@article{DietrichBehrensWilke2018,
  author    = {Dietrich, Marcel and Behrens, Harald and Wilke, Max},
  title     = {A new optical cell for in situ Raman spectroscopy, and its application to study sulfur-bearing fluids at elevated pressures and temperatures},
  series = {American mineralogist : an international journal of earth and planetary materials},
  volume    = {103},
  journal   = {American mineralogist : an international journal of earth and planetary materials},
  number    = {3},
  publisher = {Mineralogical Society of America},
  address   = {Chantilly},
  issn      = {0003-004X},
  doi       = {10.2138/am-2018-6244},
  pages     = {418 -- 429},
  year      = {2018},
  abstract  = {Sulfur is an important component in volcanic gases at the Earth surface but also present in the deep Earth in hydrothermal or magmatic fluids. Little is known about the evolution of such fluids during ascent in the crust. A new optical cell was developed for in situ Raman spectroscopic investigations on fluids allowing abrupt or continuous changes of pressure up to 200 MPa at temperatures up to 750 degrees C. The concept is based on a flexible gold bellow, which separates the sample fluid from the pressure medium water. To avoid reactions between aggressive fluids and the pressure cell, steel components in contact with the fluid are shielded by gold foil. The cell was tested to study redox reactions in fluids using aqueous ammonium sulfate solutions as a model system. During heating at constant pressure of 130 MPa, sulfate ions transform first to HSO4- ions and then to molecular units such as H2SO4. Variation of pressure shows that the stability of sulfate species relies on fluid density, i.e., highly charged species are stable only in high-density fluids. Partial decomposition of ammonium was evident above 550 degrees C by the occurrence of a nitrogen peak in the Raman spectra. Reduced sulfur species were observed above 700 degrees C by Raman signals near 2590 cm(-1) assigned to HS- and H2S. No clear evidence for the formation of sulfur dioxide was found in contrary to previous studies on aqueous H2SO4, suggesting very reducing conditions in our experiments. Fluid-mineral interaction was studied by inserting into the cell a small, semi-open capsule filled with a mixture of pyrite and pyrrhotite. Oxidation of the sample assembly was evident by transformation of pyrite to pyrrhotite. As a consequence, sulfide species were observed in the fluid already at temperatures of similar to 600 degrees C.},
  language  = {en}
}
@article{SublettSendulaLamadridetal.2019,
  author    = {Sublett, David Matthew and Sendula, Eszter and Lamadrid, Hector and Steele-MacInnis, Matthew and Spiekermann, Georg and Burruss, Robert C. and Bodnar, Robert J.},
  title     = {Shift in the Raman symmetric stretching band of N-2, CO2, and CH4 as a function of temperature, pressure, and density},
  series = {Journal of Raman spectroscopy : JRS},
  volume    = {51},
  journal   = {Journal of Raman spectroscopy : JRS},
  number    = {3},
  publisher = {Wiley},
  address   = {Hoboken},
  issn      = {0377-0486},
  doi       = {10.1002/jrs.5805},
  pages     = {555 -- 568},
  year      = {2019},
  abstract  = {The Raman spectra of pure N-2, CO2, and CH4 were analyzed over the range 10 to 500 bars and from -160 degrees C to 200 degrees C (N-2), 22 degrees C to 350 degrees C (CO2), and -100 degrees C to 450 degrees C (CH4). At constant temperature, Raman peak position, including the more intense CO2 peak (nu+), decreases (shifts to lower wave number) with increasing pressure for all three gases over the entire pressure and temperature (PT) range studied. At constant pressure, the peak position for CO2 and CH4 increases (shifts to higher wave number) with increasing temperature over the entire PT range studied. In contrast, N-2 first shows an increase in peak position with increasing temperature at constant pressure, followed by a decrease in peak position with increasing temperature. The inflection temperature at which the trend reverses for N-2 is located between 0 degrees C and 50 degrees C at pressures above similar to 50 bars and is pressure dependent. Below similar to 50 bars, the inflection temperature was observed as low as -120 degrees C. The shifts in Raman peak positions with PT are related to relative density changes, which reflect changes in intermolecular attraction and repulsion. A conceptual model relating the Raman spectral properties of N-2, CO2, and CH4 to relative density (volume) changes and attractive and repulsive forces is presented here. Additionally, reduced temperature-dependent densimeters and barometers are presented for each pure component over the respective PT ranges. The Raman spectral behavior of the pure gases as a function of temperature and pressure is assessed to provide a framework for understanding the behavior of each component in multicomponent N-2-CO2-CH4 gas systems in a future study.},
  language  = {en}
}