Mineralogical Research On Cretaceous Analcime In Ordos Basin

There is a wide distribution of Cretaceous analcime mineral resorce with an area of about about 120,000 km2 and a huge reserve in Ordos Basin. Previous studies show that analcime has ion absorption capacity and can be developed and utilized as water purification material in this area. However, the reason for its ion absorption capacity is not understood and the formation for this Cretaceous analcime also needs further study.This thesis is funded by two projects:1) The Relationship Between Cretaceous Sedimentary Rock Porosity and Underground water in Ordos Basin(Project number:121201331302), which is a sub-project of one major national resource investigation projects----Underground water investigation in Ordos Basin; and 2) Basic Research Fund for Central Universities----Study on Crystal Structure of Analcime in Ordos Basin. With the help of the funds, the author systematically investigated the Cretaceous analcime by studying the drill core, and got a clear idea about its distribution, content and occurrence. And these research findings can provide important geological support for the exploration and development of analcime in this area. In addition, the author picked out one valuable rock sample which is very useful for mineralogical studies---- analcite (sample number:B8-50) in B8 drill core. Then the author studied its petrography, mineral chemistry, crystallology, raman spcctroscopy and high-temperature thermal behavior, then assured the evolution sequence of different minerals in this analcime, confirmed its chemical ingredients, refined its crystal structure and gained the phase transition process and temperature. All the above findings are meaningful in both theory and practice for the absorption mechanism of Cretaceous analcime, processing technology of mineral materials, and also provide several important evidences for the formation of sedimentary rocks and analysis of sedimentary environment.The main findings of this thesis after a systematic mineralogical study include:1. Determination of crystal structure model of Cretaceous analcime in Ordos Basin and approach to the possible change of physics property1) Compared with ideal analcime, the electron probe analysis shows that euhedral analcime contains more Si, less Al and half of Na, and it also contains a few ions like Fe3+, Ca2+ and Mg2+. Calculation of crystal chemistry shows that the coefficient of high electricity cations in per chemical formula is 6.2 and it is over 6 which is the coefficient of major cations. Therefore, the author suggests that part of Al need to enter the tunnels to take position of Na and calculates the following chemical formula:(Ca0.001Na0.95Mg0.003Fe0.021Al0.175)1.155[Al1.55Si4.45O12]·2H2O.2) The author analyzed and studied the X-ray single crystal diffraction and achieved several stable crystal structure models where Al3+ takes the position of Na in the tunnels. The crystal structure of analcime is:space group Ia3d, unit axis length ao= 13.6927(15) A, cell volume 2567.25(50) A3 and simplefied crystal chemical formula is (Na1.00Al0.20)1.20[Al1.60Si4.40O12] ·2H2O, Z=8 with a calculation density of 2.200 g/cm3 and a structure factor of R1=0.059.3) Raman spectra are mainly used for the determination of phase of analcime but as to its crystal structure, it needs further study. There are two assured vibration modulus:spectrum peak at 483 cm-1 corresponding O-Si-O bending vibration, and 1105cm-1 corresponding Si-O stretching vibration.4) The model of analcime in this study has the following characteristics compared with ideal analcime:(1) Most Al and all Si in analcime are in 48g equivalent point system with tetrahedral cordination, and a small amount of Al possesses 24c in the tunnel with six-fold coordination.(2) The Al substitutes Na and enters the tunnel, so the occupying ratio of cations at 24c changed from 0.66 at ideal state to 0.40. Therefore, the ratio of coordinating H2O turning to free water becomes higher and its calculation density is lowered. The occurrence of Al in the tunnel may cause growth in tunnel capacity and increase of absorption ability. Quantity of Na is lowered and the exchange ability of Al cation becomes weak and unlikely to be exchanged, so the exchangeable cation of analcime is lowered. (3) Compared with ideal ones, the analcime in the study shows higher quantity of Si and lower Al. This change can cause decrease of cell volume and likely to increase the heat stability. And this change of contents also appears in the Raman spectrum.2. Establishment of thermal behavior process of analcime in Ordos Basin and investigation for symmetry of its variation of crystal structure of high-temperature analcime1) The major phase transformationsThere are two phase transformations during the rise of temperature:dehydration and non-crystallization transformation respectively. And dehydration transformation is reversible and non-crystallization is non-reversible.2) The transformation temperaturesThere are two transformation temperatures:dehydration transformation when temperature is between 120～400℃. The valley bottom temperature for transformation is around 306℃, and the dehydration process is slow which may be related to tiny one-dimensional tunnel in the analcime; and the temperature varies between 900～1250℃ for non-crystallization transformation. The valley bottom temperature is around 1180℃. The stable temperature varies between 400～900℃ for dehydrated analcime.3) The crystal symmetry during the heating processesAt ambient temperature, analcime is cubic system, and lattice type is body centered lattice. After dehydration, the symmetry changes to tetragonal crystal system, but lattice remains the same. During the process of crystal destruction, the symmetry reduces to monoclinic crystal system and lattice type turns to primitive lattice.3. Analcime occurrence and mineral evolution sequence for the reconstruction process of sedimentary rock formation and reveal of sedimentary environment1) Morphology, size and optical properties of euhedral analcimeThe Cretaceous analcime mainly outcropped in euhedral or semi-euhedral, equiaxed, granular aggregation which usually are tetragonal trisoctahedron{211}, and a few combination of tetragonal trisoctahedron{211} and cube{100}. The ordinary diameters of granules range from 5 μm to 20 μm, rarely some with a diameter of over 30μm. The polarizer shows colorless, transparent, which disappears under orthogonal polarizer.2) Cause of formation in consideration of shape of outcrops, other detrital minerals and occurrenceAnalcime in the rocks is both authigenic mineral and cement. It distributed among detrital particles with a cement base and its contents is over 50%. The detrital minerals mainly are quartz and feldspar, and some mica and hornblende. Some detrital minerals show dissolution harbor structure. And the way of gypsum filling pores look like protolith pores. Based on the above analysis, the author concluded that the original material of analcime is glassy substance from acidic volcanic materials while quartz, feldspar, mica and hornblende form the phenocryst of those volcanic materials.3) Inference of process of sedimentary rocks and environmentAnalcimite may be formed in lacustrine sedimentary desert. The author speculated that the acidic substance first deposited in desert lakes, then the lakes dried, and gypsum crystal fills the pores of those volcanic materials. Later new water flooded into those lakes and those glassy substances were resolved and re-crystallized into analcime.4. Clarification of distribution of Cretaceous analcime in Ordos Basin1) There is a larger distribution and more contents in north Ordos Basin while there are limited distribution areas and low mineral contents in the south.2) There have been different outcrops of Cretaceous analcime in the whole Ordos Basin and the middle-to-lower part of Huanhe-Huachi group shows stability. Outcrops also distributed in some areas of Luohandong group, and in 2005, researchers found rocks containing analcime in lower part of Luohe group which contains a maximum of over 50% analcime and can be called analcite (B8 drill).3) Analcime appears in silty mudstone and argillaceous siltstone, and also in siltstone.Analcime outcrops are articles with diameters of 5～15μm but in some areas it occurred alone in larger and round articles with a diameter of over 30μm. In general, there are three different outcropped shapes:round and euhedral, cemented granular, and granular coexisting with albite and potassium feldspar.5. The hypothesis of absorption and water purification mechanism of analcimeBased upon the crystal structure and water purification experiment of analcime in Ordos Basin, the hypothesis of absorption and water purification of analcime are as follow:1) Analcime purifies high-fluorine water and water with high quantities of minerals by electrostatic absorption of SO42- and F.2) The mechanism of absorption is that the cations in water with high quantity of minerals go into analcime and take the equivalent 24c position in the tunnel, so the analcime shows electropositive and absorbs those negative SO42- on its surface.3) The analcime also can absorb F" with the help of Al(H2O)2+ in the tunnel to purify fluorine.4) Reconstructed aluminum potassium sulfate solution can use Al (OH) 2+ to reconstruct analcime in combination of absorbed F.