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Determination of opal in marine sediments by X-ray diffraction
Xethcrl~ds ,7our~l of &a P~earch $ (3): 382-389 (1971)
DETERMINATION OF OPAL IN MARINE S E D I M E N T S BY X - R A Y D I F F R A C T I O N by D. EISMA and S.J. VAN DER GAAST (Netlurlands Institu~ for Sea P~seareh, Texel, The Nett~rlands) CONTENTS I. II. III. IV. V.
Introduction . Experiments . Results . . . Summary . . References . .
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382 383 384 388 388
I. I N T R O D U C T I O N Natural opal ranges in cristallinity from amorphous SiO~, which can be considered a random network of tetrahedraUy bound Si-atoms (CALWRT, 1966), to microcristaUine disordered low-cristobalite-tridymite (Swn~m~om) & F p ~ , 1959). Opal in marine sediments is primarily biogenous and rather pure amorphous SiO~: diatoms, radiolarians, sponge spicules and land-derived phytoliths (KoT.~e~, 1955). Only some types of phytoliths contain ~-quartz (LANNINO, PONNAIYA & CRUMPTON, 1958). Upon heating for several hours at 950 ° C the biogenous opal is transformed into ~¢-cristobalite with a sharp (101) diffraction peak at 4.04 A. GOLDBSRG (1958) used the height of this peak to determine the amount of biogenous opal present in the sediment. Before heating the sample to 950 ° C organic matter, CaCO s and sea salts were removed. Known amounts of opal were added and the original (unknown) amount calculated. CALVERT (1966) used a modified version, adding ~-alumina as an internal standard, and comparing with a calibration curve made from natural opal, a matrix of sediment with a clay mineral composition very similar to the sediments to be analysed, and at-alumina. He found different calibration curves for diatoms, radiolarians and sponge spicules. From the work of FLSRKE (1961), among others, it is known that the cristaUization of amorphous SiO~ at high temperatures is influenced by even minute amounts of cations (in the order of0.1%) : tridymite and, in the presence of Ca, wollastonite are formed instead of cristobalite. The transformation of quartz into cristobalite at 1100 to 1500 ° C is similarly influenced especially when feldspars and kaolinite are
DETERMINATION
OF O P A L
383
present (vz KEYSER & CYPRILs, 1961). Although in the method used by GOLDBERO (1958) and CALVERT (1966) the most accessiblecations are removed with the carbonates and the sea salts,there remain the cations on exchange positions on the clays and those in the latticesof clay-minerals,feldspars,mica's, etc.Therefore the effectof the presence of cations and various minerals on the height of the crlstobalitcpeak was determined. Also a quantitative determination of opal in sediments without heating was tried out. A c k n o w l e d g e m e n t s . - - W e l i k e to t h a n k Mile N. B o u r y - E s n a u l t (Banyuls) for sending us some M e d i t e r r a n e a n sponges a n d Dr. H . W. v a n der M a r e l for his cooperation. II. E X P E R I M E N T S
Sponge spiculeswere obtained from Mediterranean sponges (collected near Banyuls, France), diatoms from a wooden raftin the Dutch Wadden Sea. Organic parts wcrc removed with hypochloritc and H a O i and carbonates with diluted HCI. After washing the sample with distilled water the sand particles wcrc removed by repeated settling and decantation. The nearly pure opal was then dried at 105 ° C, and examined under a microscope. The sponge spiculeswcrc contaminated only by an occasional quartz or mica grain and wcrc found to bc at least 98 % pure opal. The diatoms wcrc somewhat less pure, and by chemical analysis wcrc found to contain 5.8 % impurities. TABLE I Purity of the various minerals used for mixing.
Minm'al
Purity
quartz albitc orthoclasc
pure
gibbsite microcline anorthite chlorite muscovite montmorillonitc allophane biotite illite ball clay kaolinite
traces of m i c a traces of m i c a pure traces of m i c a
pure pure pure pure traces of quartz pure 20% quartz 3% mica pure
384
D. E I S M A & S. J. V A N I ) E R G A A S T
Mixtures of sponge spicules with Ca-oxalate, Na-acctate, K-oxalate, Mg,acetate, various minerals (quartz, feldspars, mica, etc,)and 10 % weight of oc-A190s or BeO as standard were ground and h o m o ~ n i z e d in a steel mixer mill. Then the samples were heated at 950 ° C for 4 hours in platinum crucibles. After cooling, X-ray analysis was carried out on a Philips ~ c t o m c t e r with iron filtered C o Ka radiation. The added oxalates and acetates were chemically pure. The purity of the added minerals as determined by X-ray diffraction is given in Table I. III. RESULTS Fig. 1 gives the influence of various amounts of Na, K, Ca and Mg, added as oxalate or acetate, on the height of the 4.04 A (101) eristobalite peak corrected with BeO standard. The presence of even small amounts of these cations lowers this peak considerably. With Na, K and Mg tridymite is formed (peaks at 3.81, 4.07 and 4.30 A), with Ca wollastonite (peak at 2.96 A), With Na and Mg also small peaks appear at 3.93 A and 4.22 A, which could not be identified: With Mg a
!+0
i0
I.zo,
. . . . .
\'/
"
~
[] K+
o,,.
i
IO-
°o
;
i
i
% (weight) oddtd ions
Fig.1. Loweringof the peak-heightof cristobatitedue to the presenceof Ca ++, Mg++, K + a n d N a +.
385
D E T E R M I N A T I O N OF O P A L
decrease of the 4.04 A peak and formation of tridymite was only found for amounts in the order of 1.1% Mg or smaller; on adding amounts of 2 % and 4 % Mg no tridymite was formed and no other minerals, besides cristobalite, although also in these cases the 4.04 A peak was lower than from pure amorphous SiO~. 30.
25, A
i j 15,
| i I0,
O
0
5
I0
15
20
25 %
% (weight) added mineral
Fig.2. Lowering of the peak-height of cristobalite due to the presence of variom min-
crals.
The influence of the presence of various minerals on t h e height of the 4.04 A peak corrected with a-A120 3 as standard is given in Fig. 2. Most minerals lower the peak height; most of all the clay minerals and muscovite. Biotite, chlorite, gibbsite and feldspars have a smaller effect, while addition of quartz gives virtually no decrease in peak height. From these results it is evident that even if for calibration a sediment is used with a composition very similar to the sediment to be anlysed (as done by CALVERT, 1966), the variations within these sediments may still considerably influence the peak height found for cristobalite
386
D. EtSMA • S. J. VAN hER OAAST
upon heating. Therefore, a method of determination using the opal
"bulge" (Fig. 3) directly without h e a t i n g t h e sediment seemed preferable. All opal in marine sediments shows this bulge but there are differences in shape between diatoms, sponge spicules and radiolarians (CALVERT, 1966).
|
i o
o
+ 9 . 0 9 % a -AIzO 3
5o"
,6" ,o" ~" ~:¢ ~o" ,~" Fig.3. Method of measuring the height of the opal "bulge".
,~" 0
With Co Ka-radiation the bulge lies between 17 ° 2 0 and 46 ° 20 with the top at 26 ° 20. Its surface area can be determined by drawing a straight line between the base points at 17 ° and 46 °, and peak height can be measured at 26 ° with the straight line as a base (Fig. 3). In this way calibration curves were made for diatoms and sponge spicules (Fig. 4), using calcite as matrix and =-AlsOt as standard. Seven points were determined for each curve, each point being an average of three separate determinations. The results based on surface area and peak height are very similar: for both methods the largest error, for opal percentages higher than 10%, is in the order of + 3% of the value found. Below 10% opal the method becomes less precise, as can be seen from the calibration curves (Fig. 4). Calibration curves for diatoms and sponge spicules differ only slightly. T h e calibration curves cannot be appli~i directly to sediments that
DETERMINATION
387
OF OPAL
contain kaolinite (halloysite) and to a smaller extent mica, iUite and montmorillonite, since the diffractograms of these minerals also show a certain bulge at 26 ° 2 O, caused by the coincidence of peaks in this region. This can be corrected by measuring the height of this bulge at l.O.
Gg.
0.7.
0.6.
03,
0.2
0.1,
0
,~
0
% dlm~,s (mill,l)
Fig.4. Calibration curve for diatoms (% weight), based on the height of the opal "bulge" and corrected with ~-AlsOs as internal standard. 26 ° 2 ® in a sediment with a mineralogical composition very similar to the sediment to be analysed, and containing no opal, after adding ~-AlsO s as standard. Substracting this value from the total found in the sample containing opal gives an estimate of the true percentage in the sample. This was tried out by adding amounts of sponge spicules to some fine-grained marine sediments containing kaolinite and mica as dominant minerals. The agreement (Table II) between the amounts added and the percentages found is good except in one case where the percentage found is rather low (but still within the calibration error). Since the natural variations in mineralogical composition of the sediment m a y in some cases also cause errors in the opal determination based on using the bulge, this method thus has the same drawback as by the heating-method, but it is easier to apply. A d d i n g known amounts of opal can increase the reliability of the observations by pro-
388
D. E I S M A
& S.
j.
VAN
DER
GAAST
TABLE II
Opal determination in four sediments after two different additions of opal. Sediment nr.
Dominant minerals
% (weight) opal added
1.
quartz (40%), calcite (20%), mica (10%), kaolinite (20%) calcite (90%)
9.4 45.9
26 59
---16 --16
10 43
10.0 46.5 10.6 46.2
11 49 42 73
---1 ---1 ---32 --32
10 48 10 41
10.2 45.5
12 50
---3 --3
9 47
2. 3. 4.
kaolinite (40%), quartz (30%), calcite (20%) quartz (40%), feldspars (30%), mica (10%), kaolinite (10%)
% (weight) Corrertion Correrted% opalfound opalfound
viding a control. Moreover a correction can be made by comparing the height of the kaolinite or mica peak in the sediment t o be analysed, with the height of this peak in the opal,free Sediment used as standard. IV. SUMMARY The determination of opal in marine sediments by measuring the 101 cristobalite peak formed upon heating at 950 ° C m a y give erroneous results because the height of the peak is strongly influenced by the presence of cations (Na, K, Mg, Ca) and minerals common in marine sediments. Therefore an X-ray method without heating is proposed, measuring the height of the opal "bulge". This method can be used without complications in sediments containing only small amounts of kaolinite, mica, illite or montmorillonite. I f these minerals dominate, corrections must be made. Trials show t h a t in such cases the largest error found is in the order of about 6 %. When the calibration curves can be applied without corrections the largest error found is in the order of about 3 %. V. R E F E R E N C E S
CALVERT,S. E , ]966. Accumulation of d l a t ~ silica in the sediments of the Gulf of Oalifemia.--Bull. 8col. Soc. Am. 7'h 569-596. FL6aXE, O. W., 1961. A ~ o n of the tridymite-cristobalite problem.--Silic. ind. ~ : 415-418. Go--o, E. D., 1958. Determination of opal in marine scdimcnts.--J, mar. Rcs. 17t 178-182.
DETERMINATION OF OPAL
389
KEYSER,W. L. DE & R. C Y P ~ , 1961. Beitrag zum Studium der Bildung des Tridymits. Mineralisatorwirkung yon Feldspar und Kaolin auf die Umwandlung des Quarzes.--Ber. dt. keram. Ges. 38(7): 303-308. KoLnS, R. W., 1955. Diatoms from equatorial Atlantic cores.--Rep. Swed. deep Sea Exped. 7(3): 151-184. LANNING,F. C., B. W. X. PONNAIYA& C. F. CRUMPTON,1958. The chemical nature of silica in plants.--P1. Physiol., Wash. 33: 339-343. SWI~ZFORD,A. & P. C. FRANKS,1959. Opal in the Ogaliala formation in Kansas. In: Silica in sediments. Spec. Pubis Soc. econ. Palaeont. Miner. 7:111-120.
DETERMINATION OF OPAL IN MARINE S E D I M E N T S BY X - R A Y D I F F R A C T I O N by D. EISMA and S.J. VAN DER GAAST (Netlurlands Institu~ for Sea P~seareh, Texel, The Nett~rlands) CONTENTS I. II. III. IV. V.
Introduction . Experiments . Results . . . Summary . . References . .
. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . .
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382 383 384 388 388
I. I N T R O D U C T I O N Natural opal ranges in cristallinity from amorphous SiO~, which can be considered a random network of tetrahedraUy bound Si-atoms (CALWRT, 1966), to microcristaUine disordered low-cristobalite-tridymite (Swn~m~om) & F p ~ , 1959). Opal in marine sediments is primarily biogenous and rather pure amorphous SiO~: diatoms, radiolarians, sponge spicules and land-derived phytoliths (KoT.~e~, 1955). Only some types of phytoliths contain ~-quartz (LANNINO, PONNAIYA & CRUMPTON, 1958). Upon heating for several hours at 950 ° C the biogenous opal is transformed into ~¢-cristobalite with a sharp (101) diffraction peak at 4.04 A. GOLDBSRG (1958) used the height of this peak to determine the amount of biogenous opal present in the sediment. Before heating the sample to 950 ° C organic matter, CaCO s and sea salts were removed. Known amounts of opal were added and the original (unknown) amount calculated. CALVERT (1966) used a modified version, adding ~-alumina as an internal standard, and comparing with a calibration curve made from natural opal, a matrix of sediment with a clay mineral composition very similar to the sediments to be analysed, and at-alumina. He found different calibration curves for diatoms, radiolarians and sponge spicules. From the work of FLSRKE (1961), among others, it is known that the cristaUization of amorphous SiO~ at high temperatures is influenced by even minute amounts of cations (in the order of0.1%) : tridymite and, in the presence of Ca, wollastonite are formed instead of cristobalite. The transformation of quartz into cristobalite at 1100 to 1500 ° C is similarly influenced especially when feldspars and kaolinite are
DETERMINATION
OF O P A L
383
present (vz KEYSER & CYPRILs, 1961). Although in the method used by GOLDBERO (1958) and CALVERT (1966) the most accessiblecations are removed with the carbonates and the sea salts,there remain the cations on exchange positions on the clays and those in the latticesof clay-minerals,feldspars,mica's, etc.Therefore the effectof the presence of cations and various minerals on the height of the crlstobalitcpeak was determined. Also a quantitative determination of opal in sediments without heating was tried out. A c k n o w l e d g e m e n t s . - - W e l i k e to t h a n k Mile N. B o u r y - E s n a u l t (Banyuls) for sending us some M e d i t e r r a n e a n sponges a n d Dr. H . W. v a n der M a r e l for his cooperation. II. E X P E R I M E N T S
Sponge spiculeswere obtained from Mediterranean sponges (collected near Banyuls, France), diatoms from a wooden raftin the Dutch Wadden Sea. Organic parts wcrc removed with hypochloritc and H a O i and carbonates with diluted HCI. After washing the sample with distilled water the sand particles wcrc removed by repeated settling and decantation. The nearly pure opal was then dried at 105 ° C, and examined under a microscope. The sponge spiculeswcrc contaminated only by an occasional quartz or mica grain and wcrc found to bc at least 98 % pure opal. The diatoms wcrc somewhat less pure, and by chemical analysis wcrc found to contain 5.8 % impurities. TABLE I Purity of the various minerals used for mixing.
Minm'al
Purity
quartz albitc orthoclasc
pure
gibbsite microcline anorthite chlorite muscovite montmorillonitc allophane biotite illite ball clay kaolinite
traces of m i c a traces of m i c a pure traces of m i c a
pure pure pure pure traces of quartz pure 20% quartz 3% mica pure
384
D. E I S M A & S. J. V A N I ) E R G A A S T
Mixtures of sponge spicules with Ca-oxalate, Na-acctate, K-oxalate, Mg,acetate, various minerals (quartz, feldspars, mica, etc,)and 10 % weight of oc-A190s or BeO as standard were ground and h o m o ~ n i z e d in a steel mixer mill. Then the samples were heated at 950 ° C for 4 hours in platinum crucibles. After cooling, X-ray analysis was carried out on a Philips ~ c t o m c t e r with iron filtered C o Ka radiation. The added oxalates and acetates were chemically pure. The purity of the added minerals as determined by X-ray diffraction is given in Table I. III. RESULTS Fig. 1 gives the influence of various amounts of Na, K, Ca and Mg, added as oxalate or acetate, on the height of the 4.04 A (101) eristobalite peak corrected with BeO standard. The presence of even small amounts of these cations lowers this peak considerably. With Na, K and Mg tridymite is formed (peaks at 3.81, 4.07 and 4.30 A), with Ca wollastonite (peak at 2.96 A), With Na and Mg also small peaks appear at 3.93 A and 4.22 A, which could not be identified: With Mg a
!+0
i0
I.zo,
. . . . .
\'/
"
~
[] K+
o,,.
i
IO-
°o
;
i
i
% (weight) oddtd ions
Fig.1. Loweringof the peak-heightof cristobatitedue to the presenceof Ca ++, Mg++, K + a n d N a +.
385
D E T E R M I N A T I O N OF O P A L
decrease of the 4.04 A peak and formation of tridymite was only found for amounts in the order of 1.1% Mg or smaller; on adding amounts of 2 % and 4 % Mg no tridymite was formed and no other minerals, besides cristobalite, although also in these cases the 4.04 A peak was lower than from pure amorphous SiO~. 30.
25, A
i j 15,
| i I0,
O
0
5
I0
15
20
25 %
% (weight) added mineral
Fig.2. Lowering of the peak-height of cristobalite due to the presence of variom min-
crals.
The influence of the presence of various minerals on t h e height of the 4.04 A peak corrected with a-A120 3 as standard is given in Fig. 2. Most minerals lower the peak height; most of all the clay minerals and muscovite. Biotite, chlorite, gibbsite and feldspars have a smaller effect, while addition of quartz gives virtually no decrease in peak height. From these results it is evident that even if for calibration a sediment is used with a composition very similar to the sediment to be anlysed (as done by CALVERT, 1966), the variations within these sediments may still considerably influence the peak height found for cristobalite
386
D. EtSMA • S. J. VAN hER OAAST
upon heating. Therefore, a method of determination using the opal
"bulge" (Fig. 3) directly without h e a t i n g t h e sediment seemed preferable. All opal in marine sediments shows this bulge but there are differences in shape between diatoms, sponge spicules and radiolarians (CALVERT, 1966).
|
i o
o
+ 9 . 0 9 % a -AIzO 3
5o"
,6" ,o" ~" ~:¢ ~o" ,~" Fig.3. Method of measuring the height of the opal "bulge".
,~" 0
With Co Ka-radiation the bulge lies between 17 ° 2 0 and 46 ° 20 with the top at 26 ° 20. Its surface area can be determined by drawing a straight line between the base points at 17 ° and 46 °, and peak height can be measured at 26 ° with the straight line as a base (Fig. 3). In this way calibration curves were made for diatoms and sponge spicules (Fig. 4), using calcite as matrix and =-AlsOt as standard. Seven points were determined for each curve, each point being an average of three separate determinations. The results based on surface area and peak height are very similar: for both methods the largest error, for opal percentages higher than 10%, is in the order of + 3% of the value found. Below 10% opal the method becomes less precise, as can be seen from the calibration curves (Fig. 4). Calibration curves for diatoms and sponge spicules differ only slightly. T h e calibration curves cannot be appli~i directly to sediments that
DETERMINATION
387
OF OPAL
contain kaolinite (halloysite) and to a smaller extent mica, iUite and montmorillonite, since the diffractograms of these minerals also show a certain bulge at 26 ° 2 O, caused by the coincidence of peaks in this region. This can be corrected by measuring the height of this bulge at l.O.
Gg.
0.7.
0.6.
03,
0.2
0.1,
0
,~
0
% dlm~,s (mill,l)
Fig.4. Calibration curve for diatoms (% weight), based on the height of the opal "bulge" and corrected with ~-AlsOs as internal standard. 26 ° 2 ® in a sediment with a mineralogical composition very similar to the sediment to be analysed, and containing no opal, after adding ~-AlsO s as standard. Substracting this value from the total found in the sample containing opal gives an estimate of the true percentage in the sample. This was tried out by adding amounts of sponge spicules to some fine-grained marine sediments containing kaolinite and mica as dominant minerals. The agreement (Table II) between the amounts added and the percentages found is good except in one case where the percentage found is rather low (but still within the calibration error). Since the natural variations in mineralogical composition of the sediment m a y in some cases also cause errors in the opal determination based on using the bulge, this method thus has the same drawback as by the heating-method, but it is easier to apply. A d d i n g known amounts of opal can increase the reliability of the observations by pro-
388
D. E I S M A
& S.
j.
VAN
DER
GAAST
TABLE II
Opal determination in four sediments after two different additions of opal. Sediment nr.
Dominant minerals
% (weight) opal added
1.
quartz (40%), calcite (20%), mica (10%), kaolinite (20%) calcite (90%)
9.4 45.9
26 59
---16 --16
10 43
10.0 46.5 10.6 46.2
11 49 42 73
---1 ---1 ---32 --32
10 48 10 41
10.2 45.5
12 50
---3 --3
9 47
2. 3. 4.
kaolinite (40%), quartz (30%), calcite (20%) quartz (40%), feldspars (30%), mica (10%), kaolinite (10%)
% (weight) Corrertion Correrted% opalfound opalfound
viding a control. Moreover a correction can be made by comparing the height of the kaolinite or mica peak in the sediment t o be analysed, with the height of this peak in the opal,free Sediment used as standard. IV. SUMMARY The determination of opal in marine sediments by measuring the 101 cristobalite peak formed upon heating at 950 ° C m a y give erroneous results because the height of the peak is strongly influenced by the presence of cations (Na, K, Mg, Ca) and minerals common in marine sediments. Therefore an X-ray method without heating is proposed, measuring the height of the opal "bulge". This method can be used without complications in sediments containing only small amounts of kaolinite, mica, illite or montmorillonite. I f these minerals dominate, corrections must be made. Trials show t h a t in such cases the largest error found is in the order of about 6 %. When the calibration curves can be applied without corrections the largest error found is in the order of about 3 %. V. R E F E R E N C E S
CALVERT,S. E , ]966. Accumulation of d l a t ~ silica in the sediments of the Gulf of Oalifemia.--Bull. 8col. Soc. Am. 7'h 569-596. FL6aXE, O. W., 1961. A ~ o n of the tridymite-cristobalite problem.--Silic. ind. ~ : 415-418. Go--o, E. D., 1958. Determination of opal in marine scdimcnts.--J, mar. Rcs. 17t 178-182.
DETERMINATION OF OPAL
389
KEYSER,W. L. DE & R. C Y P ~ , 1961. Beitrag zum Studium der Bildung des Tridymits. Mineralisatorwirkung yon Feldspar und Kaolin auf die Umwandlung des Quarzes.--Ber. dt. keram. Ges. 38(7): 303-308. KoLnS, R. W., 1955. Diatoms from equatorial Atlantic cores.--Rep. Swed. deep Sea Exped. 7(3): 151-184. LANNING,F. C., B. W. X. PONNAIYA& C. F. CRUMPTON,1958. The chemical nature of silica in plants.--P1. Physiol., Wash. 33: 339-343. SWI~ZFORD,A. & P. C. FRANKS,1959. Opal in the Ogaliala formation in Kansas. In: Silica in sediments. Spec. Pubis Soc. econ. Palaeont. Miner. 7:111-120.