Unmixing of Phosphorus-Bearing Melts on Earth and Mars

探讨了磷对硅酸盐熔体粘度反常影响的结构性原因。已发现磷的添加增加了过碱性成分熔体的粘度。进一步加入P_2O_5会降低熔体粘度。尽管以前的研究表明这些含磷熔体是未混合的，但无论是拉曼光谱还是核磁共振光谱都没有证明这一点。对目前一些含P_2O_5的过碱性Na2O-Al_2O_3-SiO_2熔体的玻璃化转变温度的初步测量表明，存在两个玻璃化转变温度，表明样品中存在两个未混合的玻璃，尽管玻璃是无色和透明的。通过扫描电子显微镜对宏观结构的进一步研究表明，玻璃样品确实是未混合的。在高(文献数据)和低(当前数据)温度下观察到的粘度的增加表明，该熔体在1446°C以下是未混合的。鉴于这些观察，这里将使用量热、粘度、拉曼和扫描电子显微镜技术来研究含P2O5的熔体可能的混合。研究的成分范围从简单的Na2O-Al_2O_3-SiO_2和CaO-Al_2O_3-SiO_2到更复杂的火星成分(古瑟夫陨石坑中的香槟岩)。这些熔体的混合不仅对粘度有影响，而且对共存的不同成分的混合岩浆的结晶过程也有影响。磷一般被归类为熔体中的助熔剂。其他助熔剂有B、F和H2O。众所周知，硼还会导致熔体混合。此外，由于在同时含有CaO和Na2O的熔体中加入B会导致过碱度增加，反应BO4=BO_3+NbO随着CaO/Na_2O的增加而右移，因此熔体中B_2O_3的存在可能会影响含P_2O_5熔体的分解。一旦解决了含P_2O_5熔体的混合问题，该项目将扩展到添加B_2O_3、F和H_2O的P_2O_5熔体的混合。

The structural reasons for the anomalous effect of phosphorus on silicate melt viscosity will be investigated. The addition of phosphorus has been found to increase the viscosity of peralkaline composition melts. Further addition of P2O5 results in a decrease in melt viscosity. Despite indications in previous studies that these phosphorus-bearing melts are unmixed, neither Raman spectroscopy, nor NMR spectroscopy has proven this. Preliminary calorimetric measurements of the glass transition temperature of some of the present P2O5-bearing peralkaline Na2O-Al2O3-SiO2 melts has shown there to be 2 glass transition temperatures, indicating that there are 2 unmixed glasses present in the sample, despite the glass being colourless and transparent. Further investigation of the macroscopic structure via scanning electron microscopy reveals that indeed the glass sample has unmixed. The increase in viscosity observed at both high (literature data) and low (present data) temperature indicates that this melt is unmixed up to 1446 °C. In view of these observations, the possible unmixing of P2O5-bearing melts will be investigated here using calorimetry, viscosity, Raman, and SEM techniques. The array of compositions investigated range from the simple Na2O-Al2O3-SiO2 and CaO-Al2O3-SiO2 to more complex Martian compositions (the Champagne rock in the Gusev Crater). The unmixing of these melts has implications not only for viscosity but also for crystallisation processes in co-existing co-mixed magmas of different composition. Phosphorus is in general classed as a fluxing agent in melts. Other fluxing agents are B, F and H2O. Boron is also known to cause melts to unmix. Further, as the addition of boron to the melt with both CaO and Na2O will result in an increase in peralkalinity with the reaction BO4=BO3+NBO shifting to the right with increasing CaO/Na2O, the presence of B2O3 in the melt is expected to affect the unmixing of the P2O5-bearing melts. Once the question of the unmixing of P2O5-bearing melts has been addressed, the project will extend to the unmixing of P2O5-bearing melts with added B2O3, F and H2O.