Page 152 - Geochemistry of Oil Field Waters

140 INORGANIC CONSTITUENTS AND PHYSICAL PROPERTIES

ary, Cretaceous, and Jurassic ages to an evaporite-associated sea water
(Collins, 1970). Fig.5.6 illustrates the same relation for some subsurface
brines taken from Pennsylvanian and Mississippian age sediments (Collins,
1969a).
The depletion of potassium in subsurface brines might be caused by its
uptake by clays. For example, montmorillonite-type clay minerals system-
atically change to illite with increasing depth of burial, due to thermal
diagenesis; and, as a result of this transformation, they lose interlayer
(bound) water (Burst, 1969). This change appears to begin at a temperature
above 90°C. (This freed interlayer water can be readily expelled, and its
movement probably is important in the first migration stage of hydrocar-
bons.) Laboratory experiments at elevated temperatures and pressures in-
dicate that montmorillonite loses its interlayer water and transforms into
illite in the presence of potassium-enriched water (Khitarov and Pugin,
1966). The structural variations of the expandable minerals in clays appar-
ently are also influenced by the potassium content of the associated waters.

Rubidium

Rubidium, like the other alkali metals is lithophilic, and its abundance in
the earth’s crust is about 3.0 x wt.%, which is greater than that of
lithium (Fleischer, 1962). It tends to be removed from solution more readily
than lithium, primarily because of its ability to replace potassium in mineral
structures. Table 5.11 indicates that it precipitates from an evaporite along
with sylvite to a greater extent than lithium, and it has a high chemical
reactivity. The radius of its ion, 1.48 a, is only about 10% larger than the
potassium ion, so it can be accommodated into the same crystal lattices.
Because of this, it forms no minerals of its own.
Rubidium and cesium occur sympathetically in nature; that is, both are
commonly found in amazonite, vorobyevite, and beryl (Goldschmidt, 1958).
Rubidium is a member of series NH4-K-Rb-Cs, and members of this series
are more similar in their chemical and physical properties than are the mem-
bers of any other group, with the exception of the halogens. Rubidium
concentrates in the late crystallates, particularly those of granitic derivation,
and it has a greater tendency to be adsorbed by clays than has potassium. It
is removed from igneous rocks by water leaching and then adsorbed by
hydrolysate sediments and soils.
Shales contain about 250 ppm of rubidium; deep-sea red clays, about 400
ppm; and some glauconites, about 500 ppm (Goldschmidt, 1958). Sea water
contains about 0.12 mg/l of rubidium; subsurface brines contain up to 4
mg/l. Higher concentrations of rubidium probably can be found in brines
associated with rocks containing potassium minerals, such as microcline
feldspars, or lepidolite mica.