Microscopy and Microanalysis

Materials Applications

Identification of Fragile Microscopic Structures during Mineral Transformations in Wet Supercritical CO2

Bruce W. Areya1 c1, Libor Kovarika1, Odeta Qafokua1, Zheming Wanga1, Nancy J. Hessa1 and Andrew R. Felmya1

a1 Pacific Northwest National Laboratory, P.O. Box 999, K8-80, Richland, WA 99354, USA

Abstract

This study examines the nature of highly fragile reaction products that form in low water content supercritical carbon dioxide (scCO2) using a combination of focus ion beam/scanning electron microscopy, confocal Raman spectroscopy, helium ion microscopy (HeIM), and transmission electron microscopy (TEM). HeIM images show these precipitates are fragile rosettes. Using the TEM revealed details on the interfacial structure between the newly formed surface precipitates and the underlying initial solid phases. Detailed microscopy analysis revealed that growth of the precipitates either followed a tip growth mechanism, with precipitates forming directly on the forsterite surface if the initial solid was nonporous (natural forsterite) or growth from the surface of the precipitates, where fluid was conducted through the porous (nanoforsterite) agglomerates to the growth center. Identification of the mechanism of formation of hydrated/hydroxylated magnesium carbonate compound phases is a key factor in unraveling the impact of water recycling on mineral reactivity in low water content scCO2 solutions, which has received a great deal of attention as a result of the potential for CO2 to act as an atmospheric greenhouse gas. Techniques used here to examine these fragile structures are also used to examine a wide range of fragile material surfaces.

(Received October 28 2011)

(Accepted December 12 2012)

Key words

  • SEM;
  • TEM;
  • helium ion microscope;
  • carbon sequestration;
  • CO2 ;
  • forsterite

Correspondence

c1 Corresponding author. E-mail: bruce.arey@pnnl.gov