Determination of Potassium in Silicate Minerals and Rocks by Neutron Activation Analysis JOHN W. WINCHESTER Departmenf of Geology and Geophysics, Massachusetts Institute of Technology, Cambridge
b The determination of potassium in natural silicate minerals by pile neutron activation and by counting the induced /3 radiation in the irradiated specimen without chemical processing is described. An endwindow proportional counter and aluminum absorbers are used for radioactivity measurement. Precislon and absolute accuracy of the method are of the order of 1%, and interference by trace elements in general is less than this. Results are presented for the analysis of the standard granite G-1, diabase W-1, argillaceous Iimestone NBS No. l a , feldspar NBS No. 70, and biotite M.I.T. No. 83203.
T is
HE DETERMINATION of the alkalies in naturally occurring silicate minerals an especially difficult problem in geochemistry. Fairbairn (6) and, more recently, Ahrens (2) have discuwed errors in the analysis of natural minerals and rocks for their major constituenta and found that accuracy using conventional methods is variable, especially for the alkalies. A standard granite, G-1, and a standard diabase, W-I, were distributed to approximately 30 laboratories for major element analysis, and the results were compared. For potassium, the standard deviation of a single measurement was 6.8% for G-1 and 23% for W-1 of the mean of all values reported. For sodium, the standard deviation of a single measurement was 5.2% for G-1 and 9.3% for W-1 of the mean value. These large errors reflect the general difficulty in achieving great accuracy in silicate rock analysis. Current work on the determination of absolute age of minerals by the potassium-argon method, reviewed by Aldrich and Wetherill (J), requires the determination of potassium with much greater precision than previously attainable. Even though most age-determination laboratories have highly refined procedures, uncertainty in potwsium determination still constitutes a major source of error in absolute age measurement. In this work, potassium is determined with high precision in a wide variety of naturally-occurring silicate minerals and rocks by pile neutron
activation and measurement of 0 radioactivity induced in the irradiated sample. No chemical treatment before or after irradiation is required. The feasibility of this procedure has been indicated previously (18-20). Hine et a2. (7) and Reid et d.(13)have described procedures based on similar principlee for determination of alkalies in biological materials, Bradley and Bradley (4) have described the analyak of feldspar crystals for Na, K, and Ca by pile neutron activation, and Brownell et d. (6) have described the rapid analysis of rocka for Si, AI, and Ne by neutron activation with a &-Be source. Minerals containing BB their major elementa 0, Si, AI, Ne, K, Mg, Ca, and Fe, when irradiated in a nuclear reactor, give rise to radioisotopes of these elements in proportion to the amount of each element present. For an irradiation lasting a few minutea followed by a cooling time of 1 to 2 days, short-lived isotopes of 0, Si, Al, Mg, and Ca deoay to a very low level, and long-lived radioactivity induced in Fe is initially very low. Consequently, the bulk of the induced radioactivity remaining is that of Na and K-vie. , the radioisotopes 14.97-hour NaZ4 and 12.46-hour K4z. In the decay of Na24, particles emitted are 1.394-m.e.v. j3, 1.368-m.e.v. 7 and 2.754-m.e.v. y, each emitted in 100% of the decays (16). In the decay of K4*,particles emitted are 3.55m.e.v. 3!, in 82% of the decays and 1.99-m.e.v. 0 and 1.53-m.e.v. y, each in 18% of the decays (16). The present procedure is based on the selective counting of the 3.55-m.e.v. 0 of IC42 using an end-window proportional counter and aluminum absorbers. This procedure, which excludes the radiations of NaZ4,also excludes most of the radiations of trace elements which occur in natural minerals. A similar procedure may also be employed for the selective counting of Na24 radioactivity by measuring ,the high energy 2.754-m.e.v. y in a scintillation counter adjusted to exclude all lower encrgy y radiation. (It is fortuitous that there are practically no trace elements which exhibit such high 0 and y energies in their decays. These selective counting procedures for common silicate minerals, therefore, are
39, Mass.
quite free from interference by trace elements.) EXPERIMENTAL
Grind the sample and potassium acid phthalate standard in an agate mortar and dry a t 110' C. Weigh 30-mg. portions for analysis. The sample should be fine enough to lead to a source of thickness uniform within about 20% for 0 counting. The selfscattering and self-absorption effeot for @ radiation is relatively independent of sample thickness if samples are prepared aa described. Seal each sample and standard for irradiation and counting by either of the following procedures: Procedure A. Transfer the sample to a special1 prepared polyethylene planchet androement in place with 3 drops of ethyl acetate-polystyrene solution. The polyethylene planchet8 are made by heat sealing a ring punched from 0.030-inch polyethylene sheet to a circle punched from 0.003inch polyethylene sheet to form a cup of 0.8 sq. cm. area. Handling with the hands must be avoided, and the polyethylene should be washed before irradiation to prevent contamination by induced 15-hour Na2' radioactivity. Ethyl acetate containing 570 polystyrene is sufficient as a binder. Procedure B. Transfer the sample to a piece of 0.001-inch polyethylene sheet and contain the specimen by heat sealing a second piece of 0.001inch polyethylene. The heat sealing operation may be carried out by usin a heat sealing iron and a plastic via cap as follows: Place a square piece of clean polyethylene on a sheet of paper, transfer the specimen to the center of the square, and carefully lay another square over it. Lift the assembly with the paper onto the heat sealing iron, and press the vial cap for a few seconds onto the assembly so as to seal a ring around the specimen. Check the quality of the seal carefully. Handle with tongs or gloves to avoid contamination which will become radioactive. Mark a number on each specimen. Numbers may be scratched into the polyethylene or may be written on with ink. Since some inks become radioactive in a neutron flux, an ink marking should be cut off before counting. Pack samples and standards for neutron irradiation. To ensure t a t samples and standards receive identical
7
re
VOL. 33, NO. 8, JULY 1961
1007
neutron d o v e , arrange sample and standard specimena alternately. Pack in a waled polyethylene container. Belf-shielding of the neutron flux is negligible. Irradiate for about 6 minutes a t a slow neutron flux (nu) = 5 X 1011 neutrons per sq. cm. per second, or equivalent neutron dosage. Let cool for about 30 hours. During the period 24 to 48 hours after irradiation, some short-lived activities--e.g., 2.68-hour Mn*-have decayed to very low levels and other longer-lived activities still have a counting-rate smrtll compared to 12.5-hour K4*. Count fl radiirtion using an endwindow proportional counter and A1 absorbers of about 700 and 2000 mg. pef sq. cm. The thinner absorber shields all fl's from 15-hour NaZ4and passes w a y s and 10 to 20% of the 3.5m.e.v. B's of 12.5-hour K4*. The thicker absorber shields all B's and passes only y-rays. The net fl counting rate for 12.5-hour K4*is the difference between the two counting rates, a8 the y r a y counting rates with thick and thin absorbers are very nearly identical. If manual counting is employed, i t is convenient to count each specimen four times, using absorbers thick, thin, thin, thick, in that order. By averaging the pairs of meaRurements and subtracting, the nct K4' fl counting rate is obtained a t a time midway through the counting sequence, and i t is not necessary to consider the slightly different decay rate of the 7 and the net K4* fl components. An end-window proportional counter is superior to a G-M counter because of greater stability and shorter dead time. Compute net fl activity for each specimen at a certain time. The net K43 fl counting rate for each specimen must be corrected for decay according to the half life 12.40 hours, equivalent to a
~~~~l~
No. K1 2 3 4
Av. B1 2 3 4 5 6 7 8 9
P
RESULTS
Table I summarizes the analytical data for the determination of per cent K in M.I.T. standard biotite I33203 (IO). Sample and standard specimens were sealed in polyethylene planchets with polystyreneethyl acetate cement, according to procedure A, and were irradiated for 24 minutes in a pile neutron flux of 5 X 1011 neutrons per sq. cm. per second. The sealed sample and planchet assemblies were counted without opening. In the first column, K1 to K9 refer to potassium acid phthalate comparison standards (KHCr H,O4 Mallinckrodt primary standard, 19.146% K), and 151 to B9 to biotite specimens. The biotite was ground in an agate mortar before irradiation sufficiently to yield sources of uniform thickness. In all cases, source areas are 0.8 sq. cm., and thickneaws of sample material are proportional to sample weights. (Total source thickness includes sample material and 5 to 10 mg. of cement.) The third column lists the observed net counting rates through the Al absorber thicknesses indicated (not yet corrected for slight decay during counting). The y component, meaaured through the 2118 mg. per sq. cm. AI absorber and amounting to less
Table 1.
Counting Data for Determination of K in Biotite 63203
Sample Wt.,
Net p C.P.S.
Mg.
42.2 36.7 70.5 51.3 27.1 39.9 27.1 52.0 22.2 41.1 33.9 40.4 30.8 36.1 34.4 33.1 42.2 30.68 38.4 36.2
629" 307 272
336 138 103 124 93 102 97 93 116 99 101
795O 174 152 278 203 109 150 101 188 78 58
68 52 58 55 52 64 56 50
Net B C.P.S./Mg.
6294 6.68 6.94 6.76 6.84 7.09 6.87 6.75 6.74 0.65 6.82 f 5 2.75 2.85 2.85 2.73 2.79 2.82 2.81 2.83 2.79
Av. Trace element correction
Net % K Average % K in B3203
Thickneaa of A1 abeorber, mg. per sq. om.
1008 *
decay of 1% per 10.80 minutea for short decay period?. It is convenient to correct a1 specimens to some time near the middle of the counting period. Compute er cent K in the sample. The ratio o?net K" p counting rates in Sam le and standard is equal to the ratio o their potassium contents.
ANALYTICAL CHEMISTRY
7954 3.78 3.88 3.77 3.86 4.01 3.81 3.83 3.77 3.73 3.83 f 3 1.56 1.66 1.58 1.54 1.56 1.56 1.66 1.58 1.54
%K
629"
7966
7.73 7.78 8.00 7.81 8.00 7.92 7.66 7.71 7.82 7.81 7.90 7.82 7.89 7.78 7.94 7.88 7.70 7.83 7.86 7 7.80 f 7 0.04 0.03 7.77f 7 7.82 =t7 7.80f 0.05% K
*
than 3% of the total, has been subtracted. The net fl counts per second of polyethylene planchet blanks averaged less than 0.1 c.P.s., and the statistical counting errors were alwaya lees than 2%. Data have been corrected for coincidence ldss due to a counter dead time of 20.5 psec. Counting was carried out during the interval 28 to 31 hours after irradiation, and the net j3 counts per second per milligram have been corrected for decay of 12.46-hour K42to a time 29.75 hours after irradiation. The errors indicated for the average specific activities of the standard are standard deviations of the means; for the average per cent K in biotite they are standard deviations of the means, including the errors in the comparison standard. The agreement between the values for the two absorbers selected is seen to be within the random errors indicating that the measured fl radiations for sample and standard have the same energy characteristics. The correction for slight trace elemcnt intcrference, discussed below , is less than the random error of the result. These data may be examined for error due to variation in self-shielding and self-scattering of j3 radiation by variation in source thickness. If the specific radioactivity is plotted us. a m ple weight, no trend of change of activity with weight is observed, indicating that scattering and absorption of radiation roughly compensate for each other in the weight range 30 to 40 mg. for these sources made up according to procedure A. In a similar manner, the per cent K was determined in standard granite G-1 (6), standard diabase W-1 (6), feldspar, NUS No. 70, and argillaceous limestone, NBS No. la. Sample and standard specimens were heat sealed in 0.001-inch polyethylene packets according to procedure B and were irradiated for 6 minutes in a pile neutron flux of 5 X neutrons per sq. cm. per second. The sealed ample and packet assemblies were counted without opening. The net B radioactivity of blank polyethylene packets averaged less than 0.5 c.p.8. for G-1 and lees than 0.2 c.p.8. for the other samples. The counting was carried out during the interval 31 to 34 hours after irradiation for G-1, with decay corrections made to time 32.70 hours, and 30.5 to 35.0 hours for the others, with corrections made to 32.83 hours after irradiation. In all cases sample material was fine grained and yielded l-sq. cm. sources of uniform thickness. Comparison of the last two columns in Tables I1 and I11 indicates that samples and standards in all cases have the same p energy characteristics, and the data indicate no systematic variation of specific activity with source thickness. The fractions of the total count-
ing rates with the thinncr absorbers which w r c Auhtractcd as the -pray contribution are given in Table IV. 'I'rnc~o rlcment corrections, discussed below, arc of thc order of or lrss than the random errors for G-1 and W-1. 1let:ded data on the trace clement contcnts of the fcldspar and the limestow arc not available, and corrections h a v ~not been applied. 'l'ablc V is a comparison of the analytical data for per cent. K in the granite, diabase, feldspar, limestone, and biotitc obtained in this work and reported by others. A recent discussion of numerous analyses of G-1 and W-1 is givcn by Stevens et al. ( I 4 ) , and the medians of their preferred values arc listed in Table V. For NUS No. 70 and NBS No. l a , only the singlc values are given in the references cited. Agrcement in each case between neutron activation analysis and other analyses for G-1, W-1, and N13S No. 70 is within the random error of the analysis. For NRS No. la, however, the neutron activation result is significantly higher than the NBS value. Table V also compares the neutron activation analysis result with several analyses for the biotite standard (11, l a ) . In this case, there is a general lack of agreement among investigators indicating difficulty with conventional methods for precision potassium determination in biotite, a serious problem in geochronology by the potassium-argon method. The data suggest that, where the sample has bean dissolved by digestion with HF, the higher values seem to be associated with the more prolonged digestion or higher temperature. Failure to dissolve all of the sample, especially where the residue may retain a micaceous structure and adsorb K, may lead to low results in some analyses. A more complete investigation of the determination of potassium in micas by chemical means has been carried out by Abbey and Maxwell ( I ) . In any case, the neutron activation analysis is in good agreement with the highest of these analyses. DISCUSSION OF ERRORS
Table II. Counting Data for Determination of K Sample sample Wt,, Net 8 C.P.S. Net 8 C.P.S./Mg. No. Mg. 625" 791" 625' 791a K1 29.43 1084 619 34.68 19.66 2 30 40 1107 628 35 oi iS SS 3 28 65 1031 582 35 95 20 33 20 52 4 27 61 970 554 35 90 Av. 29.0 35.38f31 20.09*20 G1 26.72 243 140 R_ .46 4.76 - ~- _. -_ -. . -2 30:02 266 156 8.46 4.86 3 28.33 243 8.61 4.89 143 4 30.54 261 4.95 150 8.44 Av. 28.9 8.47 4.86 Trace element correction Net % K Average % K in G-1 a Thickness of Al absorber, mg. per sq. cm. Table 111.
in Granite G-1
%K
6250
791°
4 58 4.53 4.58 4.63 4.60 4.66 4.71 4.67 4.58f 4 4.63f 6 0.02 0.01 4.56 f 4 4.62 f 6 4.69zk 0.04% ' K
Counting Data for Determination of K in Diabase W - I , Feldspar NBS No. 70, and Argillaceous Limestone NBS No. 1 a
Sample Wt,, _Net 13 C.P.S. No. Mg. 626" 7910 K1 27.16 1147 659 2 26.51 1072 610 3 26.39 1042 592 4 27.36 1028 680 6 26.36 920 630 Av. 20.76 F1 26.68 604 341 2 29.44 644 386 674 324 3 27.08 4 28.04 682 330 6 27.23 538 308 Av. 27.69 Average % K in NBS No. 70 W1 30.00 33.2 18.9 2 28.36 30.2 18.7 3 28.06 29.6 10.8 29.9 16.9 4 28.91 29.4 17.4 5 28.36 Av. 28.74
Sample
Net 6 C.P.S./Mg. 6250 791* 21.58 37.63 21.69 37.91 22.10 39.08 39.u) 22.10 38.12 21.87 21.87 38.30 20.29 11.42 11.80 20.78 21.18 11.86 12.39 21.90 12.37 21.72 11.99 21.17
%K 625"
1 .m 1 1.083 1,104 1.164 1.070
0.672 0.633 0.608 0.687 0.684 0.617
1.308 1.292 1.374 1.330 1.346 1.330
0.740 0.741 0.776 0.731 0.785 0.755
*m1
Trace element correction Net 70 K Av. yo K in W-1 L1 29.19 2 29.68 3 29.19 4 29.10 5 26.49 Av. 28.73
41.1 23.3 39.2 22.5 38.8 21.9 36.9 19.8 31.7 18.5
Av. 7' K in NBS No. l a Thickness of A1 absorber, mg. per sq.cm
791"
10.13 9.99 10.38 10.32 10.68 10.46 10.93 10.84 10,84 10.82 10.57f 18 10.49f 17 10.53i 0.13 % K 0.502 0.501 0.510 0.664 0.631 0.632 0.551 0.613 0.576 0.698 0.534 0.540 f 14 f 18 0.017 0.009 0.617 0,531 i 14 i 18 0.624f 0.012% K 0 653 0.617 0.645 0.648 0 686 0.079 0 664 0.640 0 671 0 687 0 664 0.660 f 10 f: 12 0 662 rfl 0 008 7 0 K
0
The accuracy of the neutron activation procedure tor determining per cent K depends on thc ability of the thinner AI absorber to filter out all radioactivity from N:t24 and from trace element spccics. l'hc piirity of thc net I
