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evidence of the interaction of phosphorus with other Group I elements is lacking,
the common pegmatite assemblage LiAlPO
4
(OH,F)
NaAlSi
3
O
8
(rather than
1
NaAlPO
4
(OH,F)
H
2
O) suggests that P is more compatible with the
smaller, more acidic cations of Group I
[53
LiAlSi
2
O
6
1
1
57]
.
1.6.5 Behavior of Alkalis
The zonation of rare element pegmatites is manifested largely by heterogeneous
distributions of alkali aluminosilicates. Theoretical and experimental investigations
of the interactions of Group I elements with aluminosilicate melts provide a basis
for understanding the zonation of alkali aluminosilicates in pegmatites.
Both De Jong and Brown
[59]
and Navrotsky et al.
[66]
proposed that the smal-
ler alkalis Li and Na should exhibit a greater tendency to destabilize silicate melts
than K, Rb, and Cs.
A number of salient pegmatite characteristics can be explained by the effects of
high concentrations of boron, fluorine, and phosphorus on phase equilibria in
hydrous aluminosilicate melts.
1.6.6 Crystallization Temperatures
Comparatively, high concentrations of B, P, F, and Group I elements serve to
depress pegmatite magma liquid to approximately 650
C (within the stability fields
of petalite or spodumene), and solids to
500
C. The low liquidus temperatures per-
mit rare-element pegmatite magmas to migrate to metamorphic conditions of the
andalusite
,
staurolite facies
[75,76]
. The physical migration of magma
may be facilitated also by the lower melt viscosities of the H
2
O-, B-, and F-rich peg-
matite system. Because of the low temperature interval of crystallization, however,
rare element pegmatite magmas may experience rapid increases in melt viscosity
with only slight cooling, as glass transition temperatures are approached (e.g., as in
the macusanite analogue). As a result of increased kinetic barriers to crystallization,
internal disequilibrium may prevail (an important and poorly defined parameter in
pegmatite crystallization is the rate of cooling), but evidence from wall-rock studies
indicates that rare element pegmatite magmas are hosted by rocks at a temperature
,
cordierite
500
C
[77]
. Common textural features of rare element pegmatites, such as graphic
intergrowths and radial or banded fabrics, can be interpreted as disequilibrium phe-
nomena in supercooled liquids or glasses
[78]
. Experiments with dry macusanite,
however, present alternative possibilities. In these experiments, pegmatitic fabrics,
mineral assemblages, and zonation have been generated at or near equilibrium con-
ditions with high concentrations of B, P, and F but low water content.
Numerous experimental investigations of aqueous systems at high temperature and
high pressure have been undertaken using conductivity, potentiometry, spectropho-
tometry, solubility, PVT (Price and Volume Trend technical analysis indicator), and
calorimetry, neutron diffraction, EXAFS (Extended X-ray Absorption Fine Structure),
and other related methods. These studies yield a vast amount of information on
cation-oxygen pairing and their increased or decreased distances with varying
