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

Abstract

Opal precipitates have been found from eleven localities in eskers and ice-marginal formations in southern Finland. The precipitates occur above the present groundwater table in porous gravel layers near the surface. The depth is commonly around 1‒2 m. The precipitates build thin (up to 0.3 mm), very light grey, colloform coatings on pebbles, cobbles and boulders. The coatings are composed of fine opal microlayers (mean thickness 0.5 um). Optical and X-ray determinations show that the coatings are composed of opal of type-A (hyalite). Refractivity index is 1.446 (sodium light). The heated material gives X-ray diffraction lines of low-cristobalite. Optical spectroscopic observations show that traces of uranium cause the bright green fluorescence of these opals (under short-wave ultraviolet radiation). The coatings contain detrital minerals as inclusions and silicified organic remains: thin roots of plants and fungal hyphae. The model proposed for the formation of the opal coatings is hydromorphic precipitation from vadose water in the soil water zone. In the contacts of boulders and pebbles water evaporated episodically. This lead to supersaturation of silica in the water film. During each evaporation cycle silica coagulated from the water film and precipitated as one microlayer of opal. In the gravel deposits, the prerequisite for the opal formation was a porous texture with large empty voids.

Keywords

• precipitation
• gravel
• optical properties
• fluorescence
• petrography
• southern Finland