Study On Mineralogy And Geochemistry Of Polymetallic Nodules From The Central And Eastern Pacific Ocean

Polymetallic nodules taken from The Central and Eastern Pacific Ocean are used in this study. Samples from the outer and core sections of different nodules are taken for mineralogical and geochemical anylyses by various analytical techniques such as infrared spectragraphy, transmission electron microscopy (TEM), electron spin resonance (ESR), mossbauer spectragraphy, electron probing, x-ray diffraction (XRD). Element content data of polymetallic; nodules are obtained by ICP-AES and ICP-MS techniques. The TEM technique includes selected area electron diffraction (SAED) and energy dispersive X-ray analysis.The main manganese minerals in polymetallic nodules are vernadite and todorokite. Birnessite is mainly found in the manganese micronodules. Woodruffite is found in a manganese micronodule, and the Zn content in the micronodules is as high as 13.29%. The XRD and SAED data of the woodruffite meet that of JCPDS card well.Semi-C[uantitative XRD method is established and used to calculate the contents of vernadite, todorokite, plagioclase and phillipsite in the polymetallic nodules.The content of vernadite varies from 29.6% to 84.6%, and the mean value is 59.4%. The content range of todorokite is 12.2⁵2.2% and the mean value is 2.5%. The content of plagioclase ranges between 0.0%¹0.7% with the mean value 2.9%. The content of phillipsite varies between: 0.0 10.9%, and the mean value is 2.4%. The content of quartz varies from 1.1% to 4.9%, and the mean value is 2.5%.The unit cell constants of todorokite and birnessite are obtained by the least square method. The result indicates that the crystal structure of todorokite belongs to monoclinic symmetry, and the range of its unit cell constants is a₀=0.9772⁰.9836nm, b₀=0.2835 0.2853nm, c₀=0.9483⁰.9578nm, β=93.580⁹4.775°. The unit cell constants of todorokite in the polymetallic nodules are similar to that of land todorokite, but compared with land todorokite, their a₀ constants are a little higher, and the c₀ constants are a little lower. From the outer layer to the core of polymetallic nodules, the c₀ constants of todorokite increase gradually.The unit cell constants of birnessite in a manganese micronodule are computed and thenindexed. The results show that its crystal structure should be orthorhombic symmetry, but its sublattice may be hexagonal symmetry. The orthorhcmbic unit cell constants are ao =0,8534 nm, b0=1.5367 nm, c0 =1.4252nm, V=1.8690nm3and the hexagonal sublattice constants are ao =0.2844 nm co= 0.7122nm, V=0.0499nm3.The crystallite size (L) and lattice distortion (A) of todorokite and birnessite are calculated by the method of approximate function. The L values of todorokite vary from 12 nm to 320nm, and the average is 81nm. The A values of todorokite range between 0.86 % and 3.12%, and the average is 2.37%. The range of L values of bimessite is 8⁶8nm with the average 24nm.The A values of birnessite vary from 0.07% to 1.62%, and the average is 0.84%.The crystallite size of Vemadite is also computed by Scheller equation. The L values of Vemadite range from 5nm to 9nm, and the average is 7 nm. The A values of the manganese minerals indicate that their cystal structures aren't perfect and there are transitive states.The data obtained by Mossbauer spectra technique indicates that the iron phase in polymetallic nodoules shoud be amorphous phase. Under the temperature of liquefied nitrogen (77°K), the magnetic six absorbtive peaks of goethite are obtainted.The ESR Spectras of the polymetallic nodules are all broad resonance spectras. The g values of polymetallic nodules vary between 1.9 and 2.2. The first contributor is Fe3+, and the second is Mn2+.From the outer layer to the core of polymetallic nodules, the contents of Mn, Mg, Ba, Al, Cu+Co+Ni, Mo, W and the Mn/Fe ratios are increased, but the content of Fe3+ is decreased.The rare earth elements (REE) contents of polymetallic nodules are determined by ICP-MS. The total REE contents (2 REE) vary from 559.7 u g/g to 2076,7 u g/g, and the average is 1195.7 u g/g; The light REE( 2 Ce) is relatively richer than heavy REE ( ï¿¡ Y), The SCe/2 Y ratios rings between 2.91 and 5.60, with the average 3.86. The 8 Ce varies from 0.97 to 2.54, and the average is 1.40. The 2 REE, 2Ce/2 Y and 8 Ce display remarkably positive correlation with each other. In addition, these three before all display positive con-elation with vemadite and Fe repectively, and display negative correlation with todorokite and Mn respectively. These show that a relatively strong oxidizing envirement is propitious to forming vemadite and enrichment of REE in polymetallic nodules. According to the characteristics of crystallite size of vemadite and the Fe phase in amorphous state, it could be confirmed that they shoud have huge specific surface area and very strong absorbing ability, which result in 2 REE dislays remarkable positive correlation with vemadite and Fe content.According to cluster analysis of contents of elements and minerals in the nodules, it could be reached that minerals and elements can be divided into 4 groups: 1) todorokite-Mn-Mg-Cu-Ni-Zn-Mo; 2) vernadite-Fe-Ti-Sr-Pb-P-Co-Ca- 2 REE; 3) plagioclase-quartz- phillipite-Al-Si-K-Na; 4) Ba -W-V. Generally they can be divided into two larger groups, and iJiey represent the diagenetic and hydrogenic forming environment respectively.Structural stability of todorokite by heating treatment is tested. The results indicate that the d-value and intensity of XRD peaks of todorokite decrease and the peak width becomes broader after heated; whereas, the intensity of 0.7 nm peak of birnessite increases gradually with increasing temperature, but the core and outer todorokite represents different behavior. The results suggest that the stability or the crystal structure order degree of the todorokite in core is higher than that of the outer one. The stability of the todorokite is mainly influenced by ageing effect, that is, the structure stability increases with age of polymetallic nodules and its crystal structure become similar to that of land todorokite gradually. The contents of Cu, Ni, Co elements; seems to have no positive correlation to stability of crystal structure of todorokite in oceanic polymetallic nodules, The results prove that todorokite really exist in polymetallic nodules.The crystal structure of main manganese minerals in polymetallic nodules is in transitive states. Todorokite of marine origin is really a transitive product between buserite and ideal todorokite which have T(3><3) tunnel structure, and its crystal structure is much close to that of todorokite of land origin. With the aging and diagenetic process, its crystal structure can transit to that of perfect todorokite gradually. The crystal structure of birnessite can transit from much disordering hexagonal symmetry to ordering orthorhombic symmetry. Vernadite can evolve from amorphous phase to minicrystal.