Geoscience Reference
In-Depth Information
in igneous rocks are alkali feldspars, amphibole, and micas. In sedimentary rocks, most of
the K inventory is sequestered in clay minerals, notably in smectites, illite, and in detrital
feldspars.
The K + ion is very large. Although it may enter octahedral sites, it often fits 12-coordin-
ated sites as in feldspars and micas. During melting in the mantle and basalt differentiation,
K is strongly incompatible and follows other incompatible elements such as Th and U
to the extent that the K/U ratio remains essentially constant in the mantle and the crust.
Mid-ocean ridge basalts are far less concentrated (
0.1 wt %) in K than ocean island
basalts (1-2 wt %). Granitic melts typically contain 2-3 wt % K 2 O. Felsic igneous melts
are normally saturated in feldspar and biotite; K is therefore not incompatible in the genesis
of granitic rocks.
At low temperature and therefore during weathering, feldspars react with water to pro-
duce clay minerals. The K-rich clay mineral illite is left as a residue and its ubiquitous
presence keeps the concentration of K in low-temperature hydrous fluids (rivers, seawater)
at very low levels. Potassium is therefore transported to the sea with the suspended load. In
high-temperature hydrothermal fluids, K reacts with country rocks to form the K-feldspar
crystals commonly observed in the metamorphic aureoles of granitic intrusions. Upon
cooling, K is leached preferentially to Na, while the converse becomes true when the
temperature of interaction between fluids and their ambient rocks falls below about 300 C.
Solubility of KCl (sylvite, a major fertilizer) in water increases greatly with temperature.
Through evaporation of brines in cool and cold (
50 C) landlocked or lagoonar environ-
ments, sylvite becomes saturated before NaCl (halite). In seawater, K is not biolimited and
its concentration is essentially constant in the water column.
<
13.4 Sodium
Most common form: Na +
Ionic radius: 1.02 Å (octahedral)
Stable isotope: 23 (100%)
Atomic weight: 22.990
Condensation temperature: 970 K
No significant complex in waters
Residence time in seawater: 83
10 6 years
×
Sodium is a volatile and lithophile alkaline element. As with K, the Na terrestrial abun-
dance is therefore not well known. Breaking down the inventory of Na among mantle
minerals is very difficult. Most major mineral phases do not accept this element but, under
upper mantle conditions, Na solubility in clinopyroxene increases substantially with pres-
sure. In the crust, Na is essentially hosted in the albite component of plagioclase feldspar.
In contrast with potassium, there is no major Na-rich clay mineral of major geological
importance and most sedimentary Na resides in detrital feldspar. Evaporitic rock salt NaCl
represents another significant surface repository of sodium.
 
 
Search WWH ::




Custom Search