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Cristobalite and Quartz Isolation from Soils and Sediments by Hydrofluosilicie Acid Treatment and Heavy Liquid Separation
Author(s) -
Henderson J. H.,
Clayton R. N.,
Jackson M. L.,
Syers J. K.,
Rex R. W.,
Brown J. L.,
Sachs I. B.
Publication year - 1972
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1972.03615995003600050038x
Subject(s) - cristobalite , quartz , dissolution , mineralogy , tridymite , hydrothermal circulation , chemistry , nuclear chemistry , geology , organic chemistry , paleontology , seismology
A qualitative method for the isolation of cristobalite or of quartz from soils and sediments was developed for the characterization of these SiO 2 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‐SiO 2 minerals was effected by HCl, NaOH, and H 2 SiF 6 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 δ 18 O = 14 ‰, with HCl, NaOH, and H 2 SiF 6 removed amorphous silica and released the crystalline platelets of δ 18 O = 9 ‰. Little change in isotope abundance of the latter occurred on retreatment. When amorphous diatom skeletons (δ 18 O = 32.2 ‰) were treated with these reagents, 77% of the sample dissolved. The remaining skeletons had slightly lower δ 18 O 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 δ 18 O values of 9.6 to 10.0 ‰ The associated low‐cristobalite had δ 18 O = 8.3 to 9.1 ‰. The oxygen isotope data indicate that both of these SiO 2 polymorphs had a hydrothermal or volcanic origin, and were not formed in the soil.