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Microstructures of Agulhas Bank phosphorites
Marine Geology, 16 (1974): M63--M70
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands L e t t e r Section Microstructures o f Agulhas Bank
phosphorites
G.N. BATURIN and V.T. DUBINCHUK Institute of Oceanology, Academy of Sciences of the U.S.S.R., Moscow (U.S.S.R.)
(Accepted for publication April 5, 1974)
ABSTRACT Baturin, G.N. and Dubinchuk, V.T., 1974. Microstructures of Agulhas Bank phosphorites. Mar. Geol., 16:M63--M70. Electron microscopic study of Agulhas Bank phosphorites has revealed a large variety in their microstructures, including gel-like, fibrous, ultramicrogranular, ultramicrocrystallic and microcrystallic as well as microstructures of intermediate types. The crystallisation is mostly developed on carbonate-phosphate contacts and in free spaces between mineral grains and does not depend on absolute age of the phosphorite.
INTRODUCTION Electron microscopy can be successfully used to reveal microstructures in sedimentary rocks, including phosphorites, but hi t hert o this m e t h o d has been applied only to the study of land phosphorites (Bushinskii, 1952; Chepelevitskii et al., 1958; Mirtov et al., 1967). It is logical to ext end use of the tool to the comparable study of sea-floor phosphorites. Th e purpose of this paper is to describe the results of such a study made on samples o f Agulhas Bank phosphorites. T he phosphorites in this region were first discovered by the "Challenger" expedition (Murray and Renard, 1891), Descriptions of their chemical and mineralogical composition are given by Murray and Renard (1891); Collet (1905); Murray and Philippi (1908); Cayeux (1934); H a u g h t o n (1956); Avilov and Gershanovitch (1969); Parker (1970) and Parker and Siesser (1972). MATERIAL AND METHODS Th e samples that we have studied represent several fragments of a large slab o f phosphatized limestone recovered by trawl at 35 ° 49 'S, 22 ° 2 9 ' E from a depth of a b o u t 400 m during the cruise of the R/V "A cadem i c Kn ip o v ich " in 1965 (Avilov and Gershanovitch~1970).
M64
The chemical analysis of the slab has been made using routine methods of wet chemistry (Strakhov, 1957). The mineralogical study of thin sections has been carried out under a polarizing microscope. Samples for electron microscopy were cemented by a mixture of colophony (95%) and xylol (5%) at 80--90°C. The cemented samples were split and their surface coated with a coal replica by the method of Gritsaenko et al. (1961). This replica has been subsequently separated by gelatin and studied under a UEMW-IOOK electron microscope. The identification of individual microzones and mineral micrograins extracted on the replica has been checked by using a URS-50J microdiffractometer. RESULTS AND DISCUSSION
The average chemical composition of the samples given in Table I is near to that of the iron-poor variety of Agulhas Bank phosphorites, after the classification of Parker and Siesser {1972). TABLE I Average chemical composition of investigated samples of Agulhas Bank phosphorite Component
%
Component
%
P20~ CaO MgO F CO 2 SiO 2 AI~ O~
19.11 33.52 0.75 2.00 5.72 2.88 1.53
Fe~ O 3 FeO MnO TiO2 Corg lgn.loss insol, residue
5.57 0.24 0.14 0.16 0.33 17.26 2.88
Examination of thin sections showed that all samples have composite structure and include carbonate, phosphate and goethite zones mixed in varying proportions. Phosphate is represented mainly by cement filling fractures and voids including foraminifera casts. Occasionally phosphate is seen to substitute for carbonate or (rarely) goethite. In some of the thin sections separate phosphate grains of earlier generation are present. Electron microscopy revealed the following microstructures of phosphate: (1) Gel-like phosphate (Fig.l). (2) Ultramicrogranular phosphate represented either by compact masses or globules of 1--3 u in diameter (Fig.2, right). Their somewhat rugged surface is due to ultramicrogranules of less than 0.1 p in diameter. (3) Fibrous phosphate which constitutes the inner part of the globules (Fig.3, right). Microdiffraction study of the particles extracted from the inner part of
M65
Fig.1. Gel-like phosphate.
Fig.2. Ultramierogranular phosphate consisting of a compact mass (left) and globules (right).
M66
Fig.3. Fibrous phosphate (right) with its ring diffraction pattern shown in the top right corner, and ultramicrocrystalline phosphate (left).
globules gives evidence of their finely dispersed structure. The ring diffraction pattern with heavy lines of apatite (Fig.3, upper right) shows that the crystallic structure of fibrous phosphate is incomplete. (4) Ultramicrocrystalline phosphate, which often has the form of a crystalline cover over the surface of globules consisting of amorphous phosphate (collophane). The dimensions of tabular hexagon apatite crystals are usually within the limites 0.1--0.3 u (Fig.3, left; Fig.4). Apatite crystals are formed in particular on the carbonate-phosphate contact as a result of metasomatic substitution of phosphate (Fig.5, below) for carbonate (Fig.5, top). (5) Microcrystalline phosphate consisting of 1--3 • crystals (Fig.6). The well-formed crystals are frequently found in the interstices of carbonate grains (Fig.7, centre). The microdiffraction pattern of extracted particles (Fig.7, right) proves that phosphate is represented by the apatite form. (6) Multiphase microgranular cement. As evidenced by microdiffraction, the cement consists of carbonate, phosphate, quartz and phylosilicate. In Fig.8 this cement is shown together with a coccolithophoride in the centre. The presented data demonstrate the wide range of ultramicroscopic structures of Agulhas Bank phosphorites, with various stages of phosphate crystallisation. The early investigators considered these phosphorites as Recent (Murray and Renard, 1891; Collet, 1905). Later it was shown by the use of the
M67
Fig.4. Ultramicrocrystalline phosphate.
Fig.5. Carbonate grain (top) and phosphate globules (below), with phosphate crystallisation at their contact.
M68
Fig.6. Microcrystalline phosphate.
Fig.7. The apatite crystals (left) in the void inside the carbonate grain. The ring diffraction pattern of the phosphate is shown at the right.
M69
Fig.8. The mixed cement of the nodule with coccolith in the centre (Coccolithus sp., P3, as determined by Prof. S.I. Shumenko of Kharkov State University).
234 U/23aU m e t h o d that their absolute age exceeds 1.106 years (Kolodny and Kaplan, 1970). Our investigation also failed to find any phosphatisation that could be interpreted as Recent. It may be concluded therefore that the degree of phosphate crystallisation of marine phosphorites does not depend on their age, but rather on the diagenetic environment in which the phosphatisation of various macro- and micro-parts of original sediment t o o k place, including such factors as metasomatic replacements on phosphate-carbonate contacts and the presence of free spaces between mineral grains.
REFERENCES Avilov, I.K. and Gershanovitch, D.E., 1969. Investigation of relief and b o t t o m sediments of the South West Africa shelf. Okeanologiya, 1 0 : 3 0 1 - - 3 0 6 (in Russian) Bushinskii, G.I., 1952. Apatite, phosphorite, vivianite. Ed. Acad. Nauk S.S.S.R., Moscow, 90 pp. (in Russian) Cayeux, L., 1934. The phosphate nodules of Agulhas Bank. Ann. S. Aft. Mus., 31: 105--136. Chepelevitskii, M.L., Gimmelfarb, B.M., Kuperman, M.E. and Krasilnikova, Z.M., 1958. Electron microscopic study of phosphorites of Kara Tan basin. Dokl. Acad. Nauk S.S.S.R., 119(1): 133--135 (in Russian) Collet, L.W., 1905. Les concr6tions phosphates de l'Agulhas Bank (Cape of Good Hope). Proc. R. Soc. Edinburgh, 25: 862--893. Gritsaenko, G.S., Rudnitskaya, G.S. and Gorshkov, A.I., 1961. Electron microscopy of minerals. Nauka, Moscow, 309 pp. (in Russian)
M70 Haughton, S.H., 1956. Phosphatic-glauconitic deposits off the West Coast of South Africa. Ann. S. Afr. Mus., 42:329---334 Kolodny, Y. and Kaplan, I.R., 1970. Uranium isotopes in sea-floor phosphorites. Geochim. Cosmochim. Acta, 34(1): 3--24 Mirtov, Y.V., Krotov, G.A. and Simkina, M.I., 1967. On the use of replica method in studying phosphorites. Lithol. Mineral. Dep., 3:139--140 Murray, J. and Philippi, E., 1908. Die Grundproben der deutschen Tiefsee Expedition 1898--1899 auf dem Dampfer "Valdivia". Wiss. Ergeb. Dtsch. Tiefsee-Exped., Bd. 10, Jena, pp.181--187 Murray, J. and Renard, A., 1891. Scientific Results, H.M.S. "Challenger". Deep-Sea Deposits, pp. 391--400 Parker, R.J., 1970. Agulhas Bank phosphate deposits. Dep. Geol., Univ. of Cape Town, Tech. Rep. No. 2, pp.59--77 Parker, R.J. and Siesser, W.G., 1972. Petrology and origin of some phosphorites from the South African continental margin. J. Sediment. Petrol., 42(2): 434--440 Strakhov, N.M. (Editor), 1957. Methods of Investigation of Sedimentary Rocks, 2. Gosgeoltekhizdat, Moscow, 564 pp. (in Russian)
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands L e t t e r Section Microstructures o f Agulhas Bank
phosphorites
G.N. BATURIN and V.T. DUBINCHUK Institute of Oceanology, Academy of Sciences of the U.S.S.R., Moscow (U.S.S.R.)
(Accepted for publication April 5, 1974)
ABSTRACT Baturin, G.N. and Dubinchuk, V.T., 1974. Microstructures of Agulhas Bank phosphorites. Mar. Geol., 16:M63--M70. Electron microscopic study of Agulhas Bank phosphorites has revealed a large variety in their microstructures, including gel-like, fibrous, ultramicrogranular, ultramicrocrystallic and microcrystallic as well as microstructures of intermediate types. The crystallisation is mostly developed on carbonate-phosphate contacts and in free spaces between mineral grains and does not depend on absolute age of the phosphorite.
INTRODUCTION Electron microscopy can be successfully used to reveal microstructures in sedimentary rocks, including phosphorites, but hi t hert o this m e t h o d has been applied only to the study of land phosphorites (Bushinskii, 1952; Chepelevitskii et al., 1958; Mirtov et al., 1967). It is logical to ext end use of the tool to the comparable study of sea-floor phosphorites. Th e purpose of this paper is to describe the results of such a study made on samples o f Agulhas Bank phosphorites. T he phosphorites in this region were first discovered by the "Challenger" expedition (Murray and Renard, 1891), Descriptions of their chemical and mineralogical composition are given by Murray and Renard (1891); Collet (1905); Murray and Philippi (1908); Cayeux (1934); H a u g h t o n (1956); Avilov and Gershanovitch (1969); Parker (1970) and Parker and Siesser (1972). MATERIAL AND METHODS Th e samples that we have studied represent several fragments of a large slab o f phosphatized limestone recovered by trawl at 35 ° 49 'S, 22 ° 2 9 ' E from a depth of a b o u t 400 m during the cruise of the R/V "A cadem i c Kn ip o v ich " in 1965 (Avilov and Gershanovitch~1970).
M64
The chemical analysis of the slab has been made using routine methods of wet chemistry (Strakhov, 1957). The mineralogical study of thin sections has been carried out under a polarizing microscope. Samples for electron microscopy were cemented by a mixture of colophony (95%) and xylol (5%) at 80--90°C. The cemented samples were split and their surface coated with a coal replica by the method of Gritsaenko et al. (1961). This replica has been subsequently separated by gelatin and studied under a UEMW-IOOK electron microscope. The identification of individual microzones and mineral micrograins extracted on the replica has been checked by using a URS-50J microdiffractometer. RESULTS AND DISCUSSION
The average chemical composition of the samples given in Table I is near to that of the iron-poor variety of Agulhas Bank phosphorites, after the classification of Parker and Siesser {1972). TABLE I Average chemical composition of investigated samples of Agulhas Bank phosphorite Component
%
Component
%
P20~ CaO MgO F CO 2 SiO 2 AI~ O~
19.11 33.52 0.75 2.00 5.72 2.88 1.53
Fe~ O 3 FeO MnO TiO2 Corg lgn.loss insol, residue
5.57 0.24 0.14 0.16 0.33 17.26 2.88
Examination of thin sections showed that all samples have composite structure and include carbonate, phosphate and goethite zones mixed in varying proportions. Phosphate is represented mainly by cement filling fractures and voids including foraminifera casts. Occasionally phosphate is seen to substitute for carbonate or (rarely) goethite. In some of the thin sections separate phosphate grains of earlier generation are present. Electron microscopy revealed the following microstructures of phosphate: (1) Gel-like phosphate (Fig.l). (2) Ultramicrogranular phosphate represented either by compact masses or globules of 1--3 u in diameter (Fig.2, right). Their somewhat rugged surface is due to ultramicrogranules of less than 0.1 p in diameter. (3) Fibrous phosphate which constitutes the inner part of the globules (Fig.3, right). Microdiffraction study of the particles extracted from the inner part of
M65
Fig.1. Gel-like phosphate.
Fig.2. Ultramierogranular phosphate consisting of a compact mass (left) and globules (right).
M66
Fig.3. Fibrous phosphate (right) with its ring diffraction pattern shown in the top right corner, and ultramicrocrystalline phosphate (left).
globules gives evidence of their finely dispersed structure. The ring diffraction pattern with heavy lines of apatite (Fig.3, upper right) shows that the crystallic structure of fibrous phosphate is incomplete. (4) Ultramicrocrystalline phosphate, which often has the form of a crystalline cover over the surface of globules consisting of amorphous phosphate (collophane). The dimensions of tabular hexagon apatite crystals are usually within the limites 0.1--0.3 u (Fig.3, left; Fig.4). Apatite crystals are formed in particular on the carbonate-phosphate contact as a result of metasomatic substitution of phosphate (Fig.5, below) for carbonate (Fig.5, top). (5) Microcrystalline phosphate consisting of 1--3 • crystals (Fig.6). The well-formed crystals are frequently found in the interstices of carbonate grains (Fig.7, centre). The microdiffraction pattern of extracted particles (Fig.7, right) proves that phosphate is represented by the apatite form. (6) Multiphase microgranular cement. As evidenced by microdiffraction, the cement consists of carbonate, phosphate, quartz and phylosilicate. In Fig.8 this cement is shown together with a coccolithophoride in the centre. The presented data demonstrate the wide range of ultramicroscopic structures of Agulhas Bank phosphorites, with various stages of phosphate crystallisation. The early investigators considered these phosphorites as Recent (Murray and Renard, 1891; Collet, 1905). Later it was shown by the use of the
M67
Fig.4. Ultramicrocrystalline phosphate.
Fig.5. Carbonate grain (top) and phosphate globules (below), with phosphate crystallisation at their contact.
M68
Fig.6. Microcrystalline phosphate.
Fig.7. The apatite crystals (left) in the void inside the carbonate grain. The ring diffraction pattern of the phosphate is shown at the right.
M69
Fig.8. The mixed cement of the nodule with coccolith in the centre (Coccolithus sp., P3, as determined by Prof. S.I. Shumenko of Kharkov State University).
234 U/23aU m e t h o d that their absolute age exceeds 1.106 years (Kolodny and Kaplan, 1970). Our investigation also failed to find any phosphatisation that could be interpreted as Recent. It may be concluded therefore that the degree of phosphate crystallisation of marine phosphorites does not depend on their age, but rather on the diagenetic environment in which the phosphatisation of various macro- and micro-parts of original sediment t o o k place, including such factors as metasomatic replacements on phosphate-carbonate contacts and the presence of free spaces between mineral grains.
REFERENCES Avilov, I.K. and Gershanovitch, D.E., 1969. Investigation of relief and b o t t o m sediments of the South West Africa shelf. Okeanologiya, 1 0 : 3 0 1 - - 3 0 6 (in Russian) Bushinskii, G.I., 1952. Apatite, phosphorite, vivianite. Ed. Acad. Nauk S.S.S.R., Moscow, 90 pp. (in Russian) Cayeux, L., 1934. The phosphate nodules of Agulhas Bank. Ann. S. Aft. Mus., 31: 105--136. Chepelevitskii, M.L., Gimmelfarb, B.M., Kuperman, M.E. and Krasilnikova, Z.M., 1958. Electron microscopic study of phosphorites of Kara Tan basin. Dokl. Acad. Nauk S.S.S.R., 119(1): 133--135 (in Russian) Collet, L.W., 1905. Les concr6tions phosphates de l'Agulhas Bank (Cape of Good Hope). Proc. R. Soc. Edinburgh, 25: 862--893. Gritsaenko, G.S., Rudnitskaya, G.S. and Gorshkov, A.I., 1961. Electron microscopy of minerals. Nauka, Moscow, 309 pp. (in Russian)
M70 Haughton, S.H., 1956. Phosphatic-glauconitic deposits off the West Coast of South Africa. Ann. S. Afr. Mus., 42:329---334 Kolodny, Y. and Kaplan, I.R., 1970. Uranium isotopes in sea-floor phosphorites. Geochim. Cosmochim. Acta, 34(1): 3--24 Mirtov, Y.V., Krotov, G.A. and Simkina, M.I., 1967. On the use of replica method in studying phosphorites. Lithol. Mineral. Dep., 3:139--140 Murray, J. and Philippi, E., 1908. Die Grundproben der deutschen Tiefsee Expedition 1898--1899 auf dem Dampfer "Valdivia". Wiss. Ergeb. Dtsch. Tiefsee-Exped., Bd. 10, Jena, pp.181--187 Murray, J. and Renard, A., 1891. Scientific Results, H.M.S. "Challenger". Deep-Sea Deposits, pp. 391--400 Parker, R.J., 1970. Agulhas Bank phosphate deposits. Dep. Geol., Univ. of Cape Town, Tech. Rep. No. 2, pp.59--77 Parker, R.J. and Siesser, W.G., 1972. Petrology and origin of some phosphorites from the South African continental margin. J. Sediment. Petrol., 42(2): 434--440 Strakhov, N.M. (Editor), 1957. Methods of Investigation of Sedimentary Rocks, 2. Gosgeoltekhizdat, Moscow, 564 pp. (in Russian)