Enthalpies of formation of dolomite and of magnesian calcites
Enthalpies of formation of dolomite and of magnesian calcites
复制标题
白云石和镁方解石的形成焓
DOI：
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发表时间：
1987
期刊：
影响因子：
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通讯作者：
C. Capobianco
中科院分区：
文献类型：
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作者：
A. Navrotsky;C. Capobianco
(Ca已通过溶液量热法在85 ° C下在酸性溶剂（1 M IJ）I）中研究了碳酸镁，该溶剂在室温下用NaCl饱和并含有0.75 M CaCl和0.25 M MgClr。在85 ℃时，由方解石和菱镁矿生成有序白云石的焓为-5.74 ± 0.25kJ/mol，对于含1 mol阳离子的白云石分子式单位。合成的无序白云石的生成焓为+|点23 0.32 kJ lmol.在85 ℃下，MgCO摩尔分数X在0.01和0.13之间的四种合成镁方解石显示出相对于CaCO的混合放热。和MgCOr。结合积极的过剩自由能的混合，焓数据表明大量的负过剩熵的合成镁方解石的混合。在700 ℃附近平衡的合成镁方解石中，可能存在相当大的局部有序性，可能是镁聚集成随机分布的富镁阳离子层。合成镁铁质方解石的热力学行为可能不同于生物成因的方解石。
(Ca,Mg) carbonates have been studied by solution calorimetry at 85"C in an acid solvent (lM IJ)I) that was saturated at room temperature with NaCl and contained 0.7 5M CaCl, and 0.25M MgClr. The enthalpy of formation of ordered dolomite from calcite and mag' nesite at 85"C is -5.74 + 0.25 kJ/mol, for a formula unit of dolomite containing I mol of cations. A synthetically disordered dolomite has an enthalpy of formation of + | .23 ! 0.32 kJlmol. At 85"C, four synthetic magnesian calcites with mole fraction, X, of MgCO, between 0.01 and 0.13 show exothermic heats of mixing relative to CaCO. and MgCOr. Combined with positive excess free energies of mixing, the enthalpy data suggest substantial negative excess entropies of mixing in the synthetic magnesian calcites. It is possible that considerable local order, perhaps clustering of Mg into randomly spaced Mg-rich cation layers, exists in synthetic magnesian calcites equilibrated near 700'C. The thermodynamic behavior of synthetic magrresian calcites may differ from that of biogenic samples.