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Cristobalite and Quartz Isolation from Soils and Sediments by Hydrofluosilicie Acid Treatment and Heavy Liquid Separation¹

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 ¹⁸O = 14 , with HCl, NaOH, and H₂SiF₆ removed amorphous silica and released the crystalline platelets of ¹⁸O = 9 . Little change in isotope abundance of the latter occurred on retreatment. When amorphous diatom skeletons (¹⁸O = 32.2 ) were treated with these reagents, 77% of the sample dissolved. The remaining skeletons had slightly lower ¹⁸O 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 ¹⁸O values of 9.6 to 10.0 The associated low-cristobalite had ¹⁸O = 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.

¹ Research supported in part by the College of Agr. and Life Sci., Univ. of Wisconsin, Madison 53706, under project 1123; in part by the National Science Foundation Grants GP4144- and GA1108-Jackson, and GA1390-Clayton; in part by an Academic Senate Grant, Univ. of California, Riverside, California (Rex); in part by Training Grant WP-100 of the Federal Water Quality Administration of the US Dep. of the Interior; in part by the Engineering Exp. Sta., Georgia Institute of Technology, Atlanta, Georgia; and in part by the Fine Wood Structure Division, Forest Products Laboratory, USDA, Madison, Wisconsin; and through an International Consortium for Interinstitutional Cooperation in the Advancement of Learning (ICICAL). Presented before Div. S-9, Soil Science Society of America, Tucson, Arizona, Aug. 27, 1970.

² Research Chemist, The Permeator Corp., 4009 Forest Lane, Garland, TX (formerly Graduate Research Assistant); Professor of Chemistry, The Enrico Fermi Institute, University of Chicago, IL; Professor and Associate Professor of Soil Science (UW); Professor of Geological Sciences (UCR); Director, Analytical Instrumentation Laboratory (GIT); and Cytologist (USDA); respectively.

Received for publication June 14, 1971. Accepted for publication June 30, 1972.