【摘要】 单水方解石(CaCO3.H2O)是一种少见的含水碳酸盐矿物,具有重要的环境意义,由于不稳定而很少出现。X射线研究发现,纳木错湖底0~258 cm沉积物中存在单水方解石。单水方解石属三方晶系,六方晶胞,a=0.608 0 nm,c=0.753 6 nm,z=3。单水方解石的形成经过化学沉积过程,Ca2+先与HCO3-相结合,然后再与OH-结合而成Ca-CO3.H2O,反应进行的条件是生成自由能ΔG<0。根据相关公式计算,纳木错表层湖水全年都可以形成单水方解石,纳木错湖水pH=9.4,较高的Mg/Ca摩尔比值(10.03~15.03),咸水(矿化度为1.7 g/L)、细菌、绿藻、蓝藻、硅藻和介形类等生物的介入成为通过化学沉积形成单水方解石的直接原因,而碱性环境和低温则是该矿物保存的必要条件。单水方解石出现的时间约为AMS14C 2.06 cal.ka BP,说明此时纳木错湖水盐度增加,湖水开始咸化。更多还原

【Abstract】 Monohydrocalcite(MHC)(CaCO3·H2O)was a water-bearing carbonate rarely found in nature because of its instability.MHC occurred in sediment at the depth of 0~258 cm in Nam Co,Tibetan Plateau.X-ray powder diffraction data revealed that this sort of mineral is of a hexagonal unit cell,containing 3(CaCO3·H2O)with lattice parameters of a=0.608 0 nm,c=0.753 6 nm.In the area,the annual air temperature,pressure and wind speed is 0 ℃,570 hPa and 3.0 m/s~6.1 m/s,respectively.MHC was developed in saline waters(1.7 g/L)with high Mg/Ca ratio(6.20~9.02),high pH value(9.4) and with the presence of diatoms and ostracodes.According to the free energy of reaction ΔG =ΔGf°+RTln/,the average monthly temperature in the Nam Co area was able to form MHC all the year,and if the temperatures became higher,it could be formed more easily.The activities of organisms such as red algae,green algae,blue algae,diatom,ostracode and so on,may also played an important role in the formation and deposition of MHC.The alkalinity of lake water became higher at least at 2.06 ka BP.更多还原