| "Dolomite problem" is one of the most intriguing and long-standing puzzles in geology.As a common mineral in nature,dolomite is abundant in geological record but quiet rare in modern marine sediments.Even many attempts have been made in laboratory to synthesize dolomite under simulated natural conditions,there is hitherto hardly any report on successful synthesis of ordered dolomite under 100?.The failure in dolomite syntheses at low temperature restrains our understanding toward the formation mechanisms of dolomite and its cation-ordered structure,making the origin of dolomite remain in controversy.Dolomite-analogues refer to double carbonates which possess a cation ordering structure analogous to dolomite.Due to their chemical and structural similarities to dolomite,the investigations on the crystallization of dolomite analogues can help understand the formation mechanism of cation-ordered dolomite.In this dissertation,two magnesium-bearing dolomite-analogues were selected as the objects,and a series of mineralization experiments were conducted under ambient conditions by the use of a low-temperature mineralogical method.The formation pathways of dolomite-analogues as well as the evolution in their crystal structure were studied,and special attention was paid on the effect of solution chemistry on their crystallization behavior.Moreover,on the basis of mineralization experiments in laboratory,the microstructure of these magnesium-bearing dolomite-analogues were further characterized and studied,and we attempted to explain the different formation kinetics of dolomite analogues including dolomite from the perspective of crystallography.The obtained results will contribute to a more comprehensive understanding of the formation mechanism of minerals with dolomite-like structure,and give an insight into "dolomite problem".Several important results of this dissertation were summarized as follows:1.Norsethite is one of the few dolomite analogues that have been both found in natural environments and obtained in laboratory.Previous studies indicate that mechanisms that underlie the formation of norsethite exhibit some similarities to pathways of dolomite crystallization.However,the crystallization behavior of norsethite is still poorly understood,and the physicochemical factors regulating the process are not yet fully established.Herein,a series of norsethite mineralization experiments were carried out by using a CO2 gas-diffusion method at ambient temperatures,and the formation pathways of norsethite under different solution chemical conditions were systematically studied.The experimental results reveal that the initial Mg/Ba ratio in solution is the crucial factor determining the formation pathway of norsethite.At low Mg/Ba ratios(<15),a multistep pathway occurs,i.e.,a precursor witherite first forms,followed by norsethite precipitation and transformation from witherite to norsethite;while at Mg/Ba ratio higher than 20,stoichiometric and fully ordered norsethite can be directly precipitated as penetration twins with well-developed {104} faces from aqueous solutions.To the best of our knowledge,this is the first report of direct precipitation of norsethite under ambient conditions.The findings provide new insights into the formation pathways of dolomite and its analogues,indicating that dolomite may be directly precipitated under appropriate physicochemical conditions.2.Besides norsethite,PbMg(CO3)2 is another important dolomite-analogous carbonate that can form in laboratory under ambient conditions within reasonable time duration.However,since it has not been found in natural environments yet,only a few studies focused on the physicochemical conditons and mechanisms of PbMg(CO3)2 formation so far.In present study,a series of experiments for the synthesis of PbMg(CO3)2 from the solutions with different concentrations of Mg2+and Pb2+were carried out by using a CO2 gas-diffusion method.The formation pathways of PbMg(CO3)2 under different solution chemical conditions were studied,and special attention has been paid on the evolution of PbMg(CO3)2 crystal structure(crystallinity,stoichiometry and cation ordered degree)during the mineralization.The results show that,differing from norsethite,PbMg(CO3)2 always forms through a three-stage reaction at different Mg/Pb ratios,including the preprecipitation of hydrocerussite,the transformation from hydrocerussite to cerussite,and the transformation from cerussite to PbMg(CO3)2.Solution chemical analyses reveal that the transformation from cerussite to PbMg(CO3)2 occurs in mildly alkaline solutions with high[Mg2+]and extremely low[Pb2+],indicating that it is essentially a dolomitization-analogous process-"PbMg(CO3)2-ization",i.e.,the interaction between a simple carbonate and the magnesium-rich fluid.These results suggest that dolomitization-analogous processes could be a common mechanism for the formation of dolomite analogues under ambient conditions.In addition,XRD analyses shows that the early formed PbMg(CO3)2 is well-crystallized,stoichiometric but incompletely cation-ordered,and its cation ordering degree gradually increases with mineralization time prolonging.These results provide new insights into the physicochemical conditons,pathway and cation ordering process for PbMg(CO3)2 formaiton,promoting our understanding towards the formation mechanism of dolomite-analogues.3.The dehydration barrier of Mg2+ has long been considered as the main inhibiting factor for the crystallization of anhydrous Mg-containing carbonates,e.g.,dolomite and magnesite,at low temperatures.However,norsethite[BaMg(CO3)2],which is also an anhydrous mineral containing Mg,can readily form in short timescales under ambient conditions.It has been suggested that there should be an intrinsic relationship between the crystal structure and crystallization behavior,but the existing knowledge of norsethite structure is inadequate to explain its formation kintetics.To this end,here we have carefully examined the microstructure of norsethite crystals synthesized at high/low temperatures by a series of NMR techniques including 13C direct-polarization(DP)MAS NMR,13C{1H} cross-polarization(CP)MAS NMR,1H single pulse MAS NMR and 13C{1H} 2D HECTOR etc..The results demonstrate that there exist several intrinsic hydrogen defects in norsethite structure,including structural H2O,a few hydroxyl and bicarbonate.Especially,the 1H chemical shift of structural water(?h=5.10 ppm)is similar to that of water in several hydrated magnesium carbonates,thus this structural water may originate from the incompletely dehydration of Mg2+in norsethite.We speculate that allowing these partially dehydrated Mg2+ into crystal lattice makes it possible for norsethite to overcome the Mg dehydration barrier and crystallize at low temperatures.In addition,further 13C MAS NMR.characterization for a series of magnesium-bearing carbonate minerals show that similar H defects also exist in the structures of Mg-calcaite and PbMg(CO3)2 while these two carbonates can form at low temperatures.Therefore,the results reiterate the relationship between the H defects in the crystal structures of magnesium-bearing carbonate and their crystallization behavior,indicating that these intrinsic H defects may be the key for magnesium-bearing carbonate to overcome the dehydration of Mg2+ and crystallize at low temperatures. |
PDF Full Text Request