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<br />- TIIE C:\11\IA R.\1' AYD SPECTR.\L GA~I )IA R:1Y LOGS - <br />Table 7.9 Potassium content of evaporates. <br />Species Formula % Potassium Typical gamma <br />by w~eight• ray value API' <br />Sylvate KCI 52.5 500 <br />Camallite KCLMgCIr(Hr0)a 14.1 2W <br />Polyhalite KISO,MgSO, 12.9 190 <br />"Serra era(., 1980 <br />'Berta, 1979. <br />important effect on the radioactivity (Table 7.9). in these <br />salts there is between ]0% and 50% potassium by weight. <br />When it is considered that the average shale contains only <br />2%o - 3.5%o potassium, the very strong radioactivity of these <br />potassium evaporates is understandable (Table 7.9, Figure <br />7.21). <br />Uranium <br />Acid igneous rocks on average con[ain 4.65ppm of <br />uranium and are the principal original source for the <br />element. It forms soluble salts, especially in the uranyle <br />form (IJs') being stable in oxidising conditions, and as the <br />oxide UOZZ'(the uranyle ion) is transported in river water <br />(the uranous form U" also exists and is stable in reducing <br />u <br />0 <br />25 <br />E <br />L <br />C <br />O <br />a <br />50 <br />75 <br />conditions, but is less common) which contains on aver- <br />age 0.6 µe/ml of uranium in solution. However, it is <br />suggested that most (around 90%) uranium in rivers is <br />actually tamed attached (loosely?) to clay particles and <br />not in solution (Durrance, 1986). This is suggested <br />because suspended river sediment contains approxi- <br />mately 3ppm of uranium, while the bedload sediments <br />hale much lower values. Sea water, on average contains <br />about 3ppb of dissolved uranium. <br />From river or especially sea water, uranium passes <br />into sediments in three principal ways (Serra, 1979): 1, <br />chemical precipitation in acid (pH 2.5-1.0). reducing (rH <br />0-0.4) environments: 2, adsorption by organic matter, <br />or living plants and animals: 3, chemical reaction in <br />phosphorites (phosphate rich rocks). <br />The extremely acid, reducing conditions required for the <br />direct chemical precipitation of uranium (pH 2.5-1.0, rH <br />0-0.4) are found in few natural environments. They do <br />occur, however, in stagnant, anoxic waters with a relatively <br />slow rate of sediment deposi[ion, which typically produce <br />black shales (Adams and Weaver, 1958). The high gamma <br />radiation values of the North Sea Jurassic 'hot shales', <br />typical black shales, come from a high uranium content, <br />some of which was probably chemically precipitated <br />(Figure 7.1 I, Table 7.10) (BjOrlykke et ai.. 1975). <br />GAMM A RAY (TOTA L) API O THORIUM URANIUM PO TASSIUt.1 % <br />5 0 10 0 150 ~ (ppm) (ppm) <br /> ~ 0 d 6 12 16 2 g 6 8 2 3 C 5 1 <br /> ,~ <br />[ <br />~~ 0% K <br /> ~'~ <br /> .:, ~.ji. <br /> <br />hole Size 12.2 <br />5" `' <br /> <br />thorium <br />~ <br /> <br />~ <br />ogging spe etll 300m/h ura nium <br />t.5" stand-ol fs ' <br /> /potassium <br /> N <br /> W <br /> J <br />Q <br /> _ <br /> N <br /> Y <br /> V <br /> <br /> J <br />m <br /> I <br />Figure 7.11 'Black shale' radioac[ivity. A spectral gamma ray log over the Upper Jurasic black shales of the Nonh Sea showing <br />the high uranium contribution. <br />75 <br />