Page 174 - Geochemistry of Oil Field Waters

FLUORINE 161

Fluorine

Fluorine is a member of the VII A group of elements and is the most
electronegative of all the elements. Its ionic radius is 1.33 A, which is about
the size of OH- and O-*; therefore, it enters a variety of minerals. The
earth’s crust contains about 0.03 wt.% of fluorine (Fleischer, 1962). In
solutions, fluorine usually forms the fluoride F- ion; at a low pH, the HFo
form might occur. It also can form strong complexes with aluminum, beryl-
lium, and ferric iron.
Fluorine occurs in several minerals, but the only common industrial
source is fluorspar (CaF2). It occurs as HF or SiF in volcanic emanations,
and even as the free element in (stinkfluss) “stinking fluorspar” of Wolssen-
dorf, Bavaria. The solubility of calcium fluoride (fluorite) in water at 25OC is
about 8.7 ppm of fluoride (Aumeras, 1927); this solubility could be affected
by other dissolved constituents. Sodium fluoride is very soluble, and magne-
sium fluoride is more soluble than calcium fluoride; therefore, a petroleum-
associated water that is deficient in calcium and has been in contact with
rocks containing fluoride minerals will contain appreciable quantities of
fluoride.
Shales, sandstones, and carbonates contain about 740, 270 and 330 ppm
of fluorine, respectively (Mason, 1966). Sea water contains about 1.3 mg/l,
and natural waters with a dissolved solids concentration of less than 1,000
mg/l usually contain less than 1 mg/l of fluoride. However, concentrations
up to 50 mg/l have been reported (Hem, 1970). Not many subsurface
petroleum-associated brines have been analyzed for fluoride, but a few are
known to contain up to 5 mg/l.

Chlorine

Chlorine is a member of the VII A group of elements and is the most
important member of the group with respect to water. The crust of the earth
contains about 0.19 wt.% of chlorine (Fleischer, 1962); some estimates place
the fluorine abundance above the chlorine abundance. Volcanic activity
produces the gas hydrogen chloride and sometimes chlorine, but much less
frequently. The caliche evaporite deposits in Chile contain the perchlorate
ion C104-; however, the mechanism by which it formed is not clear. Several
minerals contain the chloride ion.
The chloride ion does not form low-solubility salts. It is not easily
adsorbed on clays or other mineral surfaces. It is not significant in oxidation
and reduction reactions, and it forms no important solute complexes.
Chloride is very mobile in the hydrosphere, yet it is relatively scarce in the
earth’s crust. It is the predominant anion in sea water and in most
petroleum-associated waters. It is found in all natural waters, and its average
concentration in rainwater is about 3 mg/l (Hem, 1970). Chloride salts are