERIC NISKANEN Ontario Research Foundation, Toronto 5, Ontario, Canada
b An x-ray diffraction method has been successfully applied to the quantitative determination of all the minerals detected in a particular type of kaolinized rock from northern Ontario, including kaolin, quartz, illite, calcite, and hematite. The direct analysis method, with an experimental determination of the mass absorption coefficient of the sample, was used in preference to the more generally accepted internal standard method. The effects of the important experimental variables (particle size, preferred orientation, and selection of standards) have been studied. When the method was applied to a series of synthetic mixtures whose components ranged in concentration from 3.3 to 80%, the absolute mean error o f the analytical results was 1.570.
T
HE theoretical basis for quantitative phase analysis by x-ray diffractometry has been detailed thoroughly in the literature (4, 8 ) . Generally, in these papers, the suitability of x-ray diffraction for phase analysis has been demonstrated using synthetic mixtures of pure substances. Actual applications of the technique to practical problems are numerous, but mcjst of the published reports deal with the determination of a single component (often quartz), mixtures of polymorphs (I,%?), or relatively well-defined chemical systems (6). There have been few papers on the application of the method to the complete analysis of complex mineral systems, a notable exception being the work of Black ( I ) on bausite. The present work deals with the quantitative determination of all the detectable minerals in a kaolinized rock from northern Ontario. The analyses were required for a mineral exploration program concerned with mapping the es-
1268
ANALYTICAL CHEMISTRY
tent of the kaolin deposit, and a study of the associated minerals, as a prelude to possible beneficiation procedures. The aim of the analytical program was to provide &-ithin a reasonable time, the complete mineralogical analysis of the samples as they were sent in from the field. To begin with, an accuracy of & l o % (absolute) was expected for the major components. As work progressed, efforts were made to improve the accuracy of the results by modifying the experimental procedure. The direct analysis method, first described by Lerous and coworkers (9, 10) was adopted, since it was felt that this technique was more amenable to multicomponent analysis, and probably as accurate as the internal standard method. Basically, all that is required is a measure of the diffraction intensity from a given component, the corresponding intensity from the pure component (which needs to be measured only once), and the mass absorption coefficient of the sample, using the same monochromatic radiation as for the diffraction measurement. The weight fraction of a given component, X1, can then be calculated using the equation
where
ZI
=
(I&
=
pa*
=
p1*
=
diffraction intensity of component 1 diffraction intensity of pure component 1 mass absorption coefficient of the sample mass absorption coefirient of pure component I
The difficulties involved (and these are common to any quantitative diffrartion procedure) are concerned with: controlling effects due to particle size
and sample inhomogeneity, reducing orientation effects (S), selecting suitable pure components, and correcting instrumental errors such as coincidence losses of the detector. I n practice, particle size esert,s considerable influence on both the intensity of a given diffraction peak ( 5 ) and the reproducibility with which this intensity can be measured. Accordingly, a study was made of the effect of grinding on the diffracted intensity from a given rock sample. Measurements were also made to determine under which conditions the intensity measurements were most reproducible. .A study of the effects of preferred orientat,ion on the anal minerals has recently been reported (11) by the author. I n that work, virtual elimination of preferred orientation of well-crystallized kaolin was achieved using an “end-loading” sample holder. The same holder was used in the present work for the kaolin det,erminations. The select,ion of standards for quantitative diffraction analysis has recently been discussed by 13rindley ( 2 ) . I n the present work, pure minerals were used as standards at first, but the analytical result’s, especially for quartz, were not entirely satisfactory. I t was felt that applying the same grinding procedure to both t,he pure standards and the analytical samples did not produce sufficiently similar particle size distributions, part,ly because of the moderating effect of the clay minerals on the grinding action, The suitability of using a single misture of pure minerals, approsimating the composition of a typical rock sample, was evaluat’ed. On the basis of the results obtained in these experiments, a practical analytical procedure was developed. X series of synthetic niistures of pure minerals was then analyzed to check the precision of the method.
EXPERIMENlA1
Instrumentation. A Philips x-ray diffractometer was used, normally under the following operating conditions, for the diffraction scans and intensity measurements: copper-target x-ray tube a t 30 k v . and 20 ma.; nickel filter; goniometer scan speed: 1'' per minute for full scans, 0.5" per minute over peak:, for planimeter measurements; 1 " divergence and scatter slits, 0.006-inch receiving slit; Geiger tube detector operated at 1490 volts; rate meter constants: scale factor 8, multiplier 1, time constant 4 ; and recorder chart speed of 30 inches per hour. In the final step of the grinding procedure, a Spex Model 5000 mixer-mill was used. The grinding vial, No. 5004, consisted of a Lucite cylinder with tungsten carbide-lined end caps, and two tungsten carbide grinding balls. Grinding Test. TI-e diffracted intensity was measured on samples of - 140 +200-mesh roc'< powder which had been ground fu.rther for successively longer periods in t h e Spex mill. Two-gram samples were milled for 15 seconds to 10 ninutes. Each sample was then packed into a ringtype specimen mount for t h e rotating specimen holder. The intensities of the 4.26- and 1.81-A. quartz lines were determined by accumulating 32,000 and 16,000 counts, respectively. Background counts were made beside each peak by accumulating 3200 and 1600 counts, respectively. Each peak measurement was repeated After the sample had been repacked into the holder. *\I1 counting rate calcu1atio:is nere corrected for coincidence losses according to Klug and hlexander ( 7 ) . The final results were taken as the average of the two corrected measurements. The intensities of the original coarse material were measured in the same manner, but in quadruplicate. X few drops of water were mixed with this coarse powder to help keep it in the sample holder. Reproducibility. l'he reproducibility of t h e 1.81-A. quartz line intensity was determined on a number of samples by packing the sample into a holder ten times, and measuring the peak intensity each time by accumulating 32,000 counts. On one packing the background was measured. The counting rates were corrected for coincidence losses, arid t3e relative standard deviations c a h l a t e d from the ten values of the correcled net intensity. The following five samples were studied: a rock sample crushed in a mortar to pass 325-mesh, a sample of the same rock crushed to -325-mesh as above, and then milled for 4 minutes, pure quartz crushed in a mortar to -325mesh, -325-mesh quartz milled for 4 minutes, and a synthetic mixture containing 33% quartz, 60'% kaolinit'e, 7% calcite mixed from -323-mesh material, and then milled for 4 minutes. Standards. To evaluate the suitability of using a mixture of pure minerals as a standard, the following three samples were prepared: (1)
pure quart'z, (2) a mixture containing 55.5y0 quart'z, 40yGkaolinite, and 4.5% calcite, a n d (3) a rock sample, which by chemical analysis was known to contain 62% quartz. Each sample was crushed to -325-mesh in a mortar, and then milled for 4 minutes. (The components for the mixture were crushed first, then weighed out, mixed, and milled.) The 1.81-.\. quartz line intensities were measured by counting. The quartz content of the rock sample was calculated from Equation 1, using both the pure quartz and the mixture as a standard. Analytical Procedure. T h e analytical procedure was divided into four main steps: preparation of sample, measurement of diffraction intensities, measurement of absorption coefficient, and calculation of results. T h e samples collected in the field were in the form of fairly large rock chips or pieces of drill core. These were crushed finer than 'j4-inch in a jaw crusher. About one-half pound of crushed rock was split out for x-ray analysis. After additional crushing, a 3-graIn sample was selected from this by quartering and ground in a mortar until it passed a 325-mesh screen. Gentle brushing with a small paint brush was used to speed the screening operation. h sample of the -325-mesh powder was packed into the end-loading holder for a preliminary scan from 3" to 63" (26) on the diffractometer. By comparison with known patterns, the components present were identified. For quantitative results, the kaolin and illite intensities were measured on the -325-mesh powder samples in the end-loading holder, by scanning a t 0.5" per minute over the 7.15- and 1.49-h. lines for kaolin, and 1.50-h. line for illite. Integrated intensities Lvere determined by measuring, with a planimeter, the areas under the recorder traces of these peaks. Xormally duplicate measurements were made after repacking the sample. The final result was taken as an average of the two readings. X sample of fine quartz powder molded in Lucite was used to standardize the x-ray intensity, so that comparable results could be obtained from day to day. h 2-gram sample of -325-mesh powder was milled for 4 minutes in the Spex mill before the line intensities of the quartz and calcite were measured on samples of milled powder in the rotating specimen attachment, using a ring-type holder filled from the back. Net line intensities were determined by counting a t the peak positions (1.81 ;2. for quartz and 3.03 A. for calcite) and a t suitable background positions. Counting rates were as high as 300 c.p.s. for the quartz line, and 150 c.p.s. for the calcite line. XI1 counting rate measurements were corrected for coincidence losses. The hematite intensity was normally determined satisfactorily by measuring the height of the 2.69-1. peak on the recorder trace of the preliminary full scan. The diffraction intensities of the pure
components were measured using the same experimental conditions. The mixture of quartz, kaolinite, and calcite, as described in the previous section, was used as a standard for quartz and calcite. The standard for the kaolin mineral, chosen on the basis of half width of the basal reflection, was a sample of pure dickite. Relatively pure illite was used as a standard, and a rock sample which had been chemically analyzed for FenOs was used as a hematite standard. The method used for measuring the mass absorption coefficient is basically the same as that described by Lennox (9). Each rock sample, having been milled as described, was carefully packed into a standard Philips holder, so that both surfaces were level with the faces of the holder, and the density of the powder compact was as uniform as possible. h bracket was constructed to position the holder in front of the receiving slit of the diffractometer. The nickel filter was removed. Using a lithium fluoride crystal a t the axis of the goniometer as a monochromator, the detector was set a t 44.96" to receive C u K a radiation. With an absorber of pure quartz powder in position, the tube controls were set to give a counting rate of about 400 counts per second. Each prepared rock sample was inserted in t.urn, and the transmitted intensity was determined by accumulating a fixed number of counts. Coincidence loss corrections were applied as usual. The initial intensity, Io, being much too high for direct counting, was determined indirectly, using absorbers of n and ( n 1) foils of 0.002-inch brass shim stock, which had previous!y been selected so that each foil had the same absorbance. The following equation was then applied.
+
T , and T("+')are the intensities transmitted by n and ( n 1) foils, n usually being 3. When the weight, W , of the powder in the cavity has been determined in grams, the mass absorption coefficient can be calculated using the equation:
+
where
T
intensity transmitted by the sample, c.p.s. ab = area of sample surface = 2.06 X 1.04 cm. = 2.14 sq. cm. The results were then tabulated, and the weight fraction of each component was calculated, using Equation . 1. In actual practice, the samples were analyzed in groups of a t least ten, whenever possible, so that a series of similar measurements could be made a t one time, with a consequent saving of time and labor. Under these circumstances, the average time for the complete analysis of one rock sample was approximately 2'/2 man-hours. =
VOL. 3 6 , NO. 7,JUNE 1964
1269
Table 1.
quartz remains loose in the grinding vial, a mixture of quartz and kaolin tends to pack tightly into the ends of the vial, with the result that the quartz in the mixture retains a larger average particle size and therefore produces less reproducible results. Standards. Agreement with the chemical result was closer when the quartz-kaolinite-calcite mixture was used as a standard for quartz. This synthetic mixture gave a 1.81-A. quartz line intensity of 327 C.P.S. (calculated to l O O ~ , quartz) and a n analytical result of 60% when used as a standard for the test rock sample. The 1.81-A. intensity for pure quartz was 280 c.P.s., giving an analytical result of 70% for the rock sample which by chemical analysis contained 62% quartz. This effect again appears to be due to the difference in particle size obtained when clay minerals are present in the sample being milled. Typical Results. T h e results obtained on a number of rock samples are presented in Table 11. These samples were selected to show the concentration ranges within which the component minerals normally occurred. Generally, the results indicate t h a t the direct method of analysis is satisfactory for the quantitative analysis of multicomponent mixtures, provided t h a t sufficient care is taken t o control the various experimental variables which have been discussed. The determination of illite was particularly troublesome, mainly because of its extremely poor and variable crystallinity. Often, the presence of as much as 207, of illite was suspected (by difference) but was not evident from the normal diffraction pattern. Its
Reproducibility of the 1.81 -A. Quartz Line Intensity
Relative standard deviation, 70 StaR Z tionary ing holder holder 9 1 7 2 3 6 4 3 28 1 17 2 6 3 2 0
Sample Rock (-325-mesh) Rock (milled) Quartz (-325-mesh) Quartz (milled) Quartz mixture (milled)
7 6
6 7
RESULTS A N D DISCUSSION
Grinding Test. T h e variations of the 4.26- and 1.81-A. quartz line intensities with milling are shown in Figure 1. Both lines increase in intensity rapidly, reach a peak after about 1 minute, and then decrease almost linearly as milling is continued. T h e importance of uniform, reproducible grinding can be readily seen. The grinding procedure adopted-Le., starting with - 325-mesh powder and milling for 4 minutes-produced powder samples with a n average particle size of about 5 to 10 microns, as determined by settling tests. Reproducibility. The results of the reproducibility measurements are shown in Table I. Reduction of particle size beyond -325-mesh by milling, and the use of a rotating sample holder, both contribute considerably to a n improvement in reproducibility. T h e presence of clay in the synthetic mixture reduces the reixoducibility of the quartz line inteniity. I t is felt that this effect is a t least partly due to the hindering of the milling action by the clay. While pure
Table II.
Sample A B C I> E a
Typical Analytical Results on Rock Samples
Kaolin, %
Quartz,
37 29
61 52 59 49 45
23 14 ...
so
Calcite, 70
...
4.5 0.6 1.2
0.5
Illite,
Hematite, %
Total,
...
2 2 2 2 2
100
5%
10” 13a 26
47
I-
40
0
I
2
3
MILLING
4
I
TIME
6
7
8
0
IO
(MlNUTESl
Figure 1 . Effect of milling on intensities of the 4.26- and 1.81 -A. diffraction lines of quattz in rock sample originally - 140 +200-mesh
presence could be verified only by sedimenting the fine clay fraction from the sample and preparing a n oriented specimen for a second diffraction scan. Accuracy. As a n indication of the accuracy obtainable, a series of four synthetic mixtures of kaolinite, quartz, and calcite was analyzed using the analytical procedure outlined above. These mixtures were made from the same pure minerals which had been used as standards. The results are listed in Table 111, compared with the actual percentage of each component. The mean error for each component was obtained by averaging the differences between the x-ray results and the actual percentages, and in each case, is within 2%. However, it is to be expected that the accuracy obtained with actual rock samples will be considerably poorer. From the results obtained on these synthetic mixtures and on a fairly large number of rock samples, it is estimated that the mean error of a series of analyses on rock samples is within 5%.
%
97.5 97.6 92.2 94.5
ACKNOWLEDGMENT
The author acknowledges the assistance of P. G. Stummer with the experimental work.
By difference, allowing about 3% for other minor constituents.
LITERATURE CITED
(1) Black, R. H., A x . 4 ~ .CHEW25, 743
(1953).
Table 111.
Kaolinite, yc X-ray Actual
Mixture 1
19 4 38 9 593
2
3 XZean error
1270
65 1
40 0
50 4 312
800
765
4
Quartz, To X-ray Actual
20 0
600
( 2 ) Brindley, G. W., in “The X-Ray
Analytical Results on Synthetic Mixtures
66 7 50 0 333 167
159
15
ANALYTICAL CHEMISTRY
1 1
Calcite, 7 0 X-ray Actual 13 3 10 0 6 7 3 3
8 8 8 9 7 2 3 7 1 6
Identification and Crystal Structures
Total,
YL
X-ray
Actual
93 3
100
97 7
100 100
98 2
96 1
100
of Clay Minerals,” G. Brown, ed., p. 501, Mineralogical Society, London, 1961. (3) Brindley, G. W., Kurtossy, S. S., Am. Mineral. 46, 1205 (19611. (4) Copeland, L. E., Bragg, R. H . , ..lxua~. CHEY.30, 196 (1958). (5) Gordon, R. L., Harris, G. IV., Safety
in
Mines
Research
Establishment,
Portobello St., Sheffield, England, Res. R e p t . 138 (1956).
( 6 ) Herbstein, F. N., Smuts, J., Van Niekirk, J. N., ANAL.CHEM.32, 21
(1960). ( 7 ) Klug, H. P., Alexander, L. E., “X-Ray Iliffraction Procedures for Polycrystalline and Amorphous Materials,” p. 281, Wiley, New I’ork, 1954. (8) Zbid., p. 410. (9) Lennox, D. H . , ANAL. CHEM. 29,
766 (1957).
(10) Leroux, J., Lennox, D. H., Kay, K.,
Zbid., 25, 740 (1953). (11) Niskanen, Eric, Am. Mineral., in
press. (12) Spurr, R. A., Myers, H., ANAL. CHEM.29, 760 (1957). RECEIVED for review December , 2 , 1963. Accepted March 26, 1964. 10th Ottawa
Symposium, Canadian Association of Applied Spectroscopy, September 17, 1963. Analytical work supported by Franc. R. Joubin and Associates on behalf of the AIgoma ,Central and Hudson Bay Railway and Coppercorp, Ltd. Development of techniques supported by the Department of Economics and Development, Government of the Province of Ontario.
Rapid Ancrlysis of Rocks by X-Ray Fluorescence P. R. HOOPER Department o f Geology, University College o f Swansea, Singleton Park, Swansea, Great Britain
A series of rocks and refractories of diverse chemical composition has been analyzed for Mg, AI, Si, PI K, Ca, Ti, Mnl Fe, and Sr on a Philips all-vacuum x-ray spectrograph. In all cases the precision is better than that obtained by analysts for W1 and GI. An estimate of error indicates that matrix effects can be overcome if a borax dilution method is used with concentrations exceeding 2‘% of oxide in the rock and that analyses by x-ray fluorescence are much quicker and probably more accurate for most elements than routine gravimetric, rapid, and optical spectrograph analyses undertaken by the average geologist.
U
of x-ray analysis has increased rapidly during the last 20 years and is now an accepted method for the accurate determination of many elements. Its application to geology has been delayed because of the comparatively weak radiation of the more abundant rock-forming elements (sodium to titanium) and the diverse matrices usually present in rocks, which cause a variable absorption of the radiation. Recent technical improvements, including the development of the allvacuum spectrograph, have brought most of the light elements within the range of the instrument ( f - 6 , 9, IO) while Claisse ( 7 ) has described a borax dilution method that reduces the matrix effect. SE
EXPERIMENTAL
Reagents. T h e present investigation of the accuracy and speed of x-ray fluorescent analysis of rocks and minerals involved the use of standards W I and GI and S a t i o n a l Bureau of Standards samples 1-1 (argillaceous limestone), 76, 77, 78 (burnt refractorirs), 98 (plastic clay), a n d 99 (soda-feldspar). A series of intrusive rocks from granite t o anorthite-gabbro and a number of shales have been analyzed for Mg, .llj Si, P, K, Ca, Ti, hln, Fe, and Sr on a Philips allvacuum spectrograph.
Procedure. Two methods of sample preparation were employed. F o r trace elements and low concentrations of Mg and P, pure rock powder was used. For the major elements the rock powder, fused ‘with nine times its weight of borax glass, gave R more accurate result with only a slight loss in precision. When pure powder is used it must be very fine-grained. The rock chips were ball-milled and hand-ground until they passed through a 200-mesh nylon sieve, and then ground for 10 more minutes in the ball mill. The boras dilution method ( 7 ) requires accurately weighing 1 gram of powder and 9 grams of borax glass, mixing, fusing over a Meker burner in a platinum crucible, and cooling the melt. I n the present investigation the boras beads were then ground in a ball mill for 10 minutes. A fully stabilized all-vacuum Philins x-ray spectrograph, Model PW 1.540 with tungsten target, was used. The analyzing crystal, counter, and operating conditions found most convenient for each element are listed in Table I. I n each case the major Kcr reflection was measured. Using a pure sample of the element required, the peak was located exactly by counting over it and the most suitable pulse height channel was selected on the discriminator. Background positions were found by scanning a typical rock sample on either side of the peak. The radiation was recorded in 64second counts. Seven 64-second counts were normally recorded and averaged, but this was increased for elements with a low count rate, and vice versa. One sample was used as a standard throughout and mas measured every fourth run. This allowed the instrumental precision t o be calculated and any fault to be immediately apparent. Careful temperature control is important, especially when the flow proportional counter is being used. A completely air-conditioned room with good temperature control is strongly recommended. The present work was undertaken without these benefits, but a simple water-cooling system attached to the gas lead into the flow counter gave surprisingly good results.
The concentration present was obtained from a calibration curve relating x-ray intensity to chemical composition. In most cases the calibration curve used was the straight line joining the plots for W1and GI, but in rare case9 where this was unsuitable-e.g., h1203other well-established standards were used. RESULTS
The precision was measured by counting the standard sample many times and calculating the standard and relative deviations for each element. These values are recorded in Table 11, where they can be compared with the results of the precision tests carried out by many analysts using varying methods in different laboratories ( 2 2 ) . The precision values quoted are strictly correct only for the standards, because a fixed time and not a fixed count was used. Thus lower concentrations have a lower precision, and vice versa. The experimental results indicate, however, that it is only in the case of magnesium, where precision, was sacrificed for the sake of time, that lack of precision had a significant effect on the estimated error. I n all cases the precision can be improved by increasing the total number of counts. The matrix effect--that is, the varying absorption of radiation by varying matrices-while not apparent in the precision test, is a real hazard in obtaining the true concentration of an element by x-ray methods. If the intensity of silicon radiation of simple rock poa ders is plotted rtgainst chemical analysis, a general trend is apparent but individual rocks show a considerable deviation. That this is due to relative absorption by the various matrices is indicated by the use of a borax dilution which reduces the scatter to a remarkable degree. The maximum likely error of the x-ray method has been ebtimated by plotting chemical results against x-ray intenaity, drawing a calibration curve through W,and GI, and calculating the standard and relative deviations of the individual V O L . 36, NO. 7 , J U N E 1964
1271
