Geoscience Reference
In-Depth Information
negative solubility effect on gold and silver crystallization, and CO
2
acts as a nonpo-
lar component of crustal fluids in the crystallization of many ore deposits
[43]
.
Thus, the recent hydrothermal solution speciation (solvation to ion pairing and com-
plexing) study has greatly contributed to the knowledge and better understanding of
various geological problems.
1.6 The Behavior of Volatiles and Other
Incompatible Components Under
Hydrothermal Conditions
The physical and thermodynamic properties of silicate melts depend upon melt
structure. The structure of a melt is determined both by its composition and the
ambient conditions and, with the notable exception of liquid immiscibility, may
vary continuously with changes in these parameters. The structures of crystalline
silicates vary only within restricted limits. As a result, variation in mineral
melt
equilibria caused by changes in either compositions or external parameters may
very well reflect the change in properties of the melt phase to a greater extent than
those of the crystalline silicates.
The behavior of phosphorus pentoxide is complex. Phosphorus pentoxide depoly-
merizes pure SiO
2
melts by entering the network as a fourfold coordinated cation but
polymerizes melts in which an additional metal cation, other than silicon, is present.
The effect of this polymerization is apparent in the widening of the granite
ferrobasalt
two-liquid solvus. In this complex system, P
2
O
5
acts to increase phase separation by
further enrichment of the high charge density cations Ti, Fe, Mg, and Ca, in the ferro-
basaltic liquid. Phosphorus pentoxide (P
2
O
5
) also produces an increase in the ferroba-
salt
granite REE liquid distribution coefficients. These distribution coefficients are
closeto4inP
2
O
5
-free melts but close to 15 in P
2
O
5
-bearing melts.
Several attempts to understand the internal evolution of highly fractionated peg-
matites focused on the roles of H
2
O and other components, especially rare alkalis,
B, P, F, are being carried out from time to time. Such studies yield a great amount
of data on the role of these volatiles under hydrothermal conditions and also distin-
guish rare earth pegmatites from other rocks
[44,45]
. These volatiles exert a signifi-
cant control on fluid properties, solidus and liquidus temperatures.
1.6.1 Water
Water is an important constituent of any hydrothermal system. It exhibits unique
properties, especially under supercritical conditions. These properties have been
exploited appropriately in the recent years to disintegrate toxic organics and recycle
or treat waste materials. In nature also, water plays an important role in the forma-
tion of various rocks and minerals and in the creation of life (origin of life). As a
component of granitic melts, H
2
O depresses solidus and liquidus temperatures
[46,47]
, lowers melt viscosities
[48
51]
, and promotes coarse grain size
[52]
.
