RESEARCH· ·1970·

SIMS
B31
_e
of the more soluble cations from muscovite. The mineral
with a muscovite-like birefringence and a kaolinitelike
che1nical composition 1nay represent the penultimate
stage in the transformation of muscovite to kaolinite.
These data may also be interpreted to suggest
that illite and (or) 1nuscovite are intermediate phases
in the authigenesis of kaolinite from smne other silicate
minernJ-such as !(-feldspar.
Jonas (1964) presents data suggesting that wellcrystallized
vermicular kaolinite is derived by recrystallization.
Jonas further indicates that authigenic
kaolinite derived from !(-feldspar occurs as wellcrystallized
pseudomorphs, and kaolinite derived from
muscovite is b-axis disordered. The kaolinite in the·
'~Tilcox Formation, except for kaolinite in the 1natrix,
is dominantly of the vermicular well-ordered type. X­
ray diffractograms show some b-axis disorder, but it is
not definitely attributable to the vermicular kaolinite.
l\1ullineaux, Nichols, and Speirer (1964) present a
paragenesis of authigenic clay minerals in a Pleistocene
paleosol near Seattle, 'Vash. The clay mineral suites in
this paleosol illustrate the destruction of illite and
chlorite in the weathering profile of soils to form
kaolinite and -montmorillonitic mixed-l'ayer minerals.·
Grbn ( 1968, p. 520-523) presents a further concise
trentment of the nature of the alteration of primary
minerals to clay minerals. These data support the
theoretical considerations of IGttrick ( 1968), Garrels
and Christ ( 1965), and l\1ackenzie and Garrels ( 1966).
J(ittrick ( 1968) presents diagrams that enable
visualization of the relative stability of minerals in
the system A1 2 0a - SiOz- RzO. Solubility studies on
soils indicate that the ll1Si0,1 activity is controlled, not
by equilibrium thermodynamics, but by kinetics of the
dissolution of silica. The silica activity in turn seems
to determine which secondary minerals will form.
Amorphous silica probably limits high silica levels,
with montmorillonite forming at slightly lower levels,
kaolinite at intermediate levels, and gibbsite at low
silica levels.
Rex ( 1966), I:Iess ( 1966), and l\1ackenzie and
Garrels (1966), illustrate the nature of the kaolinite­
I\:-mica-1(-feldspar equilibria (fig. 6). The geochemical
environment of the 'Vilcox Formation was
estimated by using world averages for the composition
of fresh-water rivers and lakes (Livingstone, 1963)
nnd assumed pfl = 7 .5, and is in agreement with Garrels
and Christ ( 1965) ttnd Rex ( 1966). The data suggest
that kaolinite can precipitate frmn solution when the
ion activity product of kaolinite in solution is increased
to equrul or exceed the equilibrium constant of kaolinite.
J(aolinite precipitates only where alkali and alkalineearth
metal cations are not present in sufficient quantity
+ ~~~ +
~~
tl.O
E
7
6
5
4
0
0
0
0 o 0
00 ~
og o 00
0 00
0
0 6
0
0
0 0
oo
Kaolinite 0
0
o
oo
Oo
3~------~~----~----o~g~~
6
D
0
5 4
EXPLANATION
Mean ocean water
(Mackenzie and Garrels,
1966)
Mean world river water
(Livingstone. 1963)
Water from arkosic sediments
(Garrels and Christ, 1965)
3
I
c:
0
:;;
~
E
ctl
1/)
ctl
~
'iii
1/)
::J
0
.J::.
0.
0
E
ct:
FIGURE 6.-Stability relations of some phases in
the system K20-AbOa-Si02-H20 at 25°0 and
1 atmosphere as functions of log [K+t]f[H+l)
and-log [H 4 Si0 4 ] and log [Na+l)f[H+ 1 ] =2.0.
The assumed pore-water composition for the
Wilcox Formation has the following cation/
hydrogen ion ratios:
[K+i]
[Na+t]
log [H+ 1 ]=4.73, log [H+t] =2.06,
and -log H 4 Si0 4 =4.36, derived from data of
Livingstone (1963, p. 41) and with an assumed
pH=7.5.
to produce cation alumino-silicates. Stated conversely
in the terms of IGttrick (1968), very low cation concentration
at relatively moderate silica values favors
crystallization of kaolinite. In order to meet these conditions
large quantities of cation-free water are needed
to either flush away or greatly dilute the solution products
derived from the hydrolysis of t~nstable 1ninerals.
Such a flushing mechanism has been proposed by Hay
(1963) to account for the diagenetic mineral assemblages
in the middle-Tertiary volcanogenic continental
sediments in Oregon.
The data presented indicate that the kaolinite occurring
interstitially in the "sawdust sand" crystallized in