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Determination of calcium, magnesium, manganese, strontium, sodium and iron in the carbonate fraction of limestones and dolomites
Chemical Geology, 28 (1980) 135--146 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
135
DETERMINATION OF CALCIUM, MAGNESIUM, MANGANESE, STRONTIUM, SODIUM AND IRON IN THE CARBONATE FRACTION OF LIMESTONES AND DOLOMITES
P. ROBINSON Geology Department, University of Tasmania, Hobart, Tasmania 7001 (Australia) (Received May 8, 1979; accepted for publication August 16, 1979)
ABSTRACT Robinson, P., 1980. Determinati6n of calcium, magnesium, manganese, strontium, sodium and iron in the carbonate fraction of limestones and dolomites. Chem. Geol., 28: 135-146. A rapid atomic absorption spectrophotometric (AAS) method for the determination of Ca, Mg, Mn, Sr, Na and Fe in limestones and dolomites is described. After cold 1 M hydrochloric acid digestion, potassium chloride is added as a releasing agent and all elements are analysed on a single 100 × dilution. The AAS sensitivity for Ca and Mg is reduced by using the spectral overlap of Ge 422.66-nm and V 285.17-nm non-resonance lines with Ca 422.67-nm and Mg 285.21-nm resonance lines, respectively. Results are reported for four international standards. The extent of any trace element leaching from clay minerals by hydrochloric acid was determined by treating 21 Tasmanian limestones and dolomites with both weak 0.3 M acetic acid and the stronger 1 M hydrochloric acid. Analyses of Mn, Sr and Na show little difference between acids whereas Fe is dependent on acid strength.
INTRODUCTION T h e m a j o r (Ca, Mg) a n d m i n o r (St, Na, Mn a n d Fe) e l e m e n t s in t h e c a r b o n a t e f r a c t i o n o f c a r b o n a t e r o c k s p r o v i d e valuable i n f o r m a t i o n a b o u t t h e original c a r b o n a t e m i n e r a l o g y a n d c h e m i c a l c h a r a c t e r i s t i c s o f d e p o s i t i o n a l a n d diagenetic s o l u t i o n s ( R a o a n d Naqvi, 1 9 7 7 ; R a o , 1 9 7 9 a, b). T h e d i a g e n e t i c fabrics o f l i m e s t o n e s a n d d o l o m i t e s are r e l a t e d t o salinity and t h e M g / C a r a t i o in s o l u t i o n (Folk, 1 9 7 4 ; F o l k a n d L a n d , 1975). Sr varies w i t h original c a r b o n a t e m i n e r a l o g y , facies a n d salinity (Veizer a n d D e m o v i c , 1974). Na is c o n s i d e r e d as a possible i n d e x o f salinity o f d e p o s i t i o n a l a n d diagenetic s o l u t i o n s ( L a n d a n d H o o p s , 1 9 7 3 ; Veizer et al., 1977). Mn d o e s n o t a p p e a r t o u n d e r g o significant changes d u r i n g diagenesis a n d can b e used as an i n d i c a t o r o f original c a r b o n a t e m i n e r a l o g y . T h e a m o u n t o f Fe increases f r o m m a r i n e t h r o u g h b r a c k i s h to f r e s h - w a t e r c a r b o n a t e s ( F r i e d m a n , 1 9 6 9 ) . T h e m a j o r p r o b l e m in such g e o c h e m i c a l studies o f c a r b o n a t e s is t h e uncertainty of contamination from the non-carbonate fraction of samples analysed
136 It would be useful to have a rapid and accurate m e t h o d of analysis which avoids such contamination. This paper deals with a rapid atomic absorption spectrophotometric (AAS) m e t h o d for the determination of Ca, Mg, Sr, Na, Mn and Fe in limestones and dolomites and evaluates the extent of any traceelement contamination from the non-carbonate fraction. SEPARATION OF NON-CARBONATE FRACTION -- TRACE-ELEMENT CONTAMINATION The non-carbonate fraction in carbonate rocks is usually a mixture of quartz, clays, feldspars and to a lesser extent sulphides and phosphates. The c o m m o n l y used m e t h o d of separation is chemical, involving digestion of the carbonate fraction with weak acid. The reaction of acids with non-carbonate minerals depends on the type of acid used, acid concentration and quantity, temperature, duration of reaction, particle size, solubility and crystallinity. Most workers have used X-ray diffraction techniques to study the structural breakdown of clay minerals under acid attack. Ostrom (1961) evaluated the effect of acetic and hydrochloric acids on clay minerals and concluded that 0.3 M acetic acid does not affect randomly interstratified illite--montmorillonite, chlorite, illite or kaolinite. Rai (1965) used 0.2 M acetic acid and Perrin (1964) acetic acid buffered to pH 3 with a m m o n i u m acetate to separate the noncarbonate fraction in limestones. Hirst and Nicholls (1958), and Barber (1974) followed the work of Ray et al. (1957) and used 25% acetic acid for limestones and dolomites. In the present work trace-element variations were studied and an a t t e m p t was made to find a faster m e t h o d of acid attack which would have little effect on the non-carbonate fraction. 1 M HC1 was used instead of the more usual acetic acid for three main reasons: (1) total carbonate digestion using weak acetic acid is slow, lasting up to several days for dolomites; (2) the volume of acetic acid required is large, and, after digestion, has to be reduced for traceelement analysis; and (3) there is a risk of introducing trace-element contamination from the laboratory environment during the prolonged digestion and subsequent evaporation. Few studies have been made on the changes in trace-element concentration with acid type. Hirst and Nicholls (1958) found 25% HC1 (3 M) gave higher results than 25% acetic acid for Ni, Cr, Co and V. Renard and Blanc (1972) studied changes in minor-element analyses with acid type, strength, duration and temperature. They concluded that a 1 N acetic acid digestion at 55°C for 2 hr. was the most favourable. However, only one limestone and one dolomite were tested and the clay mineralogy was not reported. In this work 21 Tasmanian limestones and dolomites were treated with cold 0.3 M acetic acid and the stronger 1 M HC1 to check the extent of trace-element contamination from the non-carbonate fraction. Weak acetic acid such as 0.3 M has no effect on clay mineral structure {Ostrom, 1961) and would be expected to have a reduced effect in leaching any absorbed cations from the clays. The
137 mixtures were filtered, residues washed and the filtrates analysed for Ca, Mg, Na, Sr, Mn and Fe. The test samples had 10--80% insoluble residues with high and differing clay contents which served to check the extreme cases of traceelement leaching. Samples from four areas within Tasmania were chosen: (1) Berriedale (Lower Permian), a fossiliferous impure limestone with illiticexpandable clays (Rao, 1979b); (2) Mole Creek (Ordovician), a variety of limestone types with illitic clays; (3) Junee--Florentine (Ordovician), predominantly algal and micritic limestones with illite; (4) Lower Gordon (a Lower Ordovician sub-surface sequence) consisting of dolomitic limestones, dolomites and dolomitic sandstones with illitic clay (Rao and Naqvi, 1977).
Experirnen tal 1 g of dry powdered sample was weighed into a 100-ml polythene beaker, 50 ml 1 M HC1 at room temperature were added, and the mixture was occasionally stirred over a period of 2 hr. The sample was filtered, washed with deionized water and the solution transferred to a 100-ml volumetric flask, and made up to that volume with deionized water. Another dry 1 g sample was treated with 500 ml of 0.3 M acetic acid in a 1-1 polythene beaker for one week. The solution was filtered and transferred to a Pyrex ® beaker. After evaporation to approximately 50 ml, it was allowed to cool and was washed into a 100-ml volumetric flask and made up to that volume with deionized water. Repeated Ca analysis by AAS indicated that these acid volumes and durations were necessary for complete carbonate fraction digestion. The solutions were analysed for Ca, Mg, Mn, Sr, Na and Fe as outlined later (see section " R a p i d Atomic Absorption Method"). Results are given in Table I along with K20, Mn, St, Na and Fe in the whole rock determined by X-ray fluorescence spectrom etry (XRF).
Results The K20 values range from 0.43--2.70% (Table I) and indicate the high clay content of the insoluble residues. AAS precision for trace elements is generally + 1.5% except for Na where contamination during the long acetic acid digestion and subsequent evaporation reduced it to approximately +- 7% in high Na (> 300 ppm) samples and + 20% in low Na (< 100 ppm) samples. Precision for low Sr is -+ 10% at the 20-ppm level. The Berriedale samples with high insoluble residues (29--71%) of expandable clays show no marked change in Mn, Na and Sr with either acid. The difference is + 4.4% for Mn (152--668 ppm), Na (305--366 ppm) and Sr (242--465 ppm). Fe values vary from 426--1727 ppm and the 1 M HC1 extracts 0.3--31.9% more Fe. Fe increases with carbonate c o n t e n t and shows similar trends with both acids. Mole Creek and Junee--Florentine limestones are purer with insoluble residues (12--28%) consisting largely of illitic clay and quartz. Once again Mn,
138 TABLE I Evaluation of Trace Element Contamination from Non-Carbonate Fraction Area*
B B B B B B MC MC MC MC MC JF JF LG LG LG LG LG LG LG LG
Sample No.
71B 63A 77A 75AB 83A 85C 37434 37474 37464 37466 MC236 42809 212825 6741/47 6741144 6741/45 2166 2246 6741/17 6741123 6741121
Insoluble residue (%)
K20 (%)
29.37 31.05 53.61 58.55 67.75 71.53 12.57 20.93 24.20 25.70 28.44 19.62 27.15 7.33 9.33 14.17 15.79 34.39 47.90 77.49 80.12
0.35 0.43 1.00 0.99 1.61 1.86 0.64 0.79 0.75 0.79 0.52 1.04 1.74 0.65 0.65 1.05 1.46 1.72 2.17 2.70 2.37
CaO (%)
37.4 37.3 24.5 21.5 16.9 14.7 45.5 42.5 41.5 40.6 39.4 44.9 39.0 47.5 46.2 41.2 31.1 18.1 14.3 5.9 5.7
MgO (%)
1.74 0.98 1.51 1.51 1.13 1.09 2.88 1.43 0.74 0.80 0.50 0.99 1.33 3.07 3.23 4.94 11.7 13.6 10.2 4.8 3.9
Mn (ppm) 0.3 M acetic
1M HCl
XRF
668 637 439 335 275 152 142 85 118 188 104 103 95 449 406 398 859 1,330 1,552 418 394
643 649 439 335 280 154 144 84 115 191 99 103 101 475 423 424 857 1,327 1,546 426 415
766 716 499 369 300 166 147 110 132 252 113 142 214 543 476 495 918 1,320 1,544 434 441
*B ~ Berriedale; MC = Mole Creek; JF = Junee--Florentine; LG -- Lower Gordon. N a a n d Sr s h o w n o i n c r e a s e w i t h t h e s t r o n g e r a c i d w h e r e a s F e d o e s increase. T h e L o w e r G o r d o n d o l o m i t i c s a m p l e s have g r e a t l y v a r y i n g i n s o l u b l e r e s i d u e s ( 9 - - 8 0 % ) c o n t a i n i n g an illite clay. T h e Mn values ( 3 9 4 - - 1 5 5 2 p p m ) are m a r g i n a l l y h i g h e r ( 0 - - 6 % ) w i t h 1 M HC1 a n d b o t h N a ( 7 6 - - 7 9 6 p p m ) a n d Sr (14--232 ppm) vary within the precision of the method. Fe (7120--25800 p p m ) r e s u l t s are p r e d o m i n a n t l y l o w e r w i t h a c e t i c a c i d (+1.5 t o - 1 8 . 8 % ) . T h e h i g h e s t F e r e s u l t s are s i m i l a r w i t h b o t h acids (+ 2%).
Discussion A c i d e x t r a c t i o n r e s u l t s f o r Mn, N a a n d Sr are s i m i l a r f o r b o t h 0.3 M a c e t i c a c i d a n d 1 M HC1. C o m p a r i s o n o f A A S r e s u l t s in t h e c a r b o n a t e f r a c t i o n w i t h t h e t o t a l X R F a n a l y s e s i n d i c a t e s t h a t t h e m a j o r i t y o f Mn a n d Sr is in t h e c a r b o n a t e . B a r b e r ( 1 9 7 4 ) n o t e d s i m i l a r results. T h u s , f o r t h e s e e l e m e n t s , t h e a c i d t y p e a n d s t r e n g t h is u n i m p o r t a n t . A c i d - s o l u b l e F e r e s u l t s s h o u l d be t r e a t e d w i t h c a u t i o n . T h e high r e s u l t s f o r HC1 e x t r a c t i o n s s h o w t h a t t h e r e is c o n t a m i n a t i o n from the clay fraction. However, Fe increases with c a r b o n a t e
139
Sr (ppm)
Na (ppm)
Fe (pprn)
0.3 M acetic
1M HC1
XRF
0.3 M acetic
1M HCI
XRF (%)
0.3 M acetic
1M HC1
XRF (%)
465 460 307 337 242 301 393 566 727 689 312 179 210 221 232 216 135 48 38 14 18
448 458 301 338 243 307 398 561 724 679 298 175 204 223 229 218 131 47 36 14 20
490 509 382 447 363 413 411 572 730 708 313 194 218 236 233 218 134 47 36 20 21
310 321 348 310 339 366 75 121 93 85 101 78 74 580 796 223 106 95 115 171 76
308 329 348 305 347 350 83 119 93 88 96 77 80 544 775 223 126 96 111 203 81
0.25 0.3 0.64 1.01 1.11 0.89 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1
1,233 1,647 1,040 723 503 426 1,021 1,116 944 879 1,053 703 438 7,120 7,120 9,260 20,490 23,070 25,320 8,870 7,440
1,237 1,727 1,266 818 638 626 1,169 1,288 1,032 1.091 1,014 853 669 8,920 8,130 11,200 20,980 22,730 25,800 9,830 9,170
0.45 0.58 1.14 0.82 1.26 1.20 0.56 0.62 0.87 0.76 0.39 0.88 1.21 1.24 1.13 1.85 3.10 3.13 3.86 2.80 1.96
c o n t e n t in t h e B e r r i e d a l e a n d M o l e C r e e k s a m p l e s a n d in all s a m p l e s s h o w s a s i m i l a r t r e n d w i t h b o t h acids. S a m p l e s w i t h h i g h F e {1--3%), e.g., s o m e d o l o m i t e s , are less a f f e c t e d b y c o n t a m i n a t i o n . RAPID ATOMIC ABSORPTION METHOD Current AAS m e t h o d s for the analysis of limestones and d o l o m i t e s (Angino a n d Billings, 1 9 7 2 ; B a r b e r , 1 9 7 4 ; W h i t e h e a d , 1 9 7 6 ) i n v o l v e t h e l e n g t h y a c e t i c acid digestion or involve the risk of t race-e l e m e n t c o n t a m i n a t i o n with stronger a c i d s s u c h as HC1 o r HNO3. T h i s is f o l l o w e d b y t w o or m o r e successive dilutions to bring the major and minor elements within the range of this highly s e n s i t i v e t e c h n i q u e . T y p i c a l l y Ca a n d Mg are a n a l y s e d in t h e r a n g e 1 - - 5 p p m f o r Ca a n d 0 . 1 - - 1 . 0 p p m f o r Mg, w h i c h in l i m e s t o n e s r e q u i r e s a d i l u t i o n in t h e o r d e r o f l 0 s X c o m p a r e d w i t h a 102 X d i l u t i o n r e q u i r e d f o r Mn, Sr, N a a n d Fe. T h e s e d i l u t i o n s are t i m e - c o n s u m i n g , a n d t h e r e is a r i s k o f e r r o r in t h e extra step involved. In t h i s s t u d y a s m a l l v o l u m e o f 1 M HC1, a t r o o m t e m p e r a t u r e , was u s e d t o
140
speed up the reaction. Except for Fe, trace-element contamination from the non-carbonate fraction was found to be unimportant (Table I). The major (Ca, Mg) and minor (Mn, Sr, Na and Fe) elements were determined by AAS using a single (102 × ) dilution. Sensitivity for Ca was reduced by using the spectral overlap of the Ge 422.66-nm non-resonance line profile with the Ca 422.67nm resonance line profile (Maruta and Sudoh, 1976). The overlap of the V 285.17-nm non-resonance line profile with Mg 285.21-nm resonance line profile was used for Mg determinations. Mutual interference between Ca and Mg can be avoided by adding LaC13 or KC1 to solutions as a releasing agent (Whitehead, 1976). The present author found 0.5% K in solution to be useful in lowering the Mg interference on Ca and removing the Ca interference on Mg. K was also useful as an ionization suppressant in Sr determinations. High salt content in solution gives rise to a physical interference because high viscosity causes a slower aspiration rate, decreased absorbance and curvature in calibration graphs (Angino and Billings, 1972). In this study approximately 2% solids are present in solution and the interference was overcome by matching the viscosity of standards and samples. Standard addition procedures can be introduced when the calibration is linear though these are time-consuming. The extent of Ca interference in trace-element analyses was determined by comparing the absorbances of 1, 5, 10 and 20 ppm Mn, Na, Sr and Fe solutions containing no Ca with the absorbances of similar solutions containing 3600 ppm Ca. A fuel-lean (oxidising) and a fuel-rich (reducing) flame were compared. AAS results for Ca, Mg, Mn, Na, Sr and Fe in the carbonate fraction of international standards ZGI-KH, GFS400, 401 and 402 are reported along with X-ray spectrometric results for the whole rocks. The latter were determined to supplement existing data on standard carbonates (Thompson et al., 1970; Flanagan, 1973).
A naly tical details One gram of dry sample was treated with 50 ml 1 M HC1 at room temperature for 2 hr. The sample was filtered to remove non-carbonate material, the residue washed with deionized water and the solution transferred to a 100-ml volumetric flask along with KC1 equivalent to 0.5% K in solution. Blanks were prepared similarly. Standards were prepared from specpure CaCO3, Mg metal, Mn powder, Sr(NO3)2, NaC1 and Fe powder. Analytical reagent grade HC1, KC1 and deionized water were used throughout. Ca standards were made up in the range 0--4000 ppm Ca; Mg 0--1400 ppm; and Mn, Sr, Na, Fe 0--20 ppm. Acid concentrations and types and K content were matched to the samples in each case. Matching of standards and samples with respect to Mg was required for Ca analysis. Several sets of Ca standards, containing various
141 TABLE II Instrument settings for atomic absorption analysis Element
Lamp
Current (mA)
Line (nm)
Slit Flame*l (nm)
Burner height (mm below optical axis)
Fuel Support rate rate (1/min.) (1/min.)
Ca Mg.2 Mn .2 Na Fe .2 Sr
Ge V Mn Na Fe Sr
16 15 5 5 5 10
422.66 285.17 279.48 589.59 248.33 460.73
0.3 0.3 0.2 0.2 0.2 0.3
20 5 5 12 5 5
2.4 2.0 2.7 2.7 2.4 4.7
air--C2H: air--C2H 2 air--C2H ~ air--C2H 2 air--C2H 2 N20--C2H 2
11.6 11.6 11.6 11.6 11.6 9.0
,1A Varian ® AA6 spectrophotometer was used with a 10-cm slot burner for air--acetylene and a 6-cm slot burner for nitrous oxide--acetylene. .2 Background (non-atomic) absorption, as measured with a hydrogen continuum lamp, was subtracted from the total absorbance for Mg, Mn and Fe.
a m o u n t s o f Mg up to 1 3 0 0 p p m (equivalent t o a p u r e d o l o m i t e ) , were prepared. T h e t r a c e - e l e m e n t s t a n d a r d s c o n t a i n e d 3 6 0 0 p p m Ca in solution. A A S i n s t r u m e n t a l c o n d i t i o n s are given in Table II. A Varian ® A A 6 spectrop h o t o m e t e r was used with a curve c o r r e c t o r . X R F analyses were p e r f o r m e d with a Philips ® P W 1 4 1 0 X-ray s p e c t r o m e t e r f o l l o w i n g m e t h o d s o f Norrish and Chappell (1977).
Results and discussion Calcium. The o p t i m u m c o n d i t i o n s for Ca listed in Table II were d e t e r m i n e d after e x p e r i m e n t i n g with artificial m i x t u r e s o f Ca and Mg, and v a r y i n g the t y p e and c o n d i t i o n s o f t h e burner. A n i t r o u s o x i d e - - a c e t y l e n e (N20--C2H2) flame gave erratic results. Using standards in w h i c h t h e Mg c o n t e n t was n o t m a t c h e d resulted in l o w Ca values, e.g. the Ca a b s o r b a n c e was depressed b y 3.6% in d o l o m i t e G F S 4 0 0 giving a value o f 29.4% CaO c o m p a r e d with 30.5% CaO. W i t h o u t 0.5% K in s o l u t i o n the a b s o r b a n c e was depressed b y 9.5%. A n o t h e r critical p a r a m e t e r was t h e h e i g h t o f the burner. It h a d to be 20 m m b e l o w t h e optical axis. With the b u r n e r at o p t i m u m setting for m a x i m u m a b s o r b a n c e (3 m m ) , d e p r e s s i o n d u e t o Mg in the d o l o m i t e was 15%. The small Mg i n t e r f e r e n c e c o u l d n o t be entirely e l i m i n a t e d and thus, for t h e analysis o f d o l o m i t i c samples, s t a n d a r d s m u s t be p r e p a r e d with Mg c o n t e n t s similar t o t h o s e o f t h e samples.
Magnesium. A fuel-lean air--C2H2 flame and 0.5% K in s o l u t i o n were f o u n d to r e m o v e Ca interference. A fuel-rich flame caused a b s o r b a n c e e n h a n c e m e n t
142 TABLE III Calcium interference in the AAS determination of Mn, Na, Fe and Sr Concentration* (ppm)
Per cent variation in absorbance with 3600 ppm Ca Mn*2
1 5 10 20
Na *~
Fe*2
Sr .3
1
2
1
2
1
2
3
4
--6.5 --6.9 --7.2 --4.3
--7.0 --4.8 --3.4 --2.0
--8.8 --9.1 --9.0 --7.4
--5.6 --5.5 --4.2 -4.0
--6.4 --7.7 -7.6 -6.8
+16.0 +12.3 +11.8 + 9.8
+2.7 +1.1 +0.9 +1.1
+4.1 +1.3 +0.9 +0.8
,10.5 M HC1 solutions containing 0.5% K as KC1. ,2 Air--acetylene burner with height adjusted to maximum sensitivity for Mn, Na, Fe: 1 = oxidising flame; acetylene 2.0 1/min., air 11.6 l/min. 2 = reducing flame; acetylene 2.7 1/min., air 11.6 1/min. .3 Nitrous oxide--acetylene burner for Sr: 3 = oxidising flame; acetylene 4.7 1/min., nitrous oxide 9.0 1/min. 4 = reducing flame; acetylene 5.4 1/min., nitrous oxide 9.0 1/min. o f 4--7%. B a c k g r o u n d ( n o n - a t o m i c ) a b s o r p t i o n m e a s u r e d with a h y d r o g e n c o n t i n u u m lamp was f o u n d t o be equivalent t o 0.06% MgO in s p e c p u r e CaCO3.
Manganese. In an air--C2H2 flame the a b s o r b a n c e s o f 1 - - 2 0 p p m Mn solutions were depressed b y 2--7% with 3 6 0 0 p p m Ca (Table III). A fuel-rich flame r e d u c e d t h e i n t e r f e r e n c e t o a small e x t e n t as well as giving a higher sensitivity. N o n - a t o m i c a b s o r p t i o n was e q u i v a l e n t to 0.07 p p m in solution. A N20--C2H2 flame r e m o v e d t h e Ca i n t e r f e r e n c e t h o u g h the sensitivity was t o o low for practical use.
Sodium. Initially, c o n t a m i n a t i o n was f o u n d t o be a p r o b l e m in trace Na analysis. All a p p a r a t u s s h o u l d be t h o r o u g h l y cleaned and r e a g e n t blanks s h o u l d be p u t t h r o u g h the analytical p r o c e d u r e . A fuel-rich air--C2H2 flame gave the best results with a b s o r b a n c e depressions o f 4 . 0 - - 5 . 6 % due to 3 6 0 0 p p m Ca (Table III). R u b e s k a et al. ( 1 9 6 3 ) , and T a k e u c h i and S u z u k i ( 1 9 6 4 ) also n o t e d a depression in Na a b s o r b a n c e w i t h Ca in solution.
Strontium. 0.5% K in s o l u t i o n was essential for suppression o f Sr ionisation. A fuel-lean N20--C2H2 p r o v e d t o be t h e m o s t sensitive and Ca i n t e r f e r e n c e was limited t o a 0 . 9 - - 2 . 7 % e n h a n c e m e n t in 1 - - 2 0 p p m Sr solutions with 3 6 0 0 p p m Ca (Table III). N o n - a t o m i c a b s o r p t i o n was n o t e n c o u n t e r e d .
Iron. High Ca caused erratic readings and a b s o r b a n c e e n h a n c e m e n t s o f 9 . 0 - 16.0% with a fuel-rich air--C2H2 flame (Table III). T h e a b s o r b a n c e s were depressed b y 6 . 4 - - 7 . 7 % using a fuel-lean flame. T h e r e was n o i n t e r f e r e n c e
143 TABLE IV Determination of CaO in standard limestones and dolomite Sample
Dolomite GFS400 Limestone GFS401 Limestone GFS402 Limestone ZGI-KH
This paper
Whitehead (1976) method
%CaO
S
CV
%CaO
S
CV
30.4 50.1 46.3 46.8
0.5 0.7 0.5 0.5
1.7 1.4 1.1 1.1
30.5 50.0 46.5 46.9
0.2 0.3 0.2 0.2
0.7 0.6 0.4 0.4
S = standard deviation of three separate samples; CV = coefficient of variation.
with an i n t e r m e d i a t e flame and n o n e with the N20--C2H2 flame t h o u g h the latter was t o o insensitive f o r l o w - F e samples. N o n - a t o m i c a b s o r p t i o n was equivalent to 0.20 p p m Fe with 3 6 0 0 p p m Ca in solution. T h e analyses o f Ca and Mg in i n t e r n a t i o n a l standards (Tables IV and V) are c o m p a r a b l e with t h o s e o f t h e dilute s o l u t i o n m e t h o d o f W h i t e h e a d ( 1 9 7 6 ) t h o u g h precision is r e d u c e d . This is a t t r i b u t e d to the high salt c o n t e n t o f the solution. Since g e o c h e m i c a l i n t e r p r e t a t i o n s in c a r b o n a t e s are usually based on large variations in e l e m e n t c o n t e n t , e.g. greater t h a n 10% (Veizer et al., 1978), the small loss in precision ( a p p r o x i m a t e l y 0.5%) is acceptable. Mn, Sr,
TABLE V Determination of MgO in standard limestones and dolomite Sample
Dolomite GFS400 Limestone GFS401 Limestone GFS402 Limestone ZGI-KH
This paper
Whitehead (1976) method
%MgO
S
CV
%MgO S
CV
21.1 3.57 5.53 0.36
0.1 0.04 0.02 0.05
0.5 1.1 0.4 13.9
21.2 3.56 5.66 0.36
0.3 0.6 0.4 5.6
0.06 0.02 0.02 0.02
S = standard deviation of three separate samples; CV = coefficient of variation.
Na a n d Fe results in i n t e r n a t i o n a l s t a n d a r d s are given in Table VI. Mn and Sr are similar t o or slightly less t h a n the t o t a l r o c k values. These elements are virtually c o n f i n e d t o t h e c a r b o n a t e f r a c t i o n as is Na in G F S 4 0 0 , 401 and 402. A A S values f o r Fe are c o n s i d e r a b l y l o w e r t h a n t h o s e in t h e t o t a l rock. As m e n t i o n e d previously, care s h o u l d be t a k e n w h e n i n t e r p r e t i n g Fe results o w i n g t o c o n t a m i n a t i o n f r o m the n o n - c a r b o n a t e fraction.
144 TABLE VI Trace elements (ppm) in international standards Sample
GFS400 GFS401 GFS402 ZGI-KH
Mn
Sr
AAS (carbonate fraction)
XRF (total)
others* ~ (total)
AAS (carbonate fraction)
XRF (total)
others* 1 (total)
46 105 149 670
47 117 165 750
51, 40 112, 85 162,147 710
626 113 95 546
624 115 92 564
550, 138, 103, 490 586,
1100 152, 140 101, 142 555 *3
GFS400, 401,402: *~ Thompson et al. (1970); and .2 % Na, Ingamells and Suhr (1967). ZGI-KH: *~ Flanagan (1973); and .3 Leoni and Saitta (1977).
CONCLUSIONS T h e p r o p o s e d A A S m e t h o d is essentially a time-saving o n e involving the use o f HC1 instead o f the m o r e p o p u l a r acetic acid in t h e s e p a r a t i o n of t h e c a r b o n a t e f r a c t i o n in l i m e s t o n e s a n d d o l o m i t e s . T h e n e t t i m e saved is d i f f i c u l t t o d e t e r m i n e as it d e p e n d s on t h e c o n c e n t r a t i o n o f acids used in o t h e r m e t h o d s . Usually s a m p l e s are l e f t at least o v e r n i g h t f o r digestion in acetic acid or even for d a y s in t h e case o f d o l o m i t e s . T h e p r e s e n t m e t h o d t a k e s 2 hr. w i t h 1 M HC1. T h e e x t e n t o f leaching o f Mn, Na, Sr a n d Fe f r o m clay minerals was d e t e r m i n e d b y analysing 21 T a s m a n i a n l i m e s t o n e s a n d d o l o m i t e s a f t e r treatm e n t w i t h b o t h w e a k 0.3M acetic acid a n d 1 M HC1. T h e Mn, Na and Sr analyses s h o w e d little d i f f e r e n c e as a f u n c t i o n o f t h e acid used w h e r e a s Fe was d e p e n d e n t o n acid strength. A cold 1 M HC1 digestion is useful f o r geoc h e m i c a l investigations involving Ca, Mg, Mn, N a and Sr in c a r b o n a t e r o c k s w h e n e x p a n d a b l e - i l l i t e a n d illite clay f r a c t i o n s are present. T h e smaller acid v o l u m e used is suitable f o r d i r e c t t r a c e - e l e m e n t analysis o f Mn, Na, Sr a n d Fe p r o v i d e d KC1 is p r e s e n t as a buffer. U n n e c e s s a r y dilutions and f u r t h e r b u f f e r a d d i t i o n s are a v o i d e d f o r Ca a n d Mg analysis. By using the n o n - r e s o n a n c e lines o f Ge 4 2 2 . 6 6 - n m a n d V 2 8 5 . 1 7 - n m , t h e A A S sensitivity f o r Ca a n d Mg is r e d u c e d so t h a t t h e y can be a n a l y s e d in t h e s a m e s o l u t i o n as the trace elements. Results for i n t e r n a t i o n a l s t a n d a r d s Z G I - K H , G F S 4 0 0 , 401 a n d 402 are c o m p a r a b l e w i t h t h o s e o f o t h e r w o r k e r s e x c e p t f o r a small decrease in precision f o r Ca a n d Mg. Such a loss o f t e n a c c o m p a n i e s r a p i d m e t h o d s o f analysis a n d in t h e p r e s e n t case s h o u l d n o t a f f e c t geological i n t e r p r e t a t i o n s .
145
Na
Fe
AAS (carbonate fraction)
XRF (total %)
others .2 (total %)
AAS (carbonate fraction)
XRF (total)
others* 1 (total)
286 143 141 327
<0.1 <0.1 <0.1 <0.1
0.03 0.015 0.015 0.082 .1
158 313 344 1512
400 1470 2730 6150
340,371,455 1407,1390,1358 2639,2590,2420 6500
ACKNOWLEDGEMENTS
The author would like to thank Dr. C.P. Rao for useful discussion and colleagues of the Geology Department, University of Tasmania for helpful criticism of the manuscript.
REFERENCES Angino, E.E. and Billings, G.K., 1972. Atomic Absorption Spectrophotometry in Geology. Elsevier, Amsterdam, 191 pp. Barber, C., 1974. Major and trace element associations in limestones and dolomites. Chem. Geol., 14: 273--280. Flanagan, F.J., 1973. 1972 values for international geochemical reference samples. Geochim. Cosmochim. Acta, 37: 1189--1200. Folk, R.L., 1974. The natural history of crystalline calcium carbonate: effect of magnesium content and salinity. J: Sediment. Petrol., 44: 40--53. Folk, R.L. and Land, L.S., 1975. Mg/Ca ratio and salinity: two controls over crystallization of dolomite. Am. Assoc. Pet. Geol. Bull., 59: 60--68. Friedman, G.M., 1969. Trace elements as possible environmental indicators in carbonate sediments. In: G.M. Friedman (Editor), Depositional Environments in Carbonate Rocks, Soc. Econ. Paleontol. Mineral., Spec. Publ. No. 14, pp. 193--198. Hirst, D.M. and Nicholls, G.D., 1958. Techniques in sedimentary geochemistry, 1. Separation of detrital and non-detrital fractions of limestones. J. Sediment. Petrol., 28: 468--481. Ingamells, C.O. and Suhr, N.H., 1967. Chemical and spectrochemical analysis of standard carbonate rocks. Geochim. Cosmochim. Acta, 31: 1347--1350. Land, L.S. and Hoops, G.K., 1973. Sodium in carbonate sediments and rocks: a possible index to the salinity of diagenetic solutions. J. Sediment. Petrol., 43: 614--617. Leoni, L. and Saitta, M., 1977. Matrix effect corrections by AgKa Co m p t o n scattered radiation in the analysis of rock samples for trace elements. X-Ray Spectrom., 6 (4): 185.
146 Maruta, T. and Sudoh, G., 1976. Determination of calcium in cements by atomic absorption spectrophotometry based on germanium--calcium spectral overlap. Anal. Chim. Acta, 86: 277--279. Norrish, K. and Chappell, B.W., 1977. X-Ray fluorescence spectrometry. In: Zussman, J. (Editor), Physical Methods in Determinative Mineralogy, Academic Press, London, 2nd ed., pp. 201--272. Ostrom, M.E., 1961. Separation of clay minerals from carbonate rocks by using acid. J. Sediment. Petrol., 31 (1): 123--129. Perrin, R.M.S., 1964. The analysis of chalk and other limestones for geochemical studies. Soc. Chem. Ind., Monogr., 18: 207--221. Rai, M., 1965. Separation and identification of clay minerals in argillaceous and siliceous limestones. Indian J. Technol., 3 (7): 215--218. Rao, C.P., 1979a. Cold water limestones: Recent and ancient Tasmanian examples. Aust.--N.Z. Assoc. Adv. Sci., Abstr., 49th Congress, N.Z., 1: 133. Rao, C.P., 1979b. Glacio-marine carbonate sedimentation: Berriedale Limestone (Lower Permian), Hobart, Tasmania, Australia. Am. Assoc. Pet. Geol. Bull., 63: 513. Rao, C.P. and Naqvi, I.H., 1977. Petrography, geochemistry and factor analysis of a lower Ordovician subsurface sequence, Tasmania, Australia. J. Sediment. Petrol., 47 (3): 1036--1055. Ray, S., Gault, H.R. and Dodd, C.G., 1957. The separation of clay minerals from carbonate rocks. Am. Mineral., 42: 681--686. Renard, M. and Blanc, P., 1972. Influence des conditions de mise en solution (choix de l'acide, temp4rature, dur6e, d'attaque) dans le dosage des 616ments en traces des roches carbonat(~es. C.R. Acad. Sci. Paris, S6r. D, 274: 632--635. Rubeska, I., Moldan, B. and Valny, Z., 1963. The determination of sodium in pure limestones by atomic absorption spectrophotometry. Anat. Chim. Acta, 29: 206--210. Takeuchi, T. and Suzuki, M., 1964. The determination of sodium, potassium, magnesium, manganese and calcium in cement by atomic absorption spectrophotometry. Talanta, 11: 1391--1397. Thompson, G., Bankston, D.C. and Pasley, S.M., 1970. Trace element data for reference carbonate rocks. Chem. Geol., 6: 165--170. Veizer, J. and Demovic, R., 1974. Strontium as a tool in facies analysis. J. Sediment. Petrol., 44: 93--115. Veizer, J., Lemieux, J., Jones, B., Gibling, M.R. and Savelle, J., 1977. Sodium: paleosalinity indicator in ancient carbonate rocks. Geology, 5: 177--179. Veizer, J., Lemieux, J., Jones, B., Gibling, M.R. and Savelle, J., 1978. Paleosalinity and dolomitization of a Lower Paleozoic carbonate sequence, Somerset and Prince of Wales Islands, Arctic Canada. Can. J. Earth. Sci., 15: 1448--1461. Whitehead, D., 1976. The determination of calcium and magnesium in carbonate rocks by atomic-absorption spectrophotometry after acetic acid decomposition. Chem. Geol., 18: 1 4 9 - 1 5 3 .
135
DETERMINATION OF CALCIUM, MAGNESIUM, MANGANESE, STRONTIUM, SODIUM AND IRON IN THE CARBONATE FRACTION OF LIMESTONES AND DOLOMITES
P. ROBINSON Geology Department, University of Tasmania, Hobart, Tasmania 7001 (Australia) (Received May 8, 1979; accepted for publication August 16, 1979)
ABSTRACT Robinson, P., 1980. Determinati6n of calcium, magnesium, manganese, strontium, sodium and iron in the carbonate fraction of limestones and dolomites. Chem. Geol., 28: 135-146. A rapid atomic absorption spectrophotometric (AAS) method for the determination of Ca, Mg, Mn, Sr, Na and Fe in limestones and dolomites is described. After cold 1 M hydrochloric acid digestion, potassium chloride is added as a releasing agent and all elements are analysed on a single 100 × dilution. The AAS sensitivity for Ca and Mg is reduced by using the spectral overlap of Ge 422.66-nm and V 285.17-nm non-resonance lines with Ca 422.67-nm and Mg 285.21-nm resonance lines, respectively. Results are reported for four international standards. The extent of any trace element leaching from clay minerals by hydrochloric acid was determined by treating 21 Tasmanian limestones and dolomites with both weak 0.3 M acetic acid and the stronger 1 M hydrochloric acid. Analyses of Mn, Sr and Na show little difference between acids whereas Fe is dependent on acid strength.
INTRODUCTION T h e m a j o r (Ca, Mg) a n d m i n o r (St, Na, Mn a n d Fe) e l e m e n t s in t h e c a r b o n a t e f r a c t i o n o f c a r b o n a t e r o c k s p r o v i d e valuable i n f o r m a t i o n a b o u t t h e original c a r b o n a t e m i n e r a l o g y a n d c h e m i c a l c h a r a c t e r i s t i c s o f d e p o s i t i o n a l a n d diagenetic s o l u t i o n s ( R a o a n d Naqvi, 1 9 7 7 ; R a o , 1 9 7 9 a, b). T h e d i a g e n e t i c fabrics o f l i m e s t o n e s a n d d o l o m i t e s are r e l a t e d t o salinity and t h e M g / C a r a t i o in s o l u t i o n (Folk, 1 9 7 4 ; F o l k a n d L a n d , 1975). Sr varies w i t h original c a r b o n a t e m i n e r a l o g y , facies a n d salinity (Veizer a n d D e m o v i c , 1974). Na is c o n s i d e r e d as a possible i n d e x o f salinity o f d e p o s i t i o n a l a n d diagenetic s o l u t i o n s ( L a n d a n d H o o p s , 1 9 7 3 ; Veizer et al., 1977). Mn d o e s n o t a p p e a r t o u n d e r g o significant changes d u r i n g diagenesis a n d can b e used as an i n d i c a t o r o f original c a r b o n a t e m i n e r a l o g y . T h e a m o u n t o f Fe increases f r o m m a r i n e t h r o u g h b r a c k i s h to f r e s h - w a t e r c a r b o n a t e s ( F r i e d m a n , 1 9 6 9 ) . T h e m a j o r p r o b l e m in such g e o c h e m i c a l studies o f c a r b o n a t e s is t h e uncertainty of contamination from the non-carbonate fraction of samples analysed
136 It would be useful to have a rapid and accurate m e t h o d of analysis which avoids such contamination. This paper deals with a rapid atomic absorption spectrophotometric (AAS) m e t h o d for the determination of Ca, Mg, Sr, Na, Mn and Fe in limestones and dolomites and evaluates the extent of any traceelement contamination from the non-carbonate fraction. SEPARATION OF NON-CARBONATE FRACTION -- TRACE-ELEMENT CONTAMINATION The non-carbonate fraction in carbonate rocks is usually a mixture of quartz, clays, feldspars and to a lesser extent sulphides and phosphates. The c o m m o n l y used m e t h o d of separation is chemical, involving digestion of the carbonate fraction with weak acid. The reaction of acids with non-carbonate minerals depends on the type of acid used, acid concentration and quantity, temperature, duration of reaction, particle size, solubility and crystallinity. Most workers have used X-ray diffraction techniques to study the structural breakdown of clay minerals under acid attack. Ostrom (1961) evaluated the effect of acetic and hydrochloric acids on clay minerals and concluded that 0.3 M acetic acid does not affect randomly interstratified illite--montmorillonite, chlorite, illite or kaolinite. Rai (1965) used 0.2 M acetic acid and Perrin (1964) acetic acid buffered to pH 3 with a m m o n i u m acetate to separate the noncarbonate fraction in limestones. Hirst and Nicholls (1958), and Barber (1974) followed the work of Ray et al. (1957) and used 25% acetic acid for limestones and dolomites. In the present work trace-element variations were studied and an a t t e m p t was made to find a faster m e t h o d of acid attack which would have little effect on the non-carbonate fraction. 1 M HC1 was used instead of the more usual acetic acid for three main reasons: (1) total carbonate digestion using weak acetic acid is slow, lasting up to several days for dolomites; (2) the volume of acetic acid required is large, and, after digestion, has to be reduced for traceelement analysis; and (3) there is a risk of introducing trace-element contamination from the laboratory environment during the prolonged digestion and subsequent evaporation. Few studies have been made on the changes in trace-element concentration with acid type. Hirst and Nicholls (1958) found 25% HC1 (3 M) gave higher results than 25% acetic acid for Ni, Cr, Co and V. Renard and Blanc (1972) studied changes in minor-element analyses with acid type, strength, duration and temperature. They concluded that a 1 N acetic acid digestion at 55°C for 2 hr. was the most favourable. However, only one limestone and one dolomite were tested and the clay mineralogy was not reported. In this work 21 Tasmanian limestones and dolomites were treated with cold 0.3 M acetic acid and the stronger 1 M HC1 to check the extent of trace-element contamination from the non-carbonate fraction. Weak acetic acid such as 0.3 M has no effect on clay mineral structure {Ostrom, 1961) and would be expected to have a reduced effect in leaching any absorbed cations from the clays. The
137 mixtures were filtered, residues washed and the filtrates analysed for Ca, Mg, Na, Sr, Mn and Fe. The test samples had 10--80% insoluble residues with high and differing clay contents which served to check the extreme cases of traceelement leaching. Samples from four areas within Tasmania were chosen: (1) Berriedale (Lower Permian), a fossiliferous impure limestone with illiticexpandable clays (Rao, 1979b); (2) Mole Creek (Ordovician), a variety of limestone types with illitic clays; (3) Junee--Florentine (Ordovician), predominantly algal and micritic limestones with illite; (4) Lower Gordon (a Lower Ordovician sub-surface sequence) consisting of dolomitic limestones, dolomites and dolomitic sandstones with illitic clay (Rao and Naqvi, 1977).
Experirnen tal 1 g of dry powdered sample was weighed into a 100-ml polythene beaker, 50 ml 1 M HC1 at room temperature were added, and the mixture was occasionally stirred over a period of 2 hr. The sample was filtered, washed with deionized water and the solution transferred to a 100-ml volumetric flask, and made up to that volume with deionized water. Another dry 1 g sample was treated with 500 ml of 0.3 M acetic acid in a 1-1 polythene beaker for one week. The solution was filtered and transferred to a Pyrex ® beaker. After evaporation to approximately 50 ml, it was allowed to cool and was washed into a 100-ml volumetric flask and made up to that volume with deionized water. Repeated Ca analysis by AAS indicated that these acid volumes and durations were necessary for complete carbonate fraction digestion. The solutions were analysed for Ca, Mg, Mn, Sr, Na and Fe as outlined later (see section " R a p i d Atomic Absorption Method"). Results are given in Table I along with K20, Mn, St, Na and Fe in the whole rock determined by X-ray fluorescence spectrom etry (XRF).
Results The K20 values range from 0.43--2.70% (Table I) and indicate the high clay content of the insoluble residues. AAS precision for trace elements is generally + 1.5% except for Na where contamination during the long acetic acid digestion and subsequent evaporation reduced it to approximately +- 7% in high Na (> 300 ppm) samples and + 20% in low Na (< 100 ppm) samples. Precision for low Sr is -+ 10% at the 20-ppm level. The Berriedale samples with high insoluble residues (29--71%) of expandable clays show no marked change in Mn, Na and Sr with either acid. The difference is + 4.4% for Mn (152--668 ppm), Na (305--366 ppm) and Sr (242--465 ppm). Fe values vary from 426--1727 ppm and the 1 M HC1 extracts 0.3--31.9% more Fe. Fe increases with carbonate c o n t e n t and shows similar trends with both acids. Mole Creek and Junee--Florentine limestones are purer with insoluble residues (12--28%) consisting largely of illitic clay and quartz. Once again Mn,
138 TABLE I Evaluation of Trace Element Contamination from Non-Carbonate Fraction Area*
B B B B B B MC MC MC MC MC JF JF LG LG LG LG LG LG LG LG
Sample No.
71B 63A 77A 75AB 83A 85C 37434 37474 37464 37466 MC236 42809 212825 6741/47 6741144 6741/45 2166 2246 6741/17 6741123 6741121
Insoluble residue (%)
K20 (%)
29.37 31.05 53.61 58.55 67.75 71.53 12.57 20.93 24.20 25.70 28.44 19.62 27.15 7.33 9.33 14.17 15.79 34.39 47.90 77.49 80.12
0.35 0.43 1.00 0.99 1.61 1.86 0.64 0.79 0.75 0.79 0.52 1.04 1.74 0.65 0.65 1.05 1.46 1.72 2.17 2.70 2.37
CaO (%)
37.4 37.3 24.5 21.5 16.9 14.7 45.5 42.5 41.5 40.6 39.4 44.9 39.0 47.5 46.2 41.2 31.1 18.1 14.3 5.9 5.7
MgO (%)
1.74 0.98 1.51 1.51 1.13 1.09 2.88 1.43 0.74 0.80 0.50 0.99 1.33 3.07 3.23 4.94 11.7 13.6 10.2 4.8 3.9
Mn (ppm) 0.3 M acetic
1M HCl
XRF
668 637 439 335 275 152 142 85 118 188 104 103 95 449 406 398 859 1,330 1,552 418 394
643 649 439 335 280 154 144 84 115 191 99 103 101 475 423 424 857 1,327 1,546 426 415
766 716 499 369 300 166 147 110 132 252 113 142 214 543 476 495 918 1,320 1,544 434 441
*B ~ Berriedale; MC = Mole Creek; JF = Junee--Florentine; LG -- Lower Gordon. N a a n d Sr s h o w n o i n c r e a s e w i t h t h e s t r o n g e r a c i d w h e r e a s F e d o e s increase. T h e L o w e r G o r d o n d o l o m i t i c s a m p l e s have g r e a t l y v a r y i n g i n s o l u b l e r e s i d u e s ( 9 - - 8 0 % ) c o n t a i n i n g an illite clay. T h e Mn values ( 3 9 4 - - 1 5 5 2 p p m ) are m a r g i n a l l y h i g h e r ( 0 - - 6 % ) w i t h 1 M HC1 a n d b o t h N a ( 7 6 - - 7 9 6 p p m ) a n d Sr (14--232 ppm) vary within the precision of the method. Fe (7120--25800 p p m ) r e s u l t s are p r e d o m i n a n t l y l o w e r w i t h a c e t i c a c i d (+1.5 t o - 1 8 . 8 % ) . T h e h i g h e s t F e r e s u l t s are s i m i l a r w i t h b o t h acids (+ 2%).
Discussion A c i d e x t r a c t i o n r e s u l t s f o r Mn, N a a n d Sr are s i m i l a r f o r b o t h 0.3 M a c e t i c a c i d a n d 1 M HC1. C o m p a r i s o n o f A A S r e s u l t s in t h e c a r b o n a t e f r a c t i o n w i t h t h e t o t a l X R F a n a l y s e s i n d i c a t e s t h a t t h e m a j o r i t y o f Mn a n d Sr is in t h e c a r b o n a t e . B a r b e r ( 1 9 7 4 ) n o t e d s i m i l a r results. T h u s , f o r t h e s e e l e m e n t s , t h e a c i d t y p e a n d s t r e n g t h is u n i m p o r t a n t . A c i d - s o l u b l e F e r e s u l t s s h o u l d be t r e a t e d w i t h c a u t i o n . T h e high r e s u l t s f o r HC1 e x t r a c t i o n s s h o w t h a t t h e r e is c o n t a m i n a t i o n from the clay fraction. However, Fe increases with c a r b o n a t e
139
Sr (ppm)
Na (ppm)
Fe (pprn)
0.3 M acetic
1M HC1
XRF
0.3 M acetic
1M HCI
XRF (%)
0.3 M acetic
1M HC1
XRF (%)
465 460 307 337 242 301 393 566 727 689 312 179 210 221 232 216 135 48 38 14 18
448 458 301 338 243 307 398 561 724 679 298 175 204 223 229 218 131 47 36 14 20
490 509 382 447 363 413 411 572 730 708 313 194 218 236 233 218 134 47 36 20 21
310 321 348 310 339 366 75 121 93 85 101 78 74 580 796 223 106 95 115 171 76
308 329 348 305 347 350 83 119 93 88 96 77 80 544 775 223 126 96 111 203 81
0.25 0.3 0.64 1.01 1.11 0.89 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1
1,233 1,647 1,040 723 503 426 1,021 1,116 944 879 1,053 703 438 7,120 7,120 9,260 20,490 23,070 25,320 8,870 7,440
1,237 1,727 1,266 818 638 626 1,169 1,288 1,032 1.091 1,014 853 669 8,920 8,130 11,200 20,980 22,730 25,800 9,830 9,170
0.45 0.58 1.14 0.82 1.26 1.20 0.56 0.62 0.87 0.76 0.39 0.88 1.21 1.24 1.13 1.85 3.10 3.13 3.86 2.80 1.96
c o n t e n t in t h e B e r r i e d a l e a n d M o l e C r e e k s a m p l e s a n d in all s a m p l e s s h o w s a s i m i l a r t r e n d w i t h b o t h acids. S a m p l e s w i t h h i g h F e {1--3%), e.g., s o m e d o l o m i t e s , are less a f f e c t e d b y c o n t a m i n a t i o n . RAPID ATOMIC ABSORPTION METHOD Current AAS m e t h o d s for the analysis of limestones and d o l o m i t e s (Angino a n d Billings, 1 9 7 2 ; B a r b e r , 1 9 7 4 ; W h i t e h e a d , 1 9 7 6 ) i n v o l v e t h e l e n g t h y a c e t i c acid digestion or involve the risk of t race-e l e m e n t c o n t a m i n a t i o n with stronger a c i d s s u c h as HC1 o r HNO3. T h i s is f o l l o w e d b y t w o or m o r e successive dilutions to bring the major and minor elements within the range of this highly s e n s i t i v e t e c h n i q u e . T y p i c a l l y Ca a n d Mg are a n a l y s e d in t h e r a n g e 1 - - 5 p p m f o r Ca a n d 0 . 1 - - 1 . 0 p p m f o r Mg, w h i c h in l i m e s t o n e s r e q u i r e s a d i l u t i o n in t h e o r d e r o f l 0 s X c o m p a r e d w i t h a 102 X d i l u t i o n r e q u i r e d f o r Mn, Sr, N a a n d Fe. T h e s e d i l u t i o n s are t i m e - c o n s u m i n g , a n d t h e r e is a r i s k o f e r r o r in t h e extra step involved. In t h i s s t u d y a s m a l l v o l u m e o f 1 M HC1, a t r o o m t e m p e r a t u r e , was u s e d t o
140
speed up the reaction. Except for Fe, trace-element contamination from the non-carbonate fraction was found to be unimportant (Table I). The major (Ca, Mg) and minor (Mn, Sr, Na and Fe) elements were determined by AAS using a single (102 × ) dilution. Sensitivity for Ca was reduced by using the spectral overlap of the Ge 422.66-nm non-resonance line profile with the Ca 422.67nm resonance line profile (Maruta and Sudoh, 1976). The overlap of the V 285.17-nm non-resonance line profile with Mg 285.21-nm resonance line profile was used for Mg determinations. Mutual interference between Ca and Mg can be avoided by adding LaC13 or KC1 to solutions as a releasing agent (Whitehead, 1976). The present author found 0.5% K in solution to be useful in lowering the Mg interference on Ca and removing the Ca interference on Mg. K was also useful as an ionization suppressant in Sr determinations. High salt content in solution gives rise to a physical interference because high viscosity causes a slower aspiration rate, decreased absorbance and curvature in calibration graphs (Angino and Billings, 1972). In this study approximately 2% solids are present in solution and the interference was overcome by matching the viscosity of standards and samples. Standard addition procedures can be introduced when the calibration is linear though these are time-consuming. The extent of Ca interference in trace-element analyses was determined by comparing the absorbances of 1, 5, 10 and 20 ppm Mn, Na, Sr and Fe solutions containing no Ca with the absorbances of similar solutions containing 3600 ppm Ca. A fuel-lean (oxidising) and a fuel-rich (reducing) flame were compared. AAS results for Ca, Mg, Mn, Na, Sr and Fe in the carbonate fraction of international standards ZGI-KH, GFS400, 401 and 402 are reported along with X-ray spectrometric results for the whole rocks. The latter were determined to supplement existing data on standard carbonates (Thompson et al., 1970; Flanagan, 1973).
A naly tical details One gram of dry sample was treated with 50 ml 1 M HC1 at room temperature for 2 hr. The sample was filtered to remove non-carbonate material, the residue washed with deionized water and the solution transferred to a 100-ml volumetric flask along with KC1 equivalent to 0.5% K in solution. Blanks were prepared similarly. Standards were prepared from specpure CaCO3, Mg metal, Mn powder, Sr(NO3)2, NaC1 and Fe powder. Analytical reagent grade HC1, KC1 and deionized water were used throughout. Ca standards were made up in the range 0--4000 ppm Ca; Mg 0--1400 ppm; and Mn, Sr, Na, Fe 0--20 ppm. Acid concentrations and types and K content were matched to the samples in each case. Matching of standards and samples with respect to Mg was required for Ca analysis. Several sets of Ca standards, containing various
141 TABLE II Instrument settings for atomic absorption analysis Element
Lamp
Current (mA)
Line (nm)
Slit Flame*l (nm)
Burner height (mm below optical axis)
Fuel Support rate rate (1/min.) (1/min.)
Ca Mg.2 Mn .2 Na Fe .2 Sr
Ge V Mn Na Fe Sr
16 15 5 5 5 10
422.66 285.17 279.48 589.59 248.33 460.73
0.3 0.3 0.2 0.2 0.2 0.3
20 5 5 12 5 5
2.4 2.0 2.7 2.7 2.4 4.7
air--C2H: air--C2H 2 air--C2H ~ air--C2H 2 air--C2H 2 N20--C2H 2
11.6 11.6 11.6 11.6 11.6 9.0
,1A Varian ® AA6 spectrophotometer was used with a 10-cm slot burner for air--acetylene and a 6-cm slot burner for nitrous oxide--acetylene. .2 Background (non-atomic) absorption, as measured with a hydrogen continuum lamp, was subtracted from the total absorbance for Mg, Mn and Fe.
a m o u n t s o f Mg up to 1 3 0 0 p p m (equivalent t o a p u r e d o l o m i t e ) , were prepared. T h e t r a c e - e l e m e n t s t a n d a r d s c o n t a i n e d 3 6 0 0 p p m Ca in solution. A A S i n s t r u m e n t a l c o n d i t i o n s are given in Table II. A Varian ® A A 6 spectrop h o t o m e t e r was used with a curve c o r r e c t o r . X R F analyses were p e r f o r m e d with a Philips ® P W 1 4 1 0 X-ray s p e c t r o m e t e r f o l l o w i n g m e t h o d s o f Norrish and Chappell (1977).
Results and discussion Calcium. The o p t i m u m c o n d i t i o n s for Ca listed in Table II were d e t e r m i n e d after e x p e r i m e n t i n g with artificial m i x t u r e s o f Ca and Mg, and v a r y i n g the t y p e and c o n d i t i o n s o f t h e burner. A n i t r o u s o x i d e - - a c e t y l e n e (N20--C2H2) flame gave erratic results. Using standards in w h i c h t h e Mg c o n t e n t was n o t m a t c h e d resulted in l o w Ca values, e.g. the Ca a b s o r b a n c e was depressed b y 3.6% in d o l o m i t e G F S 4 0 0 giving a value o f 29.4% CaO c o m p a r e d with 30.5% CaO. W i t h o u t 0.5% K in s o l u t i o n the a b s o r b a n c e was depressed b y 9.5%. A n o t h e r critical p a r a m e t e r was t h e h e i g h t o f the burner. It h a d to be 20 m m b e l o w t h e optical axis. With the b u r n e r at o p t i m u m setting for m a x i m u m a b s o r b a n c e (3 m m ) , d e p r e s s i o n d u e t o Mg in the d o l o m i t e was 15%. The small Mg i n t e r f e r e n c e c o u l d n o t be entirely e l i m i n a t e d and thus, for t h e analysis o f d o l o m i t i c samples, s t a n d a r d s m u s t be p r e p a r e d with Mg c o n t e n t s similar t o t h o s e o f t h e samples.
Magnesium. A fuel-lean air--C2H2 flame and 0.5% K in s o l u t i o n were f o u n d to r e m o v e Ca interference. A fuel-rich flame caused a b s o r b a n c e e n h a n c e m e n t
142 TABLE III Calcium interference in the AAS determination of Mn, Na, Fe and Sr Concentration* (ppm)
Per cent variation in absorbance with 3600 ppm Ca Mn*2
1 5 10 20
Na *~
Fe*2
Sr .3
1
2
1
2
1
2
3
4
--6.5 --6.9 --7.2 --4.3
--7.0 --4.8 --3.4 --2.0
--8.8 --9.1 --9.0 --7.4
--5.6 --5.5 --4.2 -4.0
--6.4 --7.7 -7.6 -6.8
+16.0 +12.3 +11.8 + 9.8
+2.7 +1.1 +0.9 +1.1
+4.1 +1.3 +0.9 +0.8
,10.5 M HC1 solutions containing 0.5% K as KC1. ,2 Air--acetylene burner with height adjusted to maximum sensitivity for Mn, Na, Fe: 1 = oxidising flame; acetylene 2.0 1/min., air 11.6 l/min. 2 = reducing flame; acetylene 2.7 1/min., air 11.6 1/min. .3 Nitrous oxide--acetylene burner for Sr: 3 = oxidising flame; acetylene 4.7 1/min., nitrous oxide 9.0 1/min. 4 = reducing flame; acetylene 5.4 1/min., nitrous oxide 9.0 1/min. o f 4--7%. B a c k g r o u n d ( n o n - a t o m i c ) a b s o r p t i o n m e a s u r e d with a h y d r o g e n c o n t i n u u m lamp was f o u n d t o be equivalent t o 0.06% MgO in s p e c p u r e CaCO3.
Manganese. In an air--C2H2 flame the a b s o r b a n c e s o f 1 - - 2 0 p p m Mn solutions were depressed b y 2--7% with 3 6 0 0 p p m Ca (Table III). A fuel-rich flame r e d u c e d t h e i n t e r f e r e n c e t o a small e x t e n t as well as giving a higher sensitivity. N o n - a t o m i c a b s o r p t i o n was e q u i v a l e n t to 0.07 p p m in solution. A N20--C2H2 flame r e m o v e d t h e Ca i n t e r f e r e n c e t h o u g h the sensitivity was t o o low for practical use.
Sodium. Initially, c o n t a m i n a t i o n was f o u n d t o be a p r o b l e m in trace Na analysis. All a p p a r a t u s s h o u l d be t h o r o u g h l y cleaned and r e a g e n t blanks s h o u l d be p u t t h r o u g h the analytical p r o c e d u r e . A fuel-rich air--C2H2 flame gave the best results with a b s o r b a n c e depressions o f 4 . 0 - - 5 . 6 % due to 3 6 0 0 p p m Ca (Table III). R u b e s k a et al. ( 1 9 6 3 ) , and T a k e u c h i and S u z u k i ( 1 9 6 4 ) also n o t e d a depression in Na a b s o r b a n c e w i t h Ca in solution.
Strontium. 0.5% K in s o l u t i o n was essential for suppression o f Sr ionisation. A fuel-lean N20--C2H2 p r o v e d t o be t h e m o s t sensitive and Ca i n t e r f e r e n c e was limited t o a 0 . 9 - - 2 . 7 % e n h a n c e m e n t in 1 - - 2 0 p p m Sr solutions with 3 6 0 0 p p m Ca (Table III). N o n - a t o m i c a b s o r p t i o n was n o t e n c o u n t e r e d .
Iron. High Ca caused erratic readings and a b s o r b a n c e e n h a n c e m e n t s o f 9 . 0 - 16.0% with a fuel-rich air--C2H2 flame (Table III). T h e a b s o r b a n c e s were depressed b y 6 . 4 - - 7 . 7 % using a fuel-lean flame. T h e r e was n o i n t e r f e r e n c e
143 TABLE IV Determination of CaO in standard limestones and dolomite Sample
Dolomite GFS400 Limestone GFS401 Limestone GFS402 Limestone ZGI-KH
This paper
Whitehead (1976) method
%CaO
S
CV
%CaO
S
CV
30.4 50.1 46.3 46.8
0.5 0.7 0.5 0.5
1.7 1.4 1.1 1.1
30.5 50.0 46.5 46.9
0.2 0.3 0.2 0.2
0.7 0.6 0.4 0.4
S = standard deviation of three separate samples; CV = coefficient of variation.
with an i n t e r m e d i a t e flame and n o n e with the N20--C2H2 flame t h o u g h the latter was t o o insensitive f o r l o w - F e samples. N o n - a t o m i c a b s o r p t i o n was equivalent to 0.20 p p m Fe with 3 6 0 0 p p m Ca in solution. T h e analyses o f Ca and Mg in i n t e r n a t i o n a l standards (Tables IV and V) are c o m p a r a b l e with t h o s e o f t h e dilute s o l u t i o n m e t h o d o f W h i t e h e a d ( 1 9 7 6 ) t h o u g h precision is r e d u c e d . This is a t t r i b u t e d to the high salt c o n t e n t o f the solution. Since g e o c h e m i c a l i n t e r p r e t a t i o n s in c a r b o n a t e s are usually based on large variations in e l e m e n t c o n t e n t , e.g. greater t h a n 10% (Veizer et al., 1978), the small loss in precision ( a p p r o x i m a t e l y 0.5%) is acceptable. Mn, Sr,
TABLE V Determination of MgO in standard limestones and dolomite Sample
Dolomite GFS400 Limestone GFS401 Limestone GFS402 Limestone ZGI-KH
This paper
Whitehead (1976) method
%MgO
S
CV
%MgO S
CV
21.1 3.57 5.53 0.36
0.1 0.04 0.02 0.05
0.5 1.1 0.4 13.9
21.2 3.56 5.66 0.36
0.3 0.6 0.4 5.6
0.06 0.02 0.02 0.02
S = standard deviation of three separate samples; CV = coefficient of variation.
Na a n d Fe results in i n t e r n a t i o n a l s t a n d a r d s are given in Table VI. Mn and Sr are similar t o or slightly less t h a n the t o t a l r o c k values. These elements are virtually c o n f i n e d t o t h e c a r b o n a t e f r a c t i o n as is Na in G F S 4 0 0 , 401 and 402. A A S values f o r Fe are c o n s i d e r a b l y l o w e r t h a n t h o s e in t h e t o t a l rock. As m e n t i o n e d previously, care s h o u l d be t a k e n w h e n i n t e r p r e t i n g Fe results o w i n g t o c o n t a m i n a t i o n f r o m the n o n - c a r b o n a t e fraction.
144 TABLE VI Trace elements (ppm) in international standards Sample
GFS400 GFS401 GFS402 ZGI-KH
Mn
Sr
AAS (carbonate fraction)
XRF (total)
others* ~ (total)
AAS (carbonate fraction)
XRF (total)
others* 1 (total)
46 105 149 670
47 117 165 750
51, 40 112, 85 162,147 710
626 113 95 546
624 115 92 564
550, 138, 103, 490 586,
1100 152, 140 101, 142 555 *3
GFS400, 401,402: *~ Thompson et al. (1970); and .2 % Na, Ingamells and Suhr (1967). ZGI-KH: *~ Flanagan (1973); and .3 Leoni and Saitta (1977).
CONCLUSIONS T h e p r o p o s e d A A S m e t h o d is essentially a time-saving o n e involving the use o f HC1 instead o f the m o r e p o p u l a r acetic acid in t h e s e p a r a t i o n of t h e c a r b o n a t e f r a c t i o n in l i m e s t o n e s a n d d o l o m i t e s . T h e n e t t i m e saved is d i f f i c u l t t o d e t e r m i n e as it d e p e n d s on t h e c o n c e n t r a t i o n o f acids used in o t h e r m e t h o d s . Usually s a m p l e s are l e f t at least o v e r n i g h t f o r digestion in acetic acid or even for d a y s in t h e case o f d o l o m i t e s . T h e p r e s e n t m e t h o d t a k e s 2 hr. w i t h 1 M HC1. T h e e x t e n t o f leaching o f Mn, Na, Sr a n d Fe f r o m clay minerals was d e t e r m i n e d b y analysing 21 T a s m a n i a n l i m e s t o n e s a n d d o l o m i t e s a f t e r treatm e n t w i t h b o t h w e a k 0.3M acetic acid a n d 1 M HC1. T h e Mn, Na and Sr analyses s h o w e d little d i f f e r e n c e as a f u n c t i o n o f t h e acid used w h e r e a s Fe was d e p e n d e n t o n acid strength. A cold 1 M HC1 digestion is useful f o r geoc h e m i c a l investigations involving Ca, Mg, Mn, N a and Sr in c a r b o n a t e r o c k s w h e n e x p a n d a b l e - i l l i t e a n d illite clay f r a c t i o n s are present. T h e smaller acid v o l u m e used is suitable f o r d i r e c t t r a c e - e l e m e n t analysis o f Mn, Na, Sr a n d Fe p r o v i d e d KC1 is p r e s e n t as a buffer. U n n e c e s s a r y dilutions and f u r t h e r b u f f e r a d d i t i o n s are a v o i d e d f o r Ca a n d Mg analysis. By using the n o n - r e s o n a n c e lines o f Ge 4 2 2 . 6 6 - n m a n d V 2 8 5 . 1 7 - n m , t h e A A S sensitivity f o r Ca a n d Mg is r e d u c e d so t h a t t h e y can be a n a l y s e d in t h e s a m e s o l u t i o n as the trace elements. Results for i n t e r n a t i o n a l s t a n d a r d s Z G I - K H , G F S 4 0 0 , 401 a n d 402 are c o m p a r a b l e w i t h t h o s e o f o t h e r w o r k e r s e x c e p t f o r a small decrease in precision f o r Ca a n d Mg. Such a loss o f t e n a c c o m p a n i e s r a p i d m e t h o d s o f analysis a n d in t h e p r e s e n t case s h o u l d n o t a f f e c t geological i n t e r p r e t a t i o n s .
145
Na
Fe
AAS (carbonate fraction)
XRF (total %)
others .2 (total %)
AAS (carbonate fraction)
XRF (total)
others* 1 (total)
286 143 141 327
<0.1 <0.1 <0.1 <0.1
0.03 0.015 0.015 0.082 .1
158 313 344 1512
400 1470 2730 6150
340,371,455 1407,1390,1358 2639,2590,2420 6500
ACKNOWLEDGEMENTS
The author would like to thank Dr. C.P. Rao for useful discussion and colleagues of the Geology Department, University of Tasmania for helpful criticism of the manuscript.
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