EG0000331
Third Arab Conference on the Peaceful Uses of Atomic Energy, Damascus 9 -13 Dec. 1996 AAEA Uranium Traps in the Phosphate Bearing Sudr Chalk, in Northeastern Sinai, Egypt
HA. Hussein, LE.ElAassy, MA. Mahdy, GA. Dabbour, AM. Morsy* and M.Gh. Mansour Nuclear materials Authority, Cairo, Egypt. * Suez Canal University, Ismailia, Egypt.
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Abstract The Maastrichtian Sudr Formation in northeastern Sinai is composed of three members, the lower chalk, the middle phosphate and chert-bearing and the upper chalk members. Lemon yellow secondary uranium mineralization, distributed in the lower chalk member and in some phosphate beds from the middle phosphate member are observed. The XRD analyses of some samples from the uranium bearing chalk and the
r«v phosphate beds showed the presence of the secondary uranium minerals carnotite, bergenite and upalite. The mode of uranium occurrences could be interpreted as a result of the phosphatic beds decomposition and their subjection to later diagenetic processes. Uranium leaching circulation from phosphate rocks led to the liberation of uranium from the phosphates, and vanadium from the bituminous material and clay minerals. These migrated and were deposited locally and within the underlying chalk beds which acted as a lithologic trap.
Introduction The study area is located between longitudes 34° 20" and 34° 40" E and latitudes 30° 15" and 30° 30 N (Fig. 1). Gabal Urayf An-Naqah a doubly plunging fold is located in the central part of the area. The core of this mountain consists of Triassic, Jurassic and Cretaceous rocks which are intensely folded in the ENE-WSW trend. The upper Cretaceous (Maastrichtian) chalk is slightly folded[l] . Sudr Formation is the main phosphate bearing sedimentary horizon in this area. El Aassy, [2] studied a phosphate occurrence east of El Qaa plain area in southwestern Sinai. He subdivided Sudr Formation into three distinct members: the lower chalk member, the middle limestone with chert and phosphate beds intercalations and the upper chalk member. Bartov et al [3], studied Gabal Urayf An Naqah area and subdivided the Sudr Formation into two formations namely: Mishash Formation of Campanian age and Ghareb Formation of Maestrichtian age. Phosphorites of Senonian age were studied in northern Negev just east of the study area, where the yellow uranium mineralization was found close to the phosphorite beds [4]. Sudr Formation has recently drawn the authors attention due to three reasons. First, it represents the host rock of the newly recorded phosphorites [2,5,6]. Second, the discovery of new secondary uranium mineralization in both the chalk and the associated phosphorite beds. Third, this formation includes considerable amounts of bituminous material which may have played a role in the epigenetic processes affecting the area. Thus, this study was carried out in order to investigate the uranium potentiality of Sudr Formation and its suitability as trap for uranium mineralizations. Geologic Setting The sedimentary cover in the studied area ranges from the Anisian (Triassic) Urayf An Naqah Formation to the Middle Eocene Mokattam Formation (Figs. 1 & 2). Triassic and Jurassic rocks are exposed in the core of the doubly plunging Urayf An Naqah anticline. Urayf An Naqah Formation is composed mainly of sandstone interbeds, dark coloured, cross-bedded and of fine to medium grained particles. The sandstone is alternating with fossiliferous limestone and shaly siltstones. Urayf An Naqah Formation is overlain conformably by Abu Nusrah Formation which is related to the lower Carnian-Ladinian (7). Abu Nusrah Formation is composed mainly of carbonate rocks such as fossiliferous limestone, dolomite, and marl interbeds near the top.
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Figure (1): Geologic map of the study area (after Geological Survey, 1993, wilh modifications) LJaacsumc, contains Nummullte gixebenjii, pale grey to wbitc* medium bard, highly disturbed with slumping structures.
Fig.< 2 )• Cooipil»d li thosUaiigraphJe s#ction,Uray1 An Naqah atma
Figure (2) : Compiled lithostratigraphic section, Urayf An Naqah area The Jurassic rocks are represented by the middle Liassic Rajabiyah Formation, composed of limestone, mainly coralline with algal components. The Jurassic rocks are unconformably overlain by the Aptian-Albian Malha Formation, composed of kaolinitic sandstone, with kaolinite and conglomerate interbeds. The section of the Upper Cretaceous rocks is completely exposed even with no missing rock unit. It starts with the Cenomanian Halal Formation at the base which conformably overlies the lower Cretaceous Malha Formation. Turanian Wata Formation is represented mainly by carbonate rocks (limestone and dolomite) with thin sandstone and shale interbeds. The Coniacian-Santonian Matalla Formation conformably overlies the Wata Formation and is composed of limestone, marl and shale. A clastic section of sandstone and shale with phosphatic bed is noticed at the top and may be related to the Campanian Duwi Formation, overlying the Matalla Formation. The Maastrichtian Sudr Formation represents the main target in this study and is composed mainly of carbonate rocks with clastic interbeds and some phosphorite and chert beds. It is conformably overlain by the Paleocene-Lower Eocene Esna Formation which is composed of two members; the lower Esna belonging to the Paleocene, and the upper Esna related to the lower Eocene. The two members are separated by limestone interbed (1 m). The Lower Eocene Thebes or Egma Formations are not recorded in this area. The Middle Eocene Mokattam Formation unconformably overlies Esna Formation and is composed of pale grey to white nummulitic limestone.
Lithostratigrapby of Sudr Formation Sudr Formation measures about 120 m in thickness and covers an area of about 400 km2. However, it unconformably overlies the Duwi Formation and is overlain by the Esna Formation, It can be subdivided into three members (Fig. 3) according to its lithology and areal extension. The following is a brief description of each member starting from the oldest. 1. Lower Chalk Member It overlies the Duwi Formation and varies laterally in thickness from 35 to 45 m. Each chalky bed varies from 30 to 70 cm in thickness. It is composed mainly of grey to reddish brown chalk with minor mad interbeds. The top most 4 m are fractured with joint planes in different trends. Visible yellow to canarian yellow secondary uranium minerals were observed with some chert concretions near the top. It also contains black centers of bituminous materials and some with metallic lusters. 2. Middle Phosphate and Chert-bearing Member The middle phosphate and chert-bearing member shows lateral variations either in thickness or in facies. It varies from 25 to 35 m, in thickness, and is well developed in the eastern part of the studied area (Figs.l & 3). The lower 6 to 10 m are composed of chert beds which vary in colour from dark brown to black. They are brecciated at the contact with the underlying unit. This contact is a marker horizon for the presence of the secondary uranium mineralizations. Some chert bands and lenses of light tones are associated with the phosphates, and show patches of the yellow secondary uranium mineralization. The basal brecciated 0.5 m of this chert may indicate an unconformity surface with the lower unit. This chert horizon is overlain by an intercalation of phosphate, marl and clayston (Fig.4). Phosphates are represented by eight beds which vary in thickness from 0.2 to 1.2 m. The two top most phosphate beds are soft to moderately hard, brown and contain relatively high P2O5 content. Visible yellow secondary uranium mineralizations were also noticed in the two upper phosphate beds. Figure (3) : (a) Middle Phosphorite Member overlain by the Upper Chalk Member (U-Ch), Looking E. (b) Intercalation of phosphorite beds with chalk and chalky marl bed which represents the northern wall of the upper photo, Looking N. P f
Figure (4): Middle Phosphorite Member with phosphorite and chalk with clay interbeds, Looking SE.
Figure (S) : Shark tooth with ferruginous cement around the phosphate pellets U200). 3. Upper Chalk Member This member is equivalent to the Abu Zeneima member and is composed mainly of thick bedded chalk, chalky limestone, marly in some parts, argillaceous and ferruginated. These chalks and marls show some black centers of bituminous material. The thickness of this member ranges from 40 to 50 m.
Petrographical and Mineralogical Investigations Petrographically, the phosphatic material in the phosphorites of the Sudr Formation occur in different forms including grains, pellets, and different skeletals such as shark teeth, bone fragments and fish scales. The grains are randomely distributed and are of coarse silt to medium sand size. The grains diameters range from 0.2 to 0.8 mm.Pellets are less abundant and are ovoidal to elliptic in shape. The shark teeth (Fig. 5) are sharply defined in the outer layer and reach 1 cm in length and 2 mm in width. They are highly fractured and filled with secondary calcite. The bone fragments have different shapes with their maximum lengths reaching 1 cm. Three types of cements were observed: phosphatic, calcareous and ferruginous. The phosphatic cement is fine and filling intercavities between the former different allochems. The recorded carbonate cement has different grades of recrystallization while the ferruginous cement (Fig. 5) is present in patches and coating other grains. Mineralogically, secondary uranium minerals in Sudr Formation were observed as tiny aggregates and crustaceous materials in the top of the lower chalk member, as well as in the two upper phosphate beds. These yellow secondary minerals were found on the joint surfaces, fractures filling and near the faulted and disrupted rocks. Three samples representing the secondary uranium mineralization in the chalk beds and one from a phosphate bed were gently crushed to the mesh size (< 40 mesh), washed by water to overcome slimes then dried and separated by bromoform (sp. gr. 2.81). The lemon yellow radio-active grains were picked from the heavy fraction and were subjected to mineralogical study, using a Phillips XRD instrument. The carbonate fluorapatite mineral is the dominant phosphate mineral present. This mineral has suffered from alteration and diagenetic processes which partialy changed its mineralogic composition to fluorapatite (Tablel).It is worthy to mention that the CO2 in the studied phosphate mineral assayed 2.7%. Nriagu et. al. (1984) reported that the CO2 in the carbonate fluorapatite ranges from 3.2% to 4.5% while it varies between not detected and 0.2% in the fluorapatite. Moreover, Nriagu et al (Op. Cit.) mentioned that the CO2 in the carbonate fluorapatite from Ben Guerir, west African coastal basins Table (I)1: X-Ray Diffraction of the Apatite Mineral
Sample Flour-apatite Carbonate Phosphate 15-876 fluorapatite 21- 141
dA Vl0 dA I/Io dA I/Io
8.06 6 8.12 8 8.04 18 5.24 3 5.25 4 - 4.03 5 4.55 8 4.04 16 3.86 6 3.872 8 3.86 2 3.44 56 3.442 40 3.43 20 3.17 15 3.167 14 3.16 6 3.05 16 3.067 18 3.05 35 2.79 100 2.800 100 2.79 55 2.77 65 2.772 55 2.77 16 2.69 50 2.702 60 2.69 100 2.62 28 2.624 30 2.62 8 2.51 3 2.517 6 2.51 4 2.28 8 2.289 8 2.28 2 2.24 20 2.250 20 2.24 45 2.13 5 2.140 6 2.13 4 2.09 4 2.128 4 2.06 6 2.061 6 2.00 4 1.997 4 2.02 4 1.93 22 .937 25 1.93 12 1.88 12 .884 14 1.89 8 1.84 30 .837 30 1.83 10 1.79 12 .797 16 1.79 12 1.76 10 .771 14 1.78 25 1.74 10 .748 14 1.75 8 1.72 14 .722 16 1.72 6 1.63 5 .637 6 1.60 2 .607 4 1.53 3 .543 6 1.52 3 .524 4
(Sidi Daoui) assayed 3.4% in the non weathered mineral while it reached 2.1 % in the weathered one. This matched with the authors opinion that Urayf An Naqah phosphate mineral is subjected to alterations and diagenetic processes. The XRD pattern (Table 2) of the visible secondary uranium minerals picked from the heavy fraction of the phosphate sample reveals the presence of secondary calcite with a barium-titanium phosphate mineral. These minerals are associated with the secondary uranium mineral carnotite V2O8 (UO2)2 (H3O)2 (ASTM Card No. 11-309). This mineral may be considered the starting material of the carnotite deposition. Table (2) : X-Ray Diffraction of Cainoiif: ami Accessor eriii1-; Pi ked from Phosphorite Sample.
Sample Carnotite Calcite Bf Phos phate | Phosphate (11-309} __£5~0S86) ( 2m " \ ."• ::-*.':. 1 dA I/Io dA I/Io dA~] I/To ! dA • I: •• « [ 8.30 59 i 8.34 JO '; 6.64 22 6.48 60 6.37 10 6.37 100 i 4.18 22 4.20 60 i 4.17 80 ; 3.51 13 3.50 60 3.59 10 [ 3.23 59 3.21 60 3.17 15 3.13 60 3.03 100 3.10 40 3.03 ! 00 2.82 24 2.70 20 ? 7H UX) | 2.49 7 2.54 40 2.49 J4 2.44 7 H S 2.28. 19 2.28 18 2 '28 4 | 2.15 19 2.14 6 | 2.09 13 2.09 18 i 1.93 19 1.92 5 1.94 4 1.91 21 1.91 17 1 1.87 29 1.87 J7 1 1.67 13 f .66 1.61 8 1.62 4
The XRD pattern (Table 3) of the picked yellow grains separated from the radioactive chalk beds shows the presence of the uranium mineral carnotite K2(UO2)2 (VO4)2-3H2O. This miner;)I is associated with minor amount of the secondary uranium minerals ergenite
(Ba,Ca)2(UO2)3(PO4)2(OH)4.5.5H2O and upaiite Al (i:O2)(PO4)2(OH)3 together with some- impurities of calcite CaCCh and possibly lime CaO. The review uranium minerals associated with (he nhu.-.pbaie beds in the surrounding countries shows that the uranium vnnadaies and phosphates are abundant in the Negev Desert, [8| while, uumrnite and pitchblende is recorded in the Syrian phosphates ai Ktmeifees and Sharqyieh mines |9). Table (3) : X-Ray Diffraction of Carnotite and Accessory Minerals Picked from a Chalk Sample.
Sample Carnoiite Upalite Calcite lime Bergenite* ; cralk (13-338) (5-0586) (4-0772) (20- 154) dA 1/10 dA 1/10 dA 1/10 dA 1/10 dA 1/10 dA 1/10 ~Mi 1 8.40 100 7.63 w100 7.73 100 6.48 100 6.46 60 6.35 92 6.36 100 5.09 W 5.06 40 5.43 40 4.26 7 4.22 60 4.19 27 4.1$ 80 3.87 8 3.86 12 3.83 80 3,51 43 3.50 80 3.43 80 3.34 15 336 40 X?3 45 3,21 80 3.20 60 J.14 43 3,16 60 3.10 S3 3,10 60 3.03 17 75 3.04 100 3.05 60 2.88 10 2.90 1X1 60 2.82 6 2.78 34 2.82 60 2.72 11 2.71 40 2.60 10 2.60 20 2.55 16 2.54 60 2.47 8 2.45 20 250 14 244 100 2.44 40 2.16 12 2.16 20 2.15 20 2,10 4 2.14 40 2.10 18 2.07 60 2.03 11 2.03 20 2.02 40 1.9S 10 139 20 1.94 19 1.92 5 1.90 60 1.91 11 1.91 17 1.88 40 1.82 2 1.82 40 1.77 3 • 1.74 40 1.67 9 1.70 45 1.68 40 1.65 4 1.63 4 1.65 20 1.61 5 1.60 8 1.59 4 1.59 12
* Nriagu and Moore, phosphate Minerals (1984) and ASTM Card No. (20 • 154).
*• Nriagu and Moore, phosphate Minerals (1984).
Geochemistry of the Mineralized Zones Twenty four samples were selected to carry out the geochemical study. Each of the chalk, marl, chert and phosphatic limestone beds were represented by three samples, while the phosphate beds were represented by twelve samples. The breakdown of the analysed samplwas carried out by HC1 attack. The P2O5 analysis was performed gravimetrically using the phosphoamonium molybdate method. The three trace elements V, Sr and Ba were analysed using atomic absorption technique, and U was analysed by fluorometric technique.
rnv Generally speaking the P2O5 content in the chalky samples ranges from 1.04 - 6.01 % (Table 4). samples show relatively high uranium and vanadium contents (Fig. 6). The sample with the highest uranium content (60ppm ) contains the highest vanadium content (352 ppm). Strontium ranges from 1188 ppm to 1235 ppm while barium ranges from 613 ppm to 758 ppm. The P2O5 content in the chert beds ranges from 0.4 to 1.85 7c, and the uranium content ranges from 33 to 125 ppm (Table 4) similar to what has been noted in the chalky beds, it is noticed that the samples which contain higher uranium content contain nearly the same vanadium content (123 ppm) with the highest Sr content (815 ppm) and the highest Ba content (927 ppm).are also recorded in the same bed.
Table (4) : P2O5 and some trace elements concentration at Uryf An-Naqah area, northeastern Sinai.
Sample Rock type V V Sr Ba
No. i%) (ppm) (ppm) (ppm) (ppm) AE- 4 Chalk 1.15 SO 210 1214 682 AE- 5 6.01 33 57 1188 613 AE- & 1.04 60 352 1235 758 AE- a Chert 67 125 544 498 AE-10 1.85 125 123 815 927 AE-12 0.42 33 121 600 349 AE-13 Phosph. limestone 2.11 100 187 1316 627 AK-lJ Phosphate (1) IK.62 33 111 1766 560 AE-15 Phosph. limestone 030 7i 25 860 6M AE-16 Phosphate (2) 11.07 ko 247 2019 4852 AE-W Phosph. limestone UO 200 59 1497 920 AE-18 Phosphate (3) 17.45 50 144 2318 896 AE-li) Phosphate 3) 18.23 63 109 2423 77
N.ll. : (I), (2) mum phosphate bed No. The phosphate samples show a high P2O5 content ranging from 9.47 to 29.79 % while the P2O5 content in the phosphatic beds range from 0.3 to 2.11 %. The relations between P2O5 and U is non-linear, while it is negative with V and'positive with Ba and Sr (Fig. 7). The highest uranium content (200 ppm) is recorded in a phosphatic limestone bed, however the highest P2O5 content in the phosphate beds assayed only 100 ppm uranium. It is worthy to mention that the average uranium content recorded within the phosphorite beds in the Negev area, is 100 ppm, |10], [11] and [1.2]. Vanadium in the phosphate beds ranges from 54 to 247 ppm, while it ranges from 25 to 187 ppm in the phosphatic beds. The highest strontium and barium contents are recorded in the phosphate beds (Table 4). One phosphate sample (AE-16), contains high barium content (4825 ppm) which is due to the presence of visible barite crystals . Figure (6) : Lithostratigraphic log with P2O5 and trace elements correlation of lower clalk and middle phosphate members. Regarding the performed chemical analyses (Table 4), the following points could be reported : a) In the chalk beds there exists a general negative correlation between P2O5 and each of U, V, Sr and Ba. This is due to the fact that the secondary uranium mineralization is epigenetic and has no relation with the original sedimentlogical processes. b) The same phenomenon is recorded in the marly beds, which means that the carbonate rocks (chalk and marl) were originally poor in uranium content. c) The situation is different with the phosphatic limestone beds where a general positive relation exists between P2O5 and each of V, Sr, U and Ba. This may be attributed to the presence of the phosphate minerals which incorporate U and V while Sr can substitute for Ca in the tricalcium phosphate minerals. The positive correlations of P2O5 with barium is due to the presence of barite mineral associated with the • phosphate beds. d) There exists a fairly good positive correlation between P2O5 and each of U, Sr and Ba in the chert beds, however V shows no correlation. This may be due to the formation of secondary calcedony in the form of chert. The calcedony is originally deposited in a gell form which adsorped V. e) Concerning the phosphorite beds, the correlation between P2O5 and V is strongly negative which means that the phosphatic components are not the source of vanadium. Sr, U and Ba are positively correlated with P2O5 (Fig.7) which means that the phosphatic components represent the source of uranium, which was liberated to form the secondary uranium minerals. Sr can replace Ca in the tricalcium phosphate, while the correlation with Ba is due to the presence of barite associated with the phosphates. f) On the other hand, the uranium content show some controversies. Thus while the phosphatic limestone bed (1.3% P2O5) contains 200 ppm uranium, the high grade phosphorite bed (29.79% P2O5) contains only 100 ppm uranium. The low uranium content in phospharites may be due to its liberation during the formation of the secondary uranium mineralization.
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Figure (7) : Scatter diagrams of P2O5% with (V(a), Sr (b), Ba (c), and U (d) in the phosphorite beds .
rv< Genetic Aspects Although, it is too early to propose a genetic model for the secondary uranium mineralization in G. Urayf An Naqah, the authors believe that according to available information from the present work the following preliminary model can be proposed: 1) Due to the presence of the bituminous materials in the lower and upper chalk members , as well as in the middle phosphate and marly member, these materials accumulate energy and sometimes they were subjected to combustion . 2) The heat of combustion was enough to alter the calcium carbonate materials in the chalk and marly beds to calcium oxide and carbon dioxide, which reacted chemically with meteoric water to produce lime,which is an exothermic reaction. 3) The produced heat altered the phosphate rocks, and changed partialy the phosphate mineral carbonate fluorapatite, to fluorapatite (Table, 1). This transformation happened most probably at 480°C, which led to the release of CO2 and the formation of secondary calcite. This considerable deformation occurred in the phosphate beds as evidenced by the geological studies. 4) The released CO2 from the decomposition of the chalky carbonate beds, or from the alteration of the phosphate mineral carbonate fluorapatite to - fluorapatite was dissolved in the meteoric water and produced carbonic acid. The bicarbonate radicals dissolved the phosphate minerals and other U-bearing constituents, in addition, they dissolved V from the bituminous materials and K from the clay minerals ^specially illite, by natural carbonate leaching. 5) During the prevailance of arid conditions (poor rain fall and high temperature), evaporite products have been deposited such as gypsum and barite. In this depositional stage the secondary uranium minerawere deposited.
Conclusion The present study reports for the first time the presence of secondary uranium minerals namely carnotite, bergenite and upalite in the carbonate and phosphate rocks of Urayf An Naqah area. It could be considered as a new type of surficial uranium mineralization, which should draw the attention due to the regional extension of its occurrence. In addition this study reports the presence of an altered carbonate fluorapatite phosphate mineral which add to the importance of Sudr Formation as a potential formation for phosphate. Reference [I] MOSTAFA, A.R. and KHALIL, M.H., North Sinai structure and tectonic evolution, MERC, Ain Shams Univ., Earth Sc, Ser. v. 3, p. 215-231,(1989). [2] El AASSY, I.E., Geology and radioactivity of some phosphate exposures,east El Qaa Plain, Sinai, Egypt; 1st Conf. of the Geology of the Arab World, Cairo, Univ., (1992). [3] BARTOV, Y., LEWY, Z., STEINITZ, G. and ZAK, I., Mesozoic and structural history of the Gabal Areif El Naga area, eastern Sinai, Israel J. Earth Sc. No. 84, p. 114-139, (1980). [4] NATHAN, Y. and SHILONI, Y.,Exploration for uranium in phosphor: a newly study on uranium in Israel phosphorites. Proc. Int. Symp. on Exploration for Uranium Ore Deposits, I.A.E.A., NEA (Nuclear Energy Agency), OECD (Org. Econ. Coop. Dev.), Vienna, p. 645-655, (1977) [5] El AASSY, I.E., BOTROS, N.H- and SHAHATA, R.M., Geology and uranium distribution in the phosphorite beds, Gabal Qabeliat, southwest Sinai (New occurrence), Proc. 3rd Conf. Geol. Sinai Develop., Ismailia, p.209 - 216, (1992). [6] El AASSY, I.E., Studies on the geology and radioactivity of phosphorites occurrence in Taba area, Eastern Sinai, Egypt, 6th Intern. Conf. Nucl. Sc. and Appl., Cairo, 17p., (1995). [7] E.G.S., Geologic map of Sinai, sheet No. 4, Scale 1: 250,000, (1993). [8] GROSS, S. and ILANI, S., Secondary uranium minerals from the Judean Desert and the Northern Negev, Israel, El Sevier Sc. Publishers, B.V. Amsterdam, Uranium 4, p. 147-158, (1987). [9] MAHFOUD, R.F. and BECK, J.N., Natural radioactin the Syrian phosphate at Khneifees and Sharquiemines, El Sevier Sc. Pub. B.V., Amsterdam Uranium 2, (1985). [10] GROSS, S., The mineralogy of the Hatrurim Formation, Israel Geol. Surv., Bull No. 70, 80 p., (1977). [II] NATHAN, Y., SHILONI, Y., RODED, R., GAL, I., and DEUTSCH, Y., The geochemistry of the northern and central Negev phosphorites, Israel, Geol. Surv., Bull. No. 73,41 p. (1979). [12] SHILONI, Y., The uranium content of phosphates and related rocks from the Negev, Israel, 1st Assoc. Adv. Miner. Eng. 7 th Conf. Proc., p. 194-199, (1984). [13] NRIAGU, J. O. and MOORE, P. B., Phosphate minerals, Springer - Verlag Berlin, Heidelberg, 444 p., (1984).