| Ikaite (CaCO3-·6H2O) is a metastable mineral of calcium carbonate, which is usually found in the environments characterized with low temperature (below 5℃), high pH, high Alkalinity, high concentration of phosphate and high organic matters. It is considered as one of the precursors to stable calcium carbonate minerals. Although the synthetic CaCO3·6H20 was known from earlier laboratory study in 1865, ikaite in nature was first observed quite late until 1963. Recently, Dr. Dieckmann et al. discovered this mineral in sea ice, who first directly supported the precipitation of CaCO3 during the sea ice forms.This discovery provides the necessary evidence for the evaluation of previous assumptions that in polar region, the formation of sea ice will result in the precipitation of calcium carbonate. And this discovery also lays the foundation for further studies to help elucidate the role of ikaite in the carbon cycle of the seasonally sea ice covered regions.However, little is known about the formation of ikaite in sea ice. This study looks for which conditions in sea ice favour the formation of ikaite, and how the conditions affect the stability of ikaite. Experiments are set up by modifying the ikaite recipe with the characteristics of high pH (～13) and zero salinity to the normal pH in sea ice (pH=8.5～9) and high salinity (up to 105) in a 0℃room, in order to see the effect of pH, salinity, concentrating rate of reaction solution and also the phosphate ion on the formation of ikaite. Besides, ikaite time series experiment (up to 4 months) is conducted at different ikaite concentration, temperature, salinity and phosphate concentration to find out under these conditions, when ikaite is transformed to calcite and how the size and shape of crystals develop with time both under Raman microscope and binocular microscope. Preliminary results are drawn as follows: (1) Ikaite can form under the conditions of pH=9.0 and pH=8.5, which suggests that the ikaite precipitation in sea ice is very likely. Besides, at a higher pH level, the induction time (the time when the crystal starts to precipitate) is shorter which means high pH favours the formation of ikaite. As the precipitation of ikaite is followed by Ca2++CO32-+6H20→?CaCO3·6H2O, at the same conditions, the concentration of CO32- increases with pH. So at high pH, the concentration production of Ca2+ and CO32- is enhanced and the solution can reach ikaite saturation much earlier.(2) The results of salinity effect indicates that ikaite can forms in a wide range of salinity from 0～105 both in Artificial Sea Water (ASW) and NaCl solution in the presence of phosphate ion. In ASW, the induction time increases with the salinity from S=0 to S=105; while in NaCl solution, the induction time first increases with salinity and then decreases with the further increase of salinity. The salinity plays both positive and negative roles in the formation of ikaite. On one hand, the increase of salinity will increase the ratio of CO32- in DIC. On the other hand, the increase of salinity means more ions are involved in the solution which will reduce the activities of Ca2+ and CO32- by forming ion pairs with them. This effect is more obvious in ASW, as there are more ion species in ASW than in NaCl solution. The effect of salinity on ikaite formation depends on the competition between these two effects.(3) The results of phosphate on the formation of ikaite experiment at high salinity (S=70) shows that in ASW, the precipitation is ikaite both with and without the presence of phosphate ion. In NaCl solution, in the presence of phosphate, the precipitation is ikaite; without phosphate, the precipitation is no longer ikaite but vaterite. These results suggest phosphate ion plays important role in the formation of ikaite. Besides, there must be other ion(s) in ASW which also favour the formation of ikaite.(4) The result of pumping rate experiment shows that ikaite can form in a wide range of pumping rate. At a higher pumping rate, the induction time is less which is obvious. But the normalized induction time increases with pumping rate. This is because at low concentrating rate, the Ca2+ ion and CO32- ion have more time to react with each other, ikaite can form at a relatively low saturation level (Ω). So low concentrating rate might favour the formation of ikaite during sea ice forms.(5) The results from ikaite time series experiment show that under room temperature, ikaite is transformed to calcite rather fast, within 1-2 h. longer time series experiment shows that without phosphate ion in the solution, ikaite is more easily dehydrated to calcite in high ikaite concentration and high salinity solution; while at the same condition but in the presence of phosphate ion, ikaite can be stable for longer time. However, different phosphate concentrations seem to make no difference on the stability of ikaite. One interesting phenomenon is while the shape of crystals starts to change (from irregular quadrilateral to needle or six-side), and size becomes larger, which also is the time when ikaite starts to convert to calcite. These two phenomenons are closely related.
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