Using Raman spectroscopy to improve our understanding of geological fluids
- 出版年：2000
- 作者：Pamela Murphy
- 单位1：School of Earth Sciences and Geography,Kingston University (London), Penryhn Road, Kingston Upon Thames, Surrey, KT1 2EE, UK.
- 语种：中文
- 起始页：716
- 总页数：1
- 刊名：岩石学报
- 是否内版：否
- 刊频：季刊
- 创刊时间：1985
- 主办单位：中国矿物岩石地球化学学会；中国科学院地质研究所
- 主编：叶大年
- 卷：16
- 期：4
- 期刊索取号：P350.66357
摘要
This paper is a review of the application of Raman spectroscopy to fluid inclusions and geological fluids. Mostfluid inclusion workers should be aware that microthermometry alone couldn't provide enough information to fullycharacterize inclusion fluids in terms of their composition and density. In order to determine pressure/temperature conditionsfrom microthermometric data, it is necessary to have independent analysis of the composition of the fluid, which must beprovided by another technique, such as Raman spectroscopy.
Raman spectroscopy uses the effect of molecular vibrations on the frequency of light from a laser. The laser can befocused directly into an inclusion through a microscope, and confocal optics allow the signal from above and below theinclusion to be rejected, concentrating just on the material of interest. The technique is non-destructive, and rapid.
The most common use of Raman spectroscopy in fluid inclusions is to analyse the vapour bubble or carbonic phase, whichallows identification and quantification of the proportions of gases such as CO**2, N**2 and CH**4. But Raman spectroscopy can dofar more than analyse the inclusion gases. Raman analysis at low temperatures, using a traditional heating/freezing stage,allows the study of gas hydrates (clathrates) (e.g.Murphy and Roberts, 1995,1997). This is important because theformation of clathrate changes both the density and the composition of the residual fluid.
Species in solution can also be analysed. Direct analysis and identification is possible for molecular ions such as SO24-andCO23-.Individual ions such as Na+ or Cl- cannot be identified directly, as they contain no bonds, but low temperature Ramananalyses of the salt hydrate can allow identification of the ions in solution. This type of analysis has shown that phase changespreviously though to be metastable eutectic melting are in fact the formation of salt hydrates at low temperature (Samson andWalker, 2000).
Analysis of daughter or trapped minerals is possible, although the most common daughter mineral, halite, is not Ramanactive. The identification of unusual daughter minerals such as Burbankite((Na, Ca)3 (Sr,REE,Ba)3(CO3)5)orFerropyrosmalite (Fe8Si6(OH,Cl)) give important information about the chemistry of the fluid and the solubility of elementssuch as rare earths or iron in the solution. In melt inclusions crystallized phases can also be easily identified. For all solidphases, the availability of searchable mineral databases makes identification simple.
Geological fluids are also studied through experimental work, involving fluids or synthetic fluid inclusions. Ramananalyses can be carried out at a range of temperatures and pressure with suitable furnaces or pressure cells. Such analysesallow identification of metal species in solution, and the determination of how these species change with change conditions ofP, T or pH. The speciation of a metal in solution is important ads it affects the solubility and precipitation mechanisms.Direct observations of speciation using Raman is straightforward and may highlight differences which cannot be detected usingsolubility experiments: for example, for gold chloride complexes the transformation from Au (Ⅳ) to Au (Ⅰ) at hightemperature has been shown to occur only in the presence of metallic gold (Murphy et al. , 2000). Solubility experiments, bydefinition, involve metallic gold and could not show this constraint. This type of Raman analysis can equally be well applied tometals in solution at low temperatures (such as environmental problems in acid mine drainage) or to other materials such assilica.
Raman spectroscopy uses the effect of molecular vibrations on the frequency of light from a laser. The laser can befocused directly into an inclusion through a microscope, and confocal optics allow the signal from above and below theinclusion to be rejected, concentrating just on the material of interest. The technique is non-destructive, and rapid.
The most common use of Raman spectroscopy in fluid inclusions is to analyse the vapour bubble or carbonic phase, whichallows identification and quantification of the proportions of gases such as CO**2, N**2 and CH**4. But Raman spectroscopy can dofar more than analyse the inclusion gases. Raman analysis at low temperatures, using a traditional heating/freezing stage,allows the study of gas hydrates (clathrates) (e.g.Murphy and Roberts, 1995,1997). This is important because theformation of clathrate changes both the density and the composition of the residual fluid.
Species in solution can also be analysed. Direct analysis and identification is possible for molecular ions such as SO24-andCO23-.Individual ions such as Na+ or Cl- cannot be identified directly, as they contain no bonds, but low temperature Ramananalyses of the salt hydrate can allow identification of the ions in solution. This type of analysis has shown that phase changespreviously though to be metastable eutectic melting are in fact the formation of salt hydrates at low temperature (Samson andWalker, 2000).
Analysis of daughter or trapped minerals is possible, although the most common daughter mineral, halite, is not Ramanactive. The identification of unusual daughter minerals such as Burbankite((Na, Ca)3 (Sr,REE,Ba)3(CO3)5)orFerropyrosmalite (Fe8Si6(OH,Cl)) give important information about the chemistry of the fluid and the solubility of elementssuch as rare earths or iron in the solution. In melt inclusions crystallized phases can also be easily identified. For all solidphases, the availability of searchable mineral databases makes identification simple.
Geological fluids are also studied through experimental work, involving fluids or synthetic fluid inclusions. Ramananalyses can be carried out at a range of temperatures and pressure with suitable furnaces or pressure cells. Such analysesallow identification of metal species in solution, and the determination of how these species change with change conditions ofP, T or pH. The speciation of a metal in solution is important ads it affects the solubility and precipitation mechanisms.Direct observations of speciation using Raman is straightforward and may highlight differences which cannot be detected usingsolubility experiments: for example, for gold chloride complexes the transformation from Au (Ⅳ) to Au (Ⅰ) at hightemperature has been shown to occur only in the presence of metallic gold (Murphy et al. , 2000). Solubility experiments, bydefinition, involve metallic gold and could not show this constraint. This type of Raman analysis can equally be well applied tometals in solution at low temperatures (such as environmental problems in acid mine drainage) or to other materials such assilica.