Geology Reference
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trating properties. Alpha particles are effectively stopped
by a sheet of paper, beta particles are stopped by a
few millimetres of aluminium and gamma rays are only
stopped by several centimetres of lead. In air, alpha parti-
cles can travel no more than a few centimetres, beta
particles only a few decimetres and gamma rays several
hundreds of metres. Alpha particles thus cannot be de-
tected in radiometric surveying and beta particles only
in ground surveys. Only gamma rays can be detected in
airborne surveys.
There are three radioactive series of uranium and tho-
rium whose parents are 9 235 U, 9 238 U and 9 232 Th . These all
decay eventually to stable isotopes of lead via intermedi-
ate, daughter radioisotopes.About 89% of 40 K decays by
beta emission to 40 Ca and 11% to 40 Ar by K-capture.
Th
Monazite-
rich rocks
Acid to
intermediate
igneous and
siliceous
clastics
50
50
K
U
10 -4
×
50
Fig. 10.1 Relative abundances of radioactive elements in
different rock types. Also shown are the relative radioactivities
of the radioelements. (After Wollenberg 1977.)
10.3 Radioactive minerals
There is a large number of radioactive minerals (for a
full list see Durrance 1986), but the more common are
given in Table 10.1 with their modes of occurrence.
The nature of the mineral in which the radioisotope
is found is irrelevant for detection purposes as the
prospecting techniques locate the element itself.
are accurately known and unaffected by external condi-
tions such as temperature, pressure and chemical com-
position forms the basis of radiometric dating.
The radioactive emissions have very different pene-
Table 10.1 Radioactive minerals.
(From Telford et al . 1990.)
Potassium
Mineral
(i) Orthoclase and microcline feldspars [KAlSi 3 O 8 ]
(ii) Muscovite [H 2 KAl(SiO 4 ) 3 ]
(iii) Alunite [K 2 Al 6 (OH) 12 SiO 4 ]
(iv) Sylvite, carnallite [KCl, MgCl 2 . 6H 2 O]
Occurrence
(i) Main constituents in acid igneous rocks and pegmatites
(ii) Main constituents in acid igneous rocks and pegmatites
(iii) Alteration in acid volcanics
(iv) Saline deposits in sediments
Thorium
Mineral
(i) Monazite [ThO 2 + rare earth phosphate]
(ii) Thorianite [(Th,U)O 2 ]
(iii) Thorite, uranothorite [ThSiO 4 + U]
Occurrence
(i) Granites, pegmatites, gneiss
(ii), (iii) Granites, pegmatites, placers
Uranium
Mineral
(i) Uraninite [oxide of U, Pb, Ra + Th, rare earths]
(ii) Carnotite [K 2 O.2UO 3 .V 2 O 5 .2H 2 O]
(iii) Gummite [uraninite alteration]
Occurrence
(i) Granites, pegmatites and with vein deposits of Ag, Pb,
Cu, etc.
(ii) Sandstones
(iii) Associated with uraninite
 
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