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The determination of trace elements in silicate rocks by a combined spectrochemical-anion exchange technique
The determination of trace elements in silicate rocks by a combined spectrochemical-anion exchange technique R. R. BROOKS,* 1,. H. AHRE~VS~-and 8. It. ‘!L~r,olt$ of Physiology*, Chemistry? and Geology:, IYniversit,y of Cape Town
Departments
IRecei?ied 13 May 1959)
Abstract-The paper describesan &tempt to combins anionic exchmge enrichmenttechniques with a spectroohemicalmethod of analysis for the purpose of determining several rare elements in silicate
rocks, soils and meteorites. The investigation has for the most part been confined to several elements which form chloro-complexes in hydrochloric acid; notably Ag, Pb, Sn, Bi, Cd, Zn, TI, MO and In. These may be absorbed on a basic anion exohanger: after slution and evaporation, the residues are analysod quantitatively by a d.c. arc method. The ability to determine several trace elements in a single general operation is an advantage of the procedure. The combined procedurehas been applirtdto the detsrminationof Cd, Zn, Bi, TI and Sn in a suite of nineteen rocks including granite G-l and diabass W-l, and specimens of gneiss and sea-mud from the Antarctic.
~NT~OD~CT~O~ EDUE et al. (1959) have briefly discussed tihe possibility of combining ion exchange enrichment techniques with spectrochemical methods of analysis for estimating trace elements in rocks, minerals, meteorites and soils. They referred particularly to the silicates and pointed out that enrichment could be achieved either by In the present paper we wish to describe cationic or by anionic techniques. investigations involving anionic enrichment; investigations on cationic enrichment will be described later. The paper is esse~tia,lly in two parts: in the first we wish to consider analytical aspects-column procedures and the development of quantitative spectrochemical methods-and in the second, we will consider the application of the combined anionic en~chment-spectrochemical procedure to the d~termin&tion of several trace elements in a selected suite of nineteen rocks. BASIS OF THE AIXIO;UICE~RICH~~~T
P~oc~~w~E
Metals can form anionic complexes with a variety of ligands but for our purpose we will consider one ligand only, Cl. Several elements form chloro-complexes and their absorption on basic exchange resins has been investigated extensively by KRAUS and MOORE (1953). CAMPBELL and NELSON (1953), NELSON and KRAWS (1954), and JEWTZSCH and FROTSCHER (1955) have measured the distribution coefficients of several elements at various HCI normalities when absorbed on an anion exchange column containing the strongly basic exchanger Dowex 1. These me~nrements show clearly that whereas some elements form chloro-complexes over an extensive normality range (for example from 0.1 N to 12 N, Bi forms complexes (BiCl,)-, (BiCl,)+, and (BiCl,)s-) others complex only over a restricted range : we will concern ourselves in this paper with 2 N HCl. At this normality the following elements, arranged in order of decreasing magnitude of distribution coefficients, form chloro-complexes: AU (II = 106), Tin*, Hgn, OS, Ir, Bi, Pt, Sn Iv, Sbm, Cd, Zn, Re, Snn, Ru, Pd, Ag, Cur, Pbn, Nb, MO, IWI, Ta, U, In (D = 1)
Trace element6 in silicate rocks by & combined spectrochemioal-anion
exchange technique
It is particularly noteworthy that of the abundant elements (0, Si, Fe, Al, Na, K, Ca, Mg and Ti) which together form 99 per cent of the earth’s crust, only Fe forms a chloro-complex; this complex, FeC1,3- is weak, (D = 10) and can be easily separated (see below) from the stronger complexes. This is most fortunate because iron is a major constituent of many minerals and rocks and must be separated in order to achieve an effective enrichment of the other metals; also, the spectrum of iron is complex and its concentration must be reduced to a very low level because of possible interference (EDGE et al., and below). With the exception of Fe, all elements listed above are comparatively rare; some (Au, Hg, OS, Pt, Re, Pd and Ir) are so rare that they cannot be detected in silicate rocks and minerals by the combined procedure unless an extremely large quantity of sample is taken. They have accordingly been omitted from the main investigation described here and are to be discussed separately in a publication concerned specifically with the rare noble metals. Ta and U are not particularly rare, but as their spectral sensitivity is low they have also been omitted. This leaves the following ten elements: Tl, Bi, Sn, Cd, Zn, Ag, Pb, MO, In and Nb The concentrations of most of these elements in rocks, soils, minerals and meteorites are usually below their spectrochemical detection limits (see discussion by EDGE et ab., 1959). If, however, it is possible to absorb the elements selectively from a hydrochloric acid solution of a comparatively large quantity (say, lo-20 g) of rock, subsequent elution may yield a product in which the concentrations of these elements are above their spectrochemical detection limits and they may therefore be quantitatively estimated by spectrochemical procedures: this is the basis of the investigations described below. It may be borne in mind that the sensitivity of the combined procedure can be increased by taking into solution larger quantities of rock until a limit is imposed by practical considerations. PRELIMINARY COLUMN WORK ON RESIN IR 400 For the purpose at hand, an ion exchange resin should be able to absorb ions quantitatively and rapidly in the first place and, when eluted with an appropriate reagent, the ions should be released quantitatively in a reasonably small number of column volumes. Several preliminary column investigations were undertaken in order to establish optimum absorption and elution conditions with a form of IR 400 (ROHM and HAAS), a strongly basic exchanger similar to, but not identical with Dowex I, the exchanger used by KRAUS and MOORE. The full designation of the resin used in these experiments was IR 400, 100-200 mesh, 8 per cent cross-linkage. The investigations were devoted mainly to Zn, Bi, Cd, Tl and Sn. The elution characteristics of an ion exchange resin can be represented by the following formula:
V d = (i + D”)A 163
where d is the distance travelled by an absorbed ion down an ion exchange columr I of cross-sectional area 12 when P ml of eluent have been passed through the oolumn. U, is the volume distrib~~tio~~ coefficient of the ion with respect to the eluent and i is the fractional interstitial space. Since D, is usually much greater than 2’, which is usually of the order of 0-5 and if we take a column with a cross-sectional area of 1 cm2, the equation reducex to: d =: g * Approximately 400 ml of 2 N hydrochloric acid are required to keep in solution about 20 g of silicate rock and as the distribution coefficients of the elements are I.1
TK-’
---
0
$
O,!
mequiv. of ion passed through
column
Fig, 1. Break-through ouxv~s for zinc, bismuth and oedminm in 2 N hydrochloric acid solution. Column dimensions 20 cm X 1 cm* cross-sectional area ContaAing 15 g of Amberlite 13%400 anion exchange resin. Flow rate 50 ml/hr. C = cone. of ion in eluate. C, =. cont. of ion in original solution.
greater than 20, substitution in the above expression shows that a column of 20 cm length is required to retain quantitatively the elements referred to above. In selecting the dimensions of the column it had been assumed that the distribution coefficients of Zn, Bi, Cd, Tl and Sn for IR 400 were the B&me as for Dowex 1 anion exchanger and in order to establish that these ions were in fact absorbed, break-through curves for Zn, Cd and Bi were determined. A solution of each element in 2 N hydrochloric acid was passed through the ion exchange column and the concentration of the element in the eluate was recorded at short volume intervals; the results are shown in Fig. 1. The break-through points for Zn, Cd and Bi were, respectively, 50 mEq., 50 mEq. and 92 mEq. They indicate a considerable and satisf~tory degree of absorption and correspond well with distribution coefficients of 2000,205O and 2260 obtained by KRAUS and MOORE on Dowex 1. From a study of the literature, and in particular, the work of KRAUS and 164
Trace elements in silicate rocks by a combined spectrochemical-anion MOORE
(1953)
exchange
technique
and of MILLER and HUNTER (1954), two possible eluting agents
are indicated, sulphuric acid and nitric acid. Micro amounts of Zn, Bi and Cd were absorbed on an anion exchange column in the chloride form and then eluted with 2 N sulphuric acid and also separately with nitric acid (0.25 N). Successive fractions were collected by means of a fraction cutter and analysed calorimetrically for Zn, Bi and Cd using the dithizone, iodide and sulphide methods, respectively; the results are shown in Fig. 2. It may be observed that 2 N sulphuric acid removes all ions sharply and within
0
100
150 Volume
of eluote,
200
250
ml
Fig. 2. El&ion curves for the elution of 600 pg quantities of zinc, cadmium and bismuth from a column 20 cm x 1 cm* cross-sectional area containing 15 g of Amberlite IR 400 anion exchange resin. The diagram shows separate elutions with 2 N sulphuric acid and 0.25 N nitric acid. Flow rate 50 ml/hr.
approximately eight column volumes. On the other hand O-25 N nitric acid requires approximately thirteen column volumes and although Zn and Cd have sharp elution peaks, bismuth tails appreciably. Nitric acid was nevertheless favoured for three main reasons: (a) 13 column volumes of 0.25 N nitric acid represent 5 g of nitric acid whereas 8 column volumes of 2 N sulphuric acid represent 15 g of acid and consequently the degree of contamination from sulphuric acid may be greater than from nitric acid. (b) The strongest and only highly sensitive spectrum line of Bi is at 3067.7 A and is liable to interference from nearby intense Fe 3067.2 8. It was therefore necessary to separate iron from the other elements and this could be achieved most satisfactorily by an elution involving a slower release of ions (other than iron) from the column. (c) Nitric acid elutes Tl much more efficiently than sulphuric acid; see below. INVESTIGATIONS ON SILICATE ROCKS AND A SILICATE METEORITE For the initial experiments a sample of granite from the Cape Peninsula area (hereinafter referred to a8 Cape granite) was used. About 1000 g of this rock was 165
scrubbed with a nylon brush, heated to about 6OVC1 in a muffie furnace and plunged into cold deionized water; this treatment rendered it so friable that, it could be easily crushed to t,he texture of coarse sand merely by rubbing between finger and t,humb. The rock was further ground in an agate mortar and sieved through a 100 mesh nylon sieve. Two 10 g samples were taken and digested in a mixture of nitric, sulphuric and hydrofluoric acids. After fuming OR the sulphuric acid, the samples were dissolved u&i1 the entire specimen was in solution. A small quantity of bromine was added as an oxidant, because most of the elements that form chloro-complexes do so only in their highest vnlency states and for thallium in particular it is essential that Tlr is oxidized to Tlrrr (see below). The solutions were absorbed on ion exchange columns which had been pretreated with hydrochloric acid of t’he same normality and then washed with a small quantity of 2 N hydrochloric acid. Elution was carried out using approximately 500 ml of 2 N sulphuric acid on one column and the same quantity of 0.25 N nitric acid on another identical column. Qualitative spectrochemical analysis (for details of spectrograph see below) showed detectable Bi, Sn, Zn and Cd in t)he sulphuric acid fractions, but no thallium. The nitric acid fractions, however, showed the four elements and also quit’e strong lines of thallium. The sensitive lines of (ltt 3261 A were, however, rather faint. Experiments with 20 and 50 g samples of Cape granite confirmed the detection of the above trace elements but no others were detected. The spectrum of the eluate residue from t)he 50 g sample is shown in Fig. 4. When the column which had been eluted with sulphuric acid was further eluted with O-25 N nitric acid, thallium appeared in the eluate thus indicating that this element is much rnore readily eluted with nitric acid than with sulphuric acid. Because of the urgent need for a procedure capable of detecting and determining thallium in rocks such as basalt, gabbro and diabase in particular, only nitric acid was used in subsequent investigations. An examination of the spectra of the successive nitric acid fractions showed that the iron spectrum together with that of indium was largely confined to the first fractions corresponding in all to 100 ml of eluate. Spectra of Ag and Pb were distributed partly in this fraction and partly in later fractions. The spectra of TI, Zn, Cd, Bi and Sn all appeared in the fractions which followed the first 100 ml of eluate. Fig. 3 shows 0.25 N HNO, elution curves for a 20 g specimen of Cape granite. Tailing is evident and is probably due to the formation of nitrato-complexes and the presence of traces of chloride ion in the column; elution is complete, at least for the purposes of the investigation, at about 2000 ml (elution experiments with 0.75 N HNO, showed that Sn, Zn and Cd were eluted faster than in 0.25 N acid, whereas Bi was eluted more slowly and Tl remained unchanged). Cd, Sn, Tl, Bi and Zn may therefore be obtained quantitatively in a concentrate virtually free from iron by collecting the 100 to 2000 ml fraction of eluate. Under these conditions, interference of intense Fe 3067.2 with Bi 3067.7 is usually insignificant; if necessary, iron can, however, be further separated by another cycle of evaporating the original eluate, reabsorbing on the column and re-eluting. This procedure has indeed been successfully applied to the determination of the platinum metals and 166
Tmce elements in silicate rocks by a combined spectrochemical-anion
nitric acid,
N/4
exchange technique
ml
Fig. 3, Eluticn of 20 g sample of cape granite with O-25 N nitric acid. Column dime~ions 20 am x 1 ema cross-seational ares aont&ning 15 g of Amberlite IR 400anion exahw resin. Flow rate 50 ml/hr. Note strong tailing of tin, bismuth snd thallium. Iron and indium c&n. be completely separeted from other ions by sepamting the &st 100 ml fraction.
Table Sample
Capegranite G-l IGbbbase W-l Gra.nite
1. S~trcche~c~
Zll 3345
Bi 3067
examination
of residues
from
HNO, eluates
Cd 3201
Sn 3262
In 3256
MO 3139
Ag 3281
X
X
0
X
X
X
X
X
X
0
X
X
X
Pb 3683
X
Tl 3775
X
Shale Chondrite
( x = detected,
0
=
not detected.)
I
gold (BROOKS and AHRENS: unpublished data) in which the iron was entirely separated from 150 g of iron-rich rock. The need to oxidize the rock solution is shown by the fact that when a solution from 20 g of Cape granite was reduced with titanous chloride, thallium could not be detected in the eluate, The reason is that thallium-1 does not form chlorocomplexes. The results of a spectrochemical examination of residues from nitric acid elm&es of 20 g samples of granite G-1, diabase W-I, a specimen of shale (lower shale bed Table Mt series) and a chondrite (Lafayette County, Texas) are summarized in Table 1. These materials cover a wide composition range and the ability to detect Zn, Cd, Bi, Tl, Sn, In, Ag, &IOand Pb in a single arcing operation in most of these specimens, is significant. Aoeordingly a quantitative spectrochemical procedure was developed for determining some of the above elements in eluate residues. 167
QUANTITATIVE
SPECTR~CHEB~IC~IL
I)ETERMINBTIO~UT
Bi 11X1) ‘1‘1 1S ELUATE
OF
Zn,
(Id, Sn,
&SIl>UES
All analyses were carried out on a large Hilger quartz-glass spectrograph. The discharge was focused on the slit by means of a Hilger E.958 quartz lens; a rotating step sector with a transmission ratio of 2 served to provide a graded series of exposures. Spectra were recorded on Kodak 103-O plates over the range 2800 A to 4800 8. Other details of procedure follow in the discussion below on the development of a quantitative method. The amounts of residue from 100 to 2000 ml eluates of 10 to 20 g quantities of rock are very small (approx. 15 mg) ; 20 mg of NaCl was therefore added to the residue in the evaporating dish to aid in collecting the residue and to serve as a matrix for the subsequent spectrochemical analysis. An alkali metal salt was chosen as CN emission must be effectively quenched otherwise sensitive Tl 3775 cannot be used (AHMWS and LIEBENBERG, 1046; AHREXS, 1954). After testing several amperages (2, 3, 49, 6, 7, 8 and IO), 6 A was chosen as this current appeared to provide the most satisfactory compromise between minimum background (low amperage) and maximum line intensity (high amperage). Sb, Ge, Na (from matrix) and In were tested as internal standards and Sb was chosen as it appeared to be the most satisfactory internal standard for the group of elements as a whole (see BROOKS, 1959). It may be noted that each of the elements under investigation is spectrochemically volatile and accordingly only one internal On the basis of replicate determinations relative deviations standard is required. of 20.4 per cent, 9.6 per cent, 17.7 per cent and 17.5 per cent were calculated, respectively, for the line pairs: Cd 3261/Sb 3267, Sn 3262/Sb 3267, Bi 3067/Sb 3267, Zn 3345/Sb 3267 and Tl 3529/Sb 3267. (For details of individual intensity measurements see BROOKS, 1959.) Although the intensity ratios are not highly reproducible (relative deviation from 9.5 to 20.4 per cent) precision is nevertheless satisfactory for the purpose in hand. Standards prepared in a NaCl matrix were arced in triplicate in graphite anodes f5 in. (external diameter) x 3.2 mm (internal diameter) x 5 mm (depth). Exposure was continued until intense Na “D” colour began to wane, which was usually after about 13 min. The small amount of iron which is normally present emits its most intense spectrum after the distillation of Na has subsided, a fortunate circumstance as interference with Bi 3067 is kept to a minimum. Working curves were prepared in the usual way; for details see BROOKS (1959). THE DETERMINATION OF Zn, Cd, Bi, Tl AND Sn IN SOME SELECTED ROCKS The combined anionic enrichment-spectrochemical procedure-was employed to estimate Zn, Bi, Cd, Tl and Sn in nineteen rock samples; the results are summarized in Table 2. A detailed description of the rocks is given in Table 3. Blank determinations were carried out on all reagents which were used for the enrichment technique. Of the five elements under investigation only Sn and Zn could be detected; their estimated concentration in the volumes of reagents used 168
tn
3345.0-
Fig. 4. Sped xurn of 50 g sample of Cape granite after anion exchange enrirhment.
Trace elements in silicate rocks by a combined spectrochemical-anion exchange technique Table 2. Estimated concentrations
(p.p.m.) of certain metals in nineteen rock samples
r
No.
-
Bi
72 96 -
0.10 0.12
0.70 2.16
0.6 I.1 -
Av. 22
84
0.11
1.4
0.9
8.0 9.6 -
100 89
w 0*05*
1.4 1.1 -
0.09 0.09
8.8
95
-
1.3
0.09
2.4 2.0 2.2
0.03 0.06
Av.
0.16 0.12
260 190 -225
22 21
Greygneiss
Av. 21.5
0.14
0.04
12 18 -
72 100
-
-
-
Av. 15
86
-
-
-
150 150
-
-
0.08 0.09
150
-
-
0.09
Rhyolite
Trachyte
Av. Ijolite
Av.
Av.
1.5 2.1 I.8 11 12
210 220
0.13 O-16
-
-
11.5
215
0.15
-
-
8.0 7.7 -
160 120
-
7.8
140
-
Olivine basalt
Av.
-
0
-
-
280 390
0.30 0.30
0.41 0.67
0.15 0.15
336
0.30
054
0.15
3.0 4.7 -
70 75 -
0.04 0.05
N 0.05
3.8
73
18 24 Av. 21
-
Tl
r
T-
Granite G-l
-
I
19 25 -
Zape granite
Dolerite (diabaae)
Cd
Zn
Sn
Type of rock
-
* Lines extremely faint and near detection limit.
169
-
o-05 -
-
-
‘I’1
10
Ikbase
W--l.
/
‘
11
4 v.
Norite
Av.
_..-
x.7
_... _
1.1 1.4
0.05 _._I -_. _
1.3
0416
12
Eclogite
0.7
-_
_--
0.07
13
Peridotite
1-O 0.8 0.9
_..-
_-.
-
1.20 1.20
AT.
_-
a9
14
Lherzolite
0.3
220
-
15
Hortonolite dunite
1.7 o-9
180 270 -22fi
-
Av. 16
Olivine bomb
17
Shale
1.3
1.20 0.12
oa3
_-
_.._
77
. ._
-
_-
17 21
240 270
-.-
0.30 0.70
2.0
19
260
_-..
0.50
2-l
110 100
0.12 .-_
105
0.12
0.06 0.21 -0.13
2.2 .
Av. Ocean mud
Av.
4.3 3.3 3.8
/ I
--
.“I
0.03 0.10
/
19
Carbonatite
12
I
_I_
24
,
0.06
0.90
i
for 15 g samples of rock are O-3 p.p.m. and 8 p,p.m., respectively. These figures are quite low since they seldom represent more than 10 per cent of the amount of Zn and Sn in the silicate rooks that have been examined. The possibility that the intensity of the Sb internal standard lines in the standards may have differed from that in the rock sample residues was investigated by arcing further samples in which the same amount of antimony was mixed with It was found that the intensities of the internal a wide variety of matrices. standard lines were slightly lower in the eluate residues from the rock samples. 170
Traceelementsin silicaterooksby a combinedspectrochemical-anion exohangetechnique
Table 3. Locality details of sabcted rocks used for the investigation NO.
T-
Nrtme of rock
1 2 3 4 6 6 7
Cape granite Granite G-l. Grey gneiss Rhyolite Trachyte Ijolite B&S&
8
Dolerite
11
Olivine basalt D&base W-l. Norite
x2 13 14 16 16 17
Eclogite Peridotite Lherzolite Hortonolite dun&e Olivine bomb in Lava Shale
18
Ocean mud
9 10
Locality
Cap0 peninsulaama. Westerley, Rhode I., U.S.A. (F&~b&~ et al. 1951) W. Ongul I, Lutzour-Hohn Bay, Antarctica. Mansons peninsula, Lyttelton, N.Z. Devils Gap, Akaroai,Banks peninsulrt,N.Z. South of Melt&g, Fen area, Norway. Mt. Herbert flow, Old ballast quarry, Diamond harbour, Lyttelton, N.Z. Top of Kloof Nek, Cape Town, at road leading to Camps Bay. Devils Gap, 2400 ft, Akaroa, N.Z. Centerville,Va., U.S.A. P&bearing pyroxenite norite, Merensky Reef, Lydenburg, Tvl. S.A. Almeklovsaeter, Almeklovdslen, Sunnmore, Norway. R&a, Tysfjord, Norway. S. end Lac Lherz, Pyrenees, Frsnce. Lydenburg, Tvl., S. Africa. The volcano “Sverresfjell, Bock Bay, Spitzbergen. Lower shale bed, Table Mt series, 100 yards north of Rhodes memorial, Cape Town, S.A. Lutzholm Bay, near Cook Peninsula, 68’S, 32”E, mud st 500 m. Ondur&korumeCarbonatite complex. Otjiwarongo District, S.W.A.
19
i
This is evidently due to the fact that whereas at least 90 per cent of the weight of each standard was transferred to the electrodes, a lower proportion (-70 per cent) of the eluate residue + added NaCl could be transferred to the electrode. The slightly lower intensity of the lines in the sample residues is due therefore, to a smaller proportionate transfer of material from evaporating basin to electrodes. It appeared that slight decomposition of the resin occurred due to the oxidative effect of the nitric acid. This resulted in progressively less efficient separations of iron from the “main” fraction during elution. This was apparent after four or five elutions from the same resin. The exchanger therefore was discsrded after every four elutions. This is not particularly wasteful as only about 15 g of resin is used. NOTES ON OTHER ELEMENTS
During the determination of the above mentioned five elements it was also possible to make qualitative observations on certain others, i.e. Ag, Pb, In, MO and Nb. The evaluation of Ag and Pb is difficult because of the high lead blank in the reagents and a fairly high silver blank in the electrodes. In Table 4, values have been given for Ag together with some for MO, In and Nb; the values are only approximate. 171
Table ttock
4. Semiquantitativo no.
1 2 3 4 5 6 7
observations
on In, MO, Nb and Ag in nineteen
10
NO
3 2 3
2
--
-
3 3
-._ .-
2
12 13 14 15 10 17 18 19
-
-
I .-
-
-
“._.._”
1
1 2 1
1
-
1
1 3 1 2
Ag
2
-
8 9 10 11
St.2
rocks
2
-
-
3
2 3
I -not
detected;
1 faint lines detected;
2 medium strength lines detected;
3 strong lines detected.
Niobium deserves a special mention because of the specific factors governing its detection. Figures for this element are given only for rock samples 4, 5 and 16 since the conditions in the treatment of the other rocks (except nos. 12 and 13 which appeared to contain no Nb) were such that niobium could not be detected. In the treatment of most of the rock samples, platinum basins had been used and had been seriously damaged by the sulphuric acid. The use of Teflon beakers was attempted for the remaining five samples. In the new treatment, HF and aqua regia were used instead of RF and sulphuric acid and since it was not possible to heat the residue to a high temperature, a considerable excess of fluoride ions remained in the material. During the ion exchange operations, far bulkier eluate residues than normal resulted due to the formation of fluoro-complexes of some of the major constituents of the rock. Intense niobium lines, hitherto not detectable, appeared in several of the spectra, and this appearance is evidently linked with the formation of ~uoro-complexes of niobium. The detection of Nb opens up new possibilities of quantitativeIy estimating this element; this is now Intense background emission in these five rocks samples under investigation. removed the possibility of determining Cd and raised the detection limits of the other elements.
As far as can be ascertained, direct determinations of bismuth in common igneous and sedimentary rocks have not hitherto been attempted. NODDACK and
Trace elementsin silicaterocks by &combinedspectrochemical-anion exchangetechnique NODDACK (1934) give 0.07 p.p.m. based apparently on the frequency of occurrence of Bi minerals whereas GOLDSCHMIDT(1937) gives a value of >0*2 p.p.m. which is based on the proportions of As, Sb and Bi in ferruginous bauxites and sedimentary iron ores. The magnitude arrived at here on the basis of a few determinations on The estimate is approximate because granite, basalt and norite is O-2 p.p.m. aside from any other considerations, the Bi content can apparently vary quite appreciably (compare the Cape granite with granite Gl for example) and to arrive at a satisfactory estimate, a fairly large number of samples would have to be analysed. From the few analyses available, little can be concluded about the distribution of Bi. BP+ could presumably substitute for Ca2+ in Ca minerals because their ionic radii (O-96 A and 1.01 A, respectively) are similar (see GOLDSCHMIDT, 1954, p. 480) in which case enrichment in basalt, diabase and norite relative to granite This is not apparent in the available data but it should in may be anticipated. any case be recalled that BinI with its high ionization potential, has chalcophilic characteristics which would complicate possible simple relationships based on replacement of ions. The trace of Bi in the specimen of carbonatite may be due to substitution of BP+ for Ca2+. A distinct enrichment (2 p.p.m.) in Table Mt. shale may be noted.
Cadmium SANDELL and GOLDICH (1943) estimated Cd in thirteen igneous rocks and composites of igneous rocks. The Cd content remained fairly uniform and the averages for gabbro, dolerite + greenstone and for granite were 0.19 p.p.m. and 0.12 p.p.m., respectively. These values are close in magnitude to those given here for igneous rocks in which it has been possible to estimate cadmium. This includes diabase W-l for which our estimate of 0.08 p.p.m. is far lower than that (5 + 2 p.p.m.) obtained polarographically by SMYTHE and GATEHOUSE (1955). The highest concentration (0.45 p.p.m.) of the specimens discussed in this paper is in the Kloof Nek dolerite. The ratio Zn/Cd is frequently of the magnitude of approximately 1000 and averages at 800, a value similar to that (500) reported by SANDELL and GOLDICH (1943). In carbonatite, however, Cd is distinctly enriched relative to Zn. This may be due to the greater ease with which Cd is accepted by carbonatite as the radius of Cd2+ (O-97 A) is close to that of Ca2+ (I.01 A) whereas Zn2+ (r = 0.69 A) is much smaller.
ONISHI and SANDELL (1957) have recently estimated tin in several igneous rocks and give a review of earlier work. Their averages of 3 p.p.m., 1.5 p.p.m., O-5 p.p.m. for granites, mafic rocks and ultramafic rocks, respectively, are in general lower than previous determinations; the contrast is particularly great in granite (for details see Tables 1 and 2 of ONISHI and SANDELL). Sn could be estimated in virtually all of the samples discussed here and our values appear in general to be higher than those of ONISHI and SANDELL; this is borne out by a 2
173
1%.It. RR~OI~S,L. H. AHRENSand S. I<. TAYLOR
direct comparison of I&l and \Ir-1 (all determinations values of OxIsHI ant1 lo;,,l)~Ll~ first,: ( ;- L TV’-- I
8,
2.r,
8.0,
!1.8
2.5,
.?.(I.
x.7.
s-7
in duplicate)
as follows-
The differences involve factors of 3-4 and the cause is not known. The relative agreement is good and G--l and W-1 evidently contain about the same amount of Sn. Thallium The thallium values agree reasonably well with determinations by other workers (for detailed review see SHAW, 1958). The spread of concentration in basic rocks is quite considerable ranging from 0.03 p.p.m. in basalt to 0.56 p.p.m. in dolerite. Thallium has not heretofore been estimated in G-l and W-l and the respective values of 1.3 and 0.05 p.p.m. are the first available on these two rocks.
The geochemistry of zinc has been studied in detail by WEDEPOHL (1953)who gives an exhaustive account of earlier work. Zn could be estimated in each of the nineteen rocks described here. In general the magnitudes given here agree reasonably well with those given by WEDEPOHL and others for similar rock types. Acknowledgements-We wish to thank Dr. H. NEUMANN of the Miner&log&k Museum, Oslo for the Norwegian samples and sample no. 16, members of the Japanese Antarctic expedition (leader Prof. T. NAGATA) for samples of gneiss and mud from Antarctica and Prof. E. S. W. SIMPSON for the specimen of Cape granite. REFERETU’CES L. H. (1954) Quantitative Spectrochemical Analylsis of Silicates. Pergamon Press, London. AHRENS L. H. and LIEBENBERO W. R. (1946) The qualitative spectrochemical analysis of minerals and rocks. Trans. Ceol. Sot. X. Africa 49. BROOKS R. R. (1959) Thesis in preparation. CAMPBELLE. C. and NELSON F. (1953) Chemical separation of 0.82 set Pbzo7 from Biao7. Phys.
AHRENS
Rev. 91, 499a. EDGE R. A., BROOKS R. R., AHRENS L. H. and AMDURER S. (1959) Some reconnaissance observations on the combined use of ion-exchange enrichment and spectrochemical analysis for the determination of trace constituencies in silicate rocks. Geochim. et Cosmochim. Acta 15, No. 4, p. 337. FAIRBAIRN H. W. et al. (1951) A co-operative investigation of precision and accuracy in chemical, spectrochemical end modal analyses of silicate rocks. U.S. Qeol. Survey. Bull. 980, pp. 1-71. GOLDSCHMIDTH. V. (1937) The geochemical distribution of the elements-IX. The abundances of the elements. Skr. Nmske Vidensk Akad. Mat-naturv. Kl No. 4. GOLDSCHMIDT V. M. (1954) Geochemidry. Clarendon Press, Oxford. KRAUS K. A. and MOORE G. E. (1953) Anion exchange studies-V. Absorption of transition elements Mn to Zn in HCl. J. Amer. Chem. Sot. 15, 1460. JENTZSCH D. and FROTSCHER I. (1955) Ion exchangers in analytical chemistry-the adsorption behaviour of elements in an anion exchanger in HCl solution. 2. Anal. Cltena. 144,17. MILLER C. C. and HUNTER J. A. (1954)The separation of zinc from other elements by anion exchange. Analyst 79, 483.
174
Traceelements iu silicate rocks by a combined spectrochemical-anion
exchange technique
NELSONF. and KRAUS K. A. (1954) Anion exchange studies VIII. J. Amer. Chem. Sot. 76, 6916. NODDACKI. and NODDACKW. (1934) The geochemical distribution coefficientof the elements. Sven-sk Kern. Tidakr. 46, 201. ONISHI H. and SANDELLE. B. (1957) Meteoritic and terrestrial abundance of tin. Geochim.et Cosmochim. Acta 12, 262-270. SANDELLE. B. and GOLDICHS. S. (1943) The rarer metallic constituents of some American igneous rocks. J. Geol. 51, 99, 167. SHAW D. M. (1957) Progress in. Physics and Chembtry of the Earth Vol. II. Pergamon Press, London. SMYTHEL. E. and GATEHOUSE B. M. (1955) Polarographicdetermination of traces of copper, nickel, cobalt, zinc and cadmium in rocks using rubeanicacid and nitroso-2naphthol. (University of Tasmania, Hobart). Analyt. C&m. 27, 901-903. WEDEPOHLK. H. (1963) Investigations into the geochemistry of zinc. Geochim. et Cosmochim. Acta 3, 93-142.
175
Departments
IRecei?ied 13 May 1959)
Abstract-The paper describesan &tempt to combins anionic exchmge enrichmenttechniques with a spectroohemicalmethod of analysis for the purpose of determining several rare elements in silicate
rocks, soils and meteorites. The investigation has for the most part been confined to several elements which form chloro-complexes in hydrochloric acid; notably Ag, Pb, Sn, Bi, Cd, Zn, TI, MO and In. These may be absorbed on a basic anion exohanger: after slution and evaporation, the residues are analysod quantitatively by a d.c. arc method. The ability to determine several trace elements in a single general operation is an advantage of the procedure. The combined procedurehas been applirtdto the detsrminationof Cd, Zn, Bi, TI and Sn in a suite of nineteen rocks including granite G-l and diabass W-l, and specimens of gneiss and sea-mud from the Antarctic.
~NT~OD~CT~O~ EDUE et al. (1959) have briefly discussed tihe possibility of combining ion exchange enrichment techniques with spectrochemical methods of analysis for estimating trace elements in rocks, minerals, meteorites and soils. They referred particularly to the silicates and pointed out that enrichment could be achieved either by In the present paper we wish to describe cationic or by anionic techniques. investigations involving anionic enrichment; investigations on cationic enrichment will be described later. The paper is esse~tia,lly in two parts: in the first we wish to consider analytical aspects-column procedures and the development of quantitative spectrochemical methods-and in the second, we will consider the application of the combined anionic en~chment-spectrochemical procedure to the d~termin&tion of several trace elements in a selected suite of nineteen rocks. BASIS OF THE AIXIO;UICE~RICH~~~T
P~oc~~w~E
Metals can form anionic complexes with a variety of ligands but for our purpose we will consider one ligand only, Cl. Several elements form chloro-complexes and their absorption on basic exchange resins has been investigated extensively by KRAUS and MOORE (1953). CAMPBELL and NELSON (1953), NELSON and KRAWS (1954), and JEWTZSCH and FROTSCHER (1955) have measured the distribution coefficients of several elements at various HCI normalities when absorbed on an anion exchange column containing the strongly basic exchanger Dowex 1. These me~nrements show clearly that whereas some elements form chloro-complexes over an extensive normality range (for example from 0.1 N to 12 N, Bi forms complexes (BiCl,)-, (BiCl,)+, and (BiCl,)s-) others complex only over a restricted range : we will concern ourselves in this paper with 2 N HCl. At this normality the following elements, arranged in order of decreasing magnitude of distribution coefficients, form chloro-complexes: AU (II = 106), Tin*, Hgn, OS, Ir, Bi, Pt, Sn Iv, Sbm, Cd, Zn, Re, Snn, Ru, Pd, Ag, Cur, Pbn, Nb, MO, IWI, Ta, U, In (D = 1)
Trace element6 in silicate rocks by & combined spectrochemioal-anion
exchange technique
It is particularly noteworthy that of the abundant elements (0, Si, Fe, Al, Na, K, Ca, Mg and Ti) which together form 99 per cent of the earth’s crust, only Fe forms a chloro-complex; this complex, FeC1,3- is weak, (D = 10) and can be easily separated (see below) from the stronger complexes. This is most fortunate because iron is a major constituent of many minerals and rocks and must be separated in order to achieve an effective enrichment of the other metals; also, the spectrum of iron is complex and its concentration must be reduced to a very low level because of possible interference (EDGE et al., and below). With the exception of Fe, all elements listed above are comparatively rare; some (Au, Hg, OS, Pt, Re, Pd and Ir) are so rare that they cannot be detected in silicate rocks and minerals by the combined procedure unless an extremely large quantity of sample is taken. They have accordingly been omitted from the main investigation described here and are to be discussed separately in a publication concerned specifically with the rare noble metals. Ta and U are not particularly rare, but as their spectral sensitivity is low they have also been omitted. This leaves the following ten elements: Tl, Bi, Sn, Cd, Zn, Ag, Pb, MO, In and Nb The concentrations of most of these elements in rocks, soils, minerals and meteorites are usually below their spectrochemical detection limits (see discussion by EDGE et ab., 1959). If, however, it is possible to absorb the elements selectively from a hydrochloric acid solution of a comparatively large quantity (say, lo-20 g) of rock, subsequent elution may yield a product in which the concentrations of these elements are above their spectrochemical detection limits and they may therefore be quantitatively estimated by spectrochemical procedures: this is the basis of the investigations described below. It may be borne in mind that the sensitivity of the combined procedure can be increased by taking into solution larger quantities of rock until a limit is imposed by practical considerations. PRELIMINARY COLUMN WORK ON RESIN IR 400 For the purpose at hand, an ion exchange resin should be able to absorb ions quantitatively and rapidly in the first place and, when eluted with an appropriate reagent, the ions should be released quantitatively in a reasonably small number of column volumes. Several preliminary column investigations were undertaken in order to establish optimum absorption and elution conditions with a form of IR 400 (ROHM and HAAS), a strongly basic exchanger similar to, but not identical with Dowex I, the exchanger used by KRAUS and MOORE. The full designation of the resin used in these experiments was IR 400, 100-200 mesh, 8 per cent cross-linkage. The investigations were devoted mainly to Zn, Bi, Cd, Tl and Sn. The elution characteristics of an ion exchange resin can be represented by the following formula:
V d = (i + D”)A 163
where d is the distance travelled by an absorbed ion down an ion exchange columr I of cross-sectional area 12 when P ml of eluent have been passed through the oolumn. U, is the volume distrib~~tio~~ coefficient of the ion with respect to the eluent and i is the fractional interstitial space. Since D, is usually much greater than 2’, which is usually of the order of 0-5 and if we take a column with a cross-sectional area of 1 cm2, the equation reducex to: d =: g * Approximately 400 ml of 2 N hydrochloric acid are required to keep in solution about 20 g of silicate rock and as the distribution coefficients of the elements are I.1
TK-’
---
0
$
O,!
mequiv. of ion passed through
column
Fig, 1. Break-through ouxv~s for zinc, bismuth and oedminm in 2 N hydrochloric acid solution. Column dimensions 20 cm X 1 cm* cross-sectional area ContaAing 15 g of Amberlite 13%400 anion exchange resin. Flow rate 50 ml/hr. C = cone. of ion in eluate. C, =. cont. of ion in original solution.
greater than 20, substitution in the above expression shows that a column of 20 cm length is required to retain quantitatively the elements referred to above. In selecting the dimensions of the column it had been assumed that the distribution coefficients of Zn, Bi, Cd, Tl and Sn for IR 400 were the B&me as for Dowex 1 anion exchanger and in order to establish that these ions were in fact absorbed, break-through curves for Zn, Cd and Bi were determined. A solution of each element in 2 N hydrochloric acid was passed through the ion exchange column and the concentration of the element in the eluate was recorded at short volume intervals; the results are shown in Fig. 1. The break-through points for Zn, Cd and Bi were, respectively, 50 mEq., 50 mEq. and 92 mEq. They indicate a considerable and satisf~tory degree of absorption and correspond well with distribution coefficients of 2000,205O and 2260 obtained by KRAUS and MOORE on Dowex 1. From a study of the literature, and in particular, the work of KRAUS and 164
Trace elements in silicate rocks by a combined spectrochemical-anion MOORE
(1953)
exchange
technique
and of MILLER and HUNTER (1954), two possible eluting agents
are indicated, sulphuric acid and nitric acid. Micro amounts of Zn, Bi and Cd were absorbed on an anion exchange column in the chloride form and then eluted with 2 N sulphuric acid and also separately with nitric acid (0.25 N). Successive fractions were collected by means of a fraction cutter and analysed calorimetrically for Zn, Bi and Cd using the dithizone, iodide and sulphide methods, respectively; the results are shown in Fig. 2. It may be observed that 2 N sulphuric acid removes all ions sharply and within
0
100
150 Volume
of eluote,
200
250
ml
Fig. 2. El&ion curves for the elution of 600 pg quantities of zinc, cadmium and bismuth from a column 20 cm x 1 cm* cross-sectional area containing 15 g of Amberlite IR 400 anion exchange resin. The diagram shows separate elutions with 2 N sulphuric acid and 0.25 N nitric acid. Flow rate 50 ml/hr.
approximately eight column volumes. On the other hand O-25 N nitric acid requires approximately thirteen column volumes and although Zn and Cd have sharp elution peaks, bismuth tails appreciably. Nitric acid was nevertheless favoured for three main reasons: (a) 13 column volumes of 0.25 N nitric acid represent 5 g of nitric acid whereas 8 column volumes of 2 N sulphuric acid represent 15 g of acid and consequently the degree of contamination from sulphuric acid may be greater than from nitric acid. (b) The strongest and only highly sensitive spectrum line of Bi is at 3067.7 A and is liable to interference from nearby intense Fe 3067.2 8. It was therefore necessary to separate iron from the other elements and this could be achieved most satisfactorily by an elution involving a slower release of ions (other than iron) from the column. (c) Nitric acid elutes Tl much more efficiently than sulphuric acid; see below. INVESTIGATIONS ON SILICATE ROCKS AND A SILICATE METEORITE For the initial experiments a sample of granite from the Cape Peninsula area (hereinafter referred to a8 Cape granite) was used. About 1000 g of this rock was 165
scrubbed with a nylon brush, heated to about 6OVC1 in a muffie furnace and plunged into cold deionized water; this treatment rendered it so friable that, it could be easily crushed to t,he texture of coarse sand merely by rubbing between finger and t,humb. The rock was further ground in an agate mortar and sieved through a 100 mesh nylon sieve. Two 10 g samples were taken and digested in a mixture of nitric, sulphuric and hydrofluoric acids. After fuming OR the sulphuric acid, the samples were dissolved u&i1 the entire specimen was in solution. A small quantity of bromine was added as an oxidant, because most of the elements that form chloro-complexes do so only in their highest vnlency states and for thallium in particular it is essential that Tlr is oxidized to Tlrrr (see below). The solutions were absorbed on ion exchange columns which had been pretreated with hydrochloric acid of t’he same normality and then washed with a small quantity of 2 N hydrochloric acid. Elution was carried out using approximately 500 ml of 2 N sulphuric acid on one column and the same quantity of 0.25 N nitric acid on another identical column. Qualitative spectrochemical analysis (for details of spectrograph see below) showed detectable Bi, Sn, Zn and Cd in t)he sulphuric acid fractions, but no thallium. The nitric acid fractions, however, showed the four elements and also quit’e strong lines of thallium. The sensitive lines of (ltt 3261 A were, however, rather faint. Experiments with 20 and 50 g samples of Cape granite confirmed the detection of the above trace elements but no others were detected. The spectrum of the eluate residue from t)he 50 g sample is shown in Fig. 4. When the column which had been eluted with sulphuric acid was further eluted with O-25 N nitric acid, thallium appeared in the eluate thus indicating that this element is much rnore readily eluted with nitric acid than with sulphuric acid. Because of the urgent need for a procedure capable of detecting and determining thallium in rocks such as basalt, gabbro and diabase in particular, only nitric acid was used in subsequent investigations. An examination of the spectra of the successive nitric acid fractions showed that the iron spectrum together with that of indium was largely confined to the first fractions corresponding in all to 100 ml of eluate. Spectra of Ag and Pb were distributed partly in this fraction and partly in later fractions. The spectra of TI, Zn, Cd, Bi and Sn all appeared in the fractions which followed the first 100 ml of eluate. Fig. 3 shows 0.25 N HNO, elution curves for a 20 g specimen of Cape granite. Tailing is evident and is probably due to the formation of nitrato-complexes and the presence of traces of chloride ion in the column; elution is complete, at least for the purposes of the investigation, at about 2000 ml (elution experiments with 0.75 N HNO, showed that Sn, Zn and Cd were eluted faster than in 0.25 N acid, whereas Bi was eluted more slowly and Tl remained unchanged). Cd, Sn, Tl, Bi and Zn may therefore be obtained quantitatively in a concentrate virtually free from iron by collecting the 100 to 2000 ml fraction of eluate. Under these conditions, interference of intense Fe 3067.2 with Bi 3067.7 is usually insignificant; if necessary, iron can, however, be further separated by another cycle of evaporating the original eluate, reabsorbing on the column and re-eluting. This procedure has indeed been successfully applied to the determination of the platinum metals and 166
Tmce elements in silicate rocks by a combined spectrochemical-anion
nitric acid,
N/4
exchange technique
ml
Fig. 3, Eluticn of 20 g sample of cape granite with O-25 N nitric acid. Column dime~ions 20 am x 1 ema cross-seational ares aont&ning 15 g of Amberlite IR 400anion exahw resin. Flow rate 50 ml/hr. Note strong tailing of tin, bismuth snd thallium. Iron and indium c&n. be completely separeted from other ions by sepamting the &st 100 ml fraction.
Table Sample
Capegranite G-l IGbbbase W-l Gra.nite
1. S~trcche~c~
Zll 3345
Bi 3067
examination
of residues
from
HNO, eluates
Cd 3201
Sn 3262
In 3256
MO 3139
Ag 3281
X
X
0
X
X
X
X
X
X
0
X
X
X
Pb 3683
X
Tl 3775
X
Shale Chondrite
( x = detected,
0
=
not detected.)
I
gold (BROOKS and AHRENS: unpublished data) in which the iron was entirely separated from 150 g of iron-rich rock. The need to oxidize the rock solution is shown by the fact that when a solution from 20 g of Cape granite was reduced with titanous chloride, thallium could not be detected in the eluate, The reason is that thallium-1 does not form chlorocomplexes. The results of a spectrochemical examination of residues from nitric acid elm&es of 20 g samples of granite G-1, diabase W-I, a specimen of shale (lower shale bed Table Mt series) and a chondrite (Lafayette County, Texas) are summarized in Table 1. These materials cover a wide composition range and the ability to detect Zn, Cd, Bi, Tl, Sn, In, Ag, &IOand Pb in a single arcing operation in most of these specimens, is significant. Aoeordingly a quantitative spectrochemical procedure was developed for determining some of the above elements in eluate residues. 167
QUANTITATIVE
SPECTR~CHEB~IC~IL
I)ETERMINBTIO~UT
Bi 11X1) ‘1‘1 1S ELUATE
OF
Zn,
(Id, Sn,
&SIl>UES
All analyses were carried out on a large Hilger quartz-glass spectrograph. The discharge was focused on the slit by means of a Hilger E.958 quartz lens; a rotating step sector with a transmission ratio of 2 served to provide a graded series of exposures. Spectra were recorded on Kodak 103-O plates over the range 2800 A to 4800 8. Other details of procedure follow in the discussion below on the development of a quantitative method. The amounts of residue from 100 to 2000 ml eluates of 10 to 20 g quantities of rock are very small (approx. 15 mg) ; 20 mg of NaCl was therefore added to the residue in the evaporating dish to aid in collecting the residue and to serve as a matrix for the subsequent spectrochemical analysis. An alkali metal salt was chosen as CN emission must be effectively quenched otherwise sensitive Tl 3775 cannot be used (AHMWS and LIEBENBERG, 1046; AHREXS, 1954). After testing several amperages (2, 3, 49, 6, 7, 8 and IO), 6 A was chosen as this current appeared to provide the most satisfactory compromise between minimum background (low amperage) and maximum line intensity (high amperage). Sb, Ge, Na (from matrix) and In were tested as internal standards and Sb was chosen as it appeared to be the most satisfactory internal standard for the group of elements as a whole (see BROOKS, 1959). It may be noted that each of the elements under investigation is spectrochemically volatile and accordingly only one internal On the basis of replicate determinations relative deviations standard is required. of 20.4 per cent, 9.6 per cent, 17.7 per cent and 17.5 per cent were calculated, respectively, for the line pairs: Cd 3261/Sb 3267, Sn 3262/Sb 3267, Bi 3067/Sb 3267, Zn 3345/Sb 3267 and Tl 3529/Sb 3267. (For details of individual intensity measurements see BROOKS, 1959.) Although the intensity ratios are not highly reproducible (relative deviation from 9.5 to 20.4 per cent) precision is nevertheless satisfactory for the purpose in hand. Standards prepared in a NaCl matrix were arced in triplicate in graphite anodes f5 in. (external diameter) x 3.2 mm (internal diameter) x 5 mm (depth). Exposure was continued until intense Na “D” colour began to wane, which was usually after about 13 min. The small amount of iron which is normally present emits its most intense spectrum after the distillation of Na has subsided, a fortunate circumstance as interference with Bi 3067 is kept to a minimum. Working curves were prepared in the usual way; for details see BROOKS (1959). THE DETERMINATION OF Zn, Cd, Bi, Tl AND Sn IN SOME SELECTED ROCKS The combined anionic enrichment-spectrochemical procedure-was employed to estimate Zn, Bi, Cd, Tl and Sn in nineteen rock samples; the results are summarized in Table 2. A detailed description of the rocks is given in Table 3. Blank determinations were carried out on all reagents which were used for the enrichment technique. Of the five elements under investigation only Sn and Zn could be detected; their estimated concentration in the volumes of reagents used 168
tn
3345.0-
Fig. 4. Sped xurn of 50 g sample of Cape granite after anion exchange enrirhment.
Trace elements in silicate rocks by a combined spectrochemical-anion exchange technique Table 2. Estimated concentrations
(p.p.m.) of certain metals in nineteen rock samples
r
No.
-
Bi
72 96 -
0.10 0.12
0.70 2.16
0.6 I.1 -
Av. 22
84
0.11
1.4
0.9
8.0 9.6 -
100 89
w 0*05*
1.4 1.1 -
0.09 0.09
8.8
95
-
1.3
0.09
2.4 2.0 2.2
0.03 0.06
Av.
0.16 0.12
260 190 -225
22 21
Greygneiss
Av. 21.5
0.14
0.04
12 18 -
72 100
-
-
-
Av. 15
86
-
-
-
150 150
-
-
0.08 0.09
150
-
-
0.09
Rhyolite
Trachyte
Av. Ijolite
Av.
Av.
1.5 2.1 I.8 11 12
210 220
0.13 O-16
-
-
11.5
215
0.15
-
-
8.0 7.7 -
160 120
-
7.8
140
-
Olivine basalt
Av.
-
0
-
-
280 390
0.30 0.30
0.41 0.67
0.15 0.15
336
0.30
054
0.15
3.0 4.7 -
70 75 -
0.04 0.05
N 0.05
3.8
73
18 24 Av. 21
-
Tl
r
T-
Granite G-l
-
I
19 25 -
Zape granite
Dolerite (diabaae)
Cd
Zn
Sn
Type of rock
-
* Lines extremely faint and near detection limit.
169
-
o-05 -
-
-
‘I’1
10
Ikbase
W--l.
/
‘
11
4 v.
Norite
Av.
_..-
x.7
_... _
1.1 1.4
0.05 _._I -_. _
1.3
0416
12
Eclogite
0.7
-_
_--
0.07
13
Peridotite
1-O 0.8 0.9
_..-
_-.
-
1.20 1.20
AT.
_-
a9
14
Lherzolite
0.3
220
-
15
Hortonolite dunite
1.7 o-9
180 270 -22fi
-
Av. 16
Olivine bomb
17
Shale
1.3
1.20 0.12
oa3
_-
_.._
77
. ._
-
_-
17 21
240 270
-.-
0.30 0.70
2.0
19
260
_-..
0.50
2-l
110 100
0.12 .-_
105
0.12
0.06 0.21 -0.13
2.2 .
Av. Ocean mud
Av.
4.3 3.3 3.8
/ I
--
.“I
0.03 0.10
/
19
Carbonatite
12
I
_I_
24
,
0.06
0.90
i
for 15 g samples of rock are O-3 p.p.m. and 8 p,p.m., respectively. These figures are quite low since they seldom represent more than 10 per cent of the amount of Zn and Sn in the silicate rooks that have been examined. The possibility that the intensity of the Sb internal standard lines in the standards may have differed from that in the rock sample residues was investigated by arcing further samples in which the same amount of antimony was mixed with It was found that the intensities of the internal a wide variety of matrices. standard lines were slightly lower in the eluate residues from the rock samples. 170
Traceelementsin silicaterooksby a combinedspectrochemical-anion exohangetechnique
Table 3. Locality details of sabcted rocks used for the investigation NO.
T-
Nrtme of rock
1 2 3 4 6 6 7
Cape granite Granite G-l. Grey gneiss Rhyolite Trachyte Ijolite B&S&
8
Dolerite
11
Olivine basalt D&base W-l. Norite
x2 13 14 16 16 17
Eclogite Peridotite Lherzolite Hortonolite dun&e Olivine bomb in Lava Shale
18
Ocean mud
9 10
Locality
Cap0 peninsulaama. Westerley, Rhode I., U.S.A. (F&~b&~ et al. 1951) W. Ongul I, Lutzour-Hohn Bay, Antarctica. Mansons peninsula, Lyttelton, N.Z. Devils Gap, Akaroai,Banks peninsulrt,N.Z. South of Melt&g, Fen area, Norway. Mt. Herbert flow, Old ballast quarry, Diamond harbour, Lyttelton, N.Z. Top of Kloof Nek, Cape Town, at road leading to Camps Bay. Devils Gap, 2400 ft, Akaroa, N.Z. Centerville,Va., U.S.A. P&bearing pyroxenite norite, Merensky Reef, Lydenburg, Tvl. S.A. Almeklovsaeter, Almeklovdslen, Sunnmore, Norway. R&a, Tysfjord, Norway. S. end Lac Lherz, Pyrenees, Frsnce. Lydenburg, Tvl., S. Africa. The volcano “Sverresfjell, Bock Bay, Spitzbergen. Lower shale bed, Table Mt series, 100 yards north of Rhodes memorial, Cape Town, S.A. Lutzholm Bay, near Cook Peninsula, 68’S, 32”E, mud st 500 m. Ondur&korumeCarbonatite complex. Otjiwarongo District, S.W.A.
19
i
This is evidently due to the fact that whereas at least 90 per cent of the weight of each standard was transferred to the electrodes, a lower proportion (-70 per cent) of the eluate residue + added NaCl could be transferred to the electrode. The slightly lower intensity of the lines in the sample residues is due therefore, to a smaller proportionate transfer of material from evaporating basin to electrodes. It appeared that slight decomposition of the resin occurred due to the oxidative effect of the nitric acid. This resulted in progressively less efficient separations of iron from the “main” fraction during elution. This was apparent after four or five elutions from the same resin. The exchanger therefore was discsrded after every four elutions. This is not particularly wasteful as only about 15 g of resin is used. NOTES ON OTHER ELEMENTS
During the determination of the above mentioned five elements it was also possible to make qualitative observations on certain others, i.e. Ag, Pb, In, MO and Nb. The evaluation of Ag and Pb is difficult because of the high lead blank in the reagents and a fairly high silver blank in the electrodes. In Table 4, values have been given for Ag together with some for MO, In and Nb; the values are only approximate. 171
Table ttock
4. Semiquantitativo no.
1 2 3 4 5 6 7
observations
on In, MO, Nb and Ag in nineteen
10
NO
3 2 3
2
--
-
3 3
-._ .-
2
12 13 14 15 10 17 18 19
-
-
I .-
-
-
“._.._”
1
1 2 1
1
-
1
1 3 1 2
Ag
2
-
8 9 10 11
St.2
rocks
2
-
-
3
2 3
I -not
detected;
1 faint lines detected;
2 medium strength lines detected;
3 strong lines detected.
Niobium deserves a special mention because of the specific factors governing its detection. Figures for this element are given only for rock samples 4, 5 and 16 since the conditions in the treatment of the other rocks (except nos. 12 and 13 which appeared to contain no Nb) were such that niobium could not be detected. In the treatment of most of the rock samples, platinum basins had been used and had been seriously damaged by the sulphuric acid. The use of Teflon beakers was attempted for the remaining five samples. In the new treatment, HF and aqua regia were used instead of RF and sulphuric acid and since it was not possible to heat the residue to a high temperature, a considerable excess of fluoride ions remained in the material. During the ion exchange operations, far bulkier eluate residues than normal resulted due to the formation of fluoro-complexes of some of the major constituents of the rock. Intense niobium lines, hitherto not detectable, appeared in several of the spectra, and this appearance is evidently linked with the formation of ~uoro-complexes of niobium. The detection of Nb opens up new possibilities of quantitativeIy estimating this element; this is now Intense background emission in these five rocks samples under investigation. removed the possibility of determining Cd and raised the detection limits of the other elements.
As far as can be ascertained, direct determinations of bismuth in common igneous and sedimentary rocks have not hitherto been attempted. NODDACK and
Trace elementsin silicaterocks by &combinedspectrochemical-anion exchangetechnique NODDACK (1934) give 0.07 p.p.m. based apparently on the frequency of occurrence of Bi minerals whereas GOLDSCHMIDT(1937) gives a value of >0*2 p.p.m. which is based on the proportions of As, Sb and Bi in ferruginous bauxites and sedimentary iron ores. The magnitude arrived at here on the basis of a few determinations on The estimate is approximate because granite, basalt and norite is O-2 p.p.m. aside from any other considerations, the Bi content can apparently vary quite appreciably (compare the Cape granite with granite Gl for example) and to arrive at a satisfactory estimate, a fairly large number of samples would have to be analysed. From the few analyses available, little can be concluded about the distribution of Bi. BP+ could presumably substitute for Ca2+ in Ca minerals because their ionic radii (O-96 A and 1.01 A, respectively) are similar (see GOLDSCHMIDT, 1954, p. 480) in which case enrichment in basalt, diabase and norite relative to granite This is not apparent in the available data but it should in may be anticipated. any case be recalled that BinI with its high ionization potential, has chalcophilic characteristics which would complicate possible simple relationships based on replacement of ions. The trace of Bi in the specimen of carbonatite may be due to substitution of BP+ for Ca2+. A distinct enrichment (2 p.p.m.) in Table Mt. shale may be noted.
Cadmium SANDELL and GOLDICH (1943) estimated Cd in thirteen igneous rocks and composites of igneous rocks. The Cd content remained fairly uniform and the averages for gabbro, dolerite + greenstone and for granite were 0.19 p.p.m. and 0.12 p.p.m., respectively. These values are close in magnitude to those given here for igneous rocks in which it has been possible to estimate cadmium. This includes diabase W-l for which our estimate of 0.08 p.p.m. is far lower than that (5 + 2 p.p.m.) obtained polarographically by SMYTHE and GATEHOUSE (1955). The highest concentration (0.45 p.p.m.) of the specimens discussed in this paper is in the Kloof Nek dolerite. The ratio Zn/Cd is frequently of the magnitude of approximately 1000 and averages at 800, a value similar to that (500) reported by SANDELL and GOLDICH (1943). In carbonatite, however, Cd is distinctly enriched relative to Zn. This may be due to the greater ease with which Cd is accepted by carbonatite as the radius of Cd2+ (O-97 A) is close to that of Ca2+ (I.01 A) whereas Zn2+ (r = 0.69 A) is much smaller.
ONISHI and SANDELL (1957) have recently estimated tin in several igneous rocks and give a review of earlier work. Their averages of 3 p.p.m., 1.5 p.p.m., O-5 p.p.m. for granites, mafic rocks and ultramafic rocks, respectively, are in general lower than previous determinations; the contrast is particularly great in granite (for details see Tables 1 and 2 of ONISHI and SANDELL). Sn could be estimated in virtually all of the samples discussed here and our values appear in general to be higher than those of ONISHI and SANDELL; this is borne out by a 2
173
1%.It. RR~OI~S,L. H. AHRENSand S. I<. TAYLOR
direct comparison of I&l and \Ir-1 (all determinations values of OxIsHI ant1 lo;,,l)~Ll~ first,: ( ;- L TV’-- I
8,
2.r,
8.0,
!1.8
2.5,
.?.(I.
x.7.
s-7
in duplicate)
as follows-
The differences involve factors of 3-4 and the cause is not known. The relative agreement is good and G--l and W-1 evidently contain about the same amount of Sn. Thallium The thallium values agree reasonably well with determinations by other workers (for detailed review see SHAW, 1958). The spread of concentration in basic rocks is quite considerable ranging from 0.03 p.p.m. in basalt to 0.56 p.p.m. in dolerite. Thallium has not heretofore been estimated in G-l and W-l and the respective values of 1.3 and 0.05 p.p.m. are the first available on these two rocks.
The geochemistry of zinc has been studied in detail by WEDEPOHL (1953)who gives an exhaustive account of earlier work. Zn could be estimated in each of the nineteen rocks described here. In general the magnitudes given here agree reasonably well with those given by WEDEPOHL and others for similar rock types. Acknowledgements-We wish to thank Dr. H. NEUMANN of the Miner&log&k Museum, Oslo for the Norwegian samples and sample no. 16, members of the Japanese Antarctic expedition (leader Prof. T. NAGATA) for samples of gneiss and mud from Antarctica and Prof. E. S. W. SIMPSON for the specimen of Cape granite. REFERETU’CES L. H. (1954) Quantitative Spectrochemical Analylsis of Silicates. Pergamon Press, London. AHRENS L. H. and LIEBENBERO W. R. (1946) The qualitative spectrochemical analysis of minerals and rocks. Trans. Ceol. Sot. X. Africa 49. BROOKS R. R. (1959) Thesis in preparation. CAMPBELLE. C. and NELSON F. (1953) Chemical separation of 0.82 set Pbzo7 from Biao7. Phys.
AHRENS
Rev. 91, 499a. EDGE R. A., BROOKS R. R., AHRENS L. H. and AMDURER S. (1959) Some reconnaissance observations on the combined use of ion-exchange enrichment and spectrochemical analysis for the determination of trace constituencies in silicate rocks. Geochim. et Cosmochim. Acta 15, No. 4, p. 337. FAIRBAIRN H. W. et al. (1951) A co-operative investigation of precision and accuracy in chemical, spectrochemical end modal analyses of silicate rocks. U.S. Qeol. Survey. Bull. 980, pp. 1-71. GOLDSCHMIDTH. V. (1937) The geochemical distribution of the elements-IX. The abundances of the elements. Skr. Nmske Vidensk Akad. Mat-naturv. Kl No. 4. GOLDSCHMIDT V. M. (1954) Geochemidry. Clarendon Press, Oxford. KRAUS K. A. and MOORE G. E. (1953) Anion exchange studies-V. Absorption of transition elements Mn to Zn in HCl. J. Amer. Chem. Sot. 15, 1460. JENTZSCH D. and FROTSCHER I. (1955) Ion exchangers in analytical chemistry-the adsorption behaviour of elements in an anion exchanger in HCl solution. 2. Anal. Cltena. 144,17. MILLER C. C. and HUNTER J. A. (1954)The separation of zinc from other elements by anion exchange. Analyst 79, 483.
174
Traceelements iu silicate rocks by a combined spectrochemical-anion
exchange technique
NELSONF. and KRAUS K. A. (1954) Anion exchange studies VIII. J. Amer. Chem. Sot. 76, 6916. NODDACKI. and NODDACKW. (1934) The geochemical distribution coefficientof the elements. Sven-sk Kern. Tidakr. 46, 201. ONISHI H. and SANDELLE. B. (1957) Meteoritic and terrestrial abundance of tin. Geochim.et Cosmochim. Acta 12, 262-270. SANDELLE. B. and GOLDICHS. S. (1943) The rarer metallic constituents of some American igneous rocks. J. Geol. 51, 99, 167. SHAW D. M. (1957) Progress in. Physics and Chembtry of the Earth Vol. II. Pergamon Press, London. SMYTHEL. E. and GATEHOUSE B. M. (1955) Polarographicdetermination of traces of copper, nickel, cobalt, zinc and cadmium in rocks using rubeanicacid and nitroso-2naphthol. (University of Tasmania, Hobart). Analyt. C&m. 27, 901-903. WEDEPOHLK. H. (1963) Investigations into the geochemistry of zinc. Geochim. et Cosmochim. Acta 3, 93-142.
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