Cristobalite and Quartz Isolation from Soils and Sediments by Hydrofluosilicie Acid Treatment and Heavy Liquid Separation

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Abstract
A qualitative method for the isolation of cristobalite or of quartz from soils and sediments was developed for the characterization of these SiO2 polymorphs by X‐ray diffraction (XRD), scanning electron microscopy (SEM), and oxygen isotope analysis by mass spectrometry. The procedure, applicable to silt or sand size fractions, involved two steps. First the quartz was separated from the cristobalite by centrifugation of a polyvinylpyrrolidone (PVP) stabilized suspension in tetrabromoethane and nitrobenzene mixtures (specific gravity range from 2.28 to 2.38). Then selective chemical dissolution of the non‐SiO2 minerals was effected by HCl, NaOH, and H2SiF6 treatments. Minerals, such as magnetite, resistant to dissolution in these reagents, were subsequently removed by heavy liquid separation.Treatment of hydrothermal low‐cristobalite (Taiwan), having blocky SEM masses and δ18O = 14 ‰, with HCl, NaOH, and H2SiF6 removed amorphous silica and released the crystalline platelets of δ18O = 9 ‰. Little change in isotope abundance of the latter occurred on retreatment. When amorphous diatom skeletons (δ18O = 32.2 ‰) were treated with these reagents, 77% of the sample dissolved. The remaining skeletons had slightly lower δ18O value (29.1 ‰) but a similar diatom skeleton morphology by SEM.Quartz isolated from the A2 and Cl horizons of Parahaki soil of New Zealand had δ18O values of 9.6 to 10.0 ‰ The associated low‐cristobalite had δ18O = 8.3 to 9.1 ‰. The oxygen isotope data indicate that both of these SiO2 polymorphs had a hydrothermal or volcanic origin, and were not formed in the soil.
Funding Information
  • U.S. Department of Agriculture
  • National Science Foundation (GP4144, GA1108, GA1390)
  • U.S. Department of the Interior
  • Georgia Institute of Technology
  • Forest Products Laboratory

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