Opal; a new hydromorphic precipitate type from gravel deposits in southern Finland

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Authors:Kinnunen, Kari A.; Ikonen, Liisa
Author Affiliations:Primary:
Geol. Surv. Finland, Espoo, Finland
Volume Title:Bulletin of the Geological Society of Finland
Source:Bulletin of the Geological Society of Finland, 63(2), p.95-104. Publisher: Geological Society of Finland, Helsinki, Finland. ISSN: 0367-5211
Publication Date:1991
Note:In English
Summary:Opal precipitates occur as thin (<= 0.3 mm), very light grey, colloform coatings on pebbles, cobbles and boulders in eskers and ice-margin formation in S Finland. Optical and XRD studies show that the coatings consist of opal-A (hyalite); refr. ind. 1.446; the heated material gives XRD lines of low cristobalite. Traces of U cause the bright green fluorescence of these opals. The coatings contain detrital minerals as inclusions and silicified organic remains. It is suggested that the opal coatings formed by precipitation from vadose water in the soil water zone, leading to supersaturation of silica in the water film. During each evaporation cycle silica coagulated from the film and precipitated as one microlayer of opal. In these gravels, the prerequisite for opal formation is a porous texture with large empty voids. [R.A.H.]
Subjects:Clastic sediments; Fluorescence; Framework silicates; Gravel; Opal; Optical properties; Petrography; Precipitation; Sediments; Silica minerals; Silicates; Europe; Finland; Scandinavia; Uusimaa Finland; Western Europe; Southern Finland
Abstract Numbers:92M/4635
Record ID:1995026010
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom, Reference includes data from FINGEO, Geological Survey of Finland
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