Main and trace element content of shales from Ankerskogen (Hamar) and Øvre Slottsgt. (Oslo), Norway
The composition of black and other shales within the Oslo Rift system may vary locally with respect to main and trace elements. Chemical analysis of major, minor and trace elements of one hundred Cambrian and Ordovician shale samples collected in Oslo and Hamar quantifies the variation in concentrations among the samples. In this report the concentration of elements determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) after digestion in strong acid, combustion assay with infrared detector (i.e. Leco stove), and x-ray fluorescence (XRF), is presented on samples from two locations in the Oslo Rift system. The data are presented in similar diagrams to those used by Broekmans and Sæther (2008) to facilitate comparison between shale samples from different localities. Data on mineralogical composition are assessed with respect to the role of sulfides and carbonates in forming gypsum and thus potentially causing swelling. The samples from Ankerskogen are not typical "Black shales". By definition"Black shales" are dark-colored mudrocks containing organic matter and silt- and clay-sized mineral grains that accumulated together, and usually contain 1% or more organic carbons. Samples in Øvre Slottsgt. are typical black shales. They contain average organic carbon around 8-9%, with distinct black color due to the high content of organic carbon. The Ankerskogen samples indicate that clay minerals account for the largest fraction of the total mineral weight, spanning from 17% to 70% (excluding two samples of limestone nodules), with an average of around 53%. Quartz comes second, ranging from 9% to 45%, averaging about 24%. The black shales from Øvre Slottsgt. contain illite/muscovite, lack chlorite, and represent a mineralogically very mature sediment. High concentrations of sulphur are found in these samples, with minimum of 2.38%, maximum of 7.96% and average of .5.47%. Semi-quantitation of the mineral content shows that quartz amounts to an average of 23% (excluding one sample containing limestone nodules) of all the mineral weight, clay (llite/muscovite) 45%, and pyrite 10%. Calcite minerals in black shales together with iron sulfides can lead to the development of gypsum when sulphuric acid reacts with the calcite. Gypsum has lower density and can cause the expansion of shales. The conversion of iron sulfides to gypsum in situ has been suggested by Hagelia et al. (2003) as the mechanism leading to the swelling of alum shales in Oslo region. Gypsum was not found in any samples. This is most probably due to the fact that shales from these areas have not yet been exposed to oxidizing aquatic environment as they were freshly drilled and excavated.
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