Geoscience Reference
In-Depth Information
some relatively rare minerals (samarskite, tantalite, gadolinite, germantite, and so
on). Individual germanium minerals or germanates are rather rare or very few,
especially in some sulfides and sulfates
[202
204]
.
Germanates could be obtained by both flux and hydrothermal methods.
However, just like silicates, even germanates insists hydrothermal conditions for
the formation of a large variety of anionic groups. The flux-grown germanates are
very few and are represented by [GeO
4
] and [Ge
2
O
7
] anionic groups, whereas the
hydrothermally grown germanates do contain a large variety of anionic groups like
[GeO
4
], [Ge
2
O
7
], [Ge
4
O
13
], [Ge
3
O
12
], [Ge
10
O
25
], [Ge
3
O
12
], and [Ge
5
O
16
]. These
germanates with different cations can be obtained at rather lower parameters
(T
500
C, P
2000 atm). Under such conditions, germanates can occupy
both tetrahedral and octahedral positions in the structure. The fixation of Si in octa-
hedral coordination demands much higher pressure (
1500
5
5
10 kbar). This fact reveals an
important aspect of the study of germanates crystallization, because the germanates
obtained under lower PT conditions may be the analogs of silicates formed in the
earth crust at greater depths.
The growth of germanates began as an attempt to synthesize silicates analogs
during the middle of this century. Onishi
[202]
and Wardani
[203]
studied the geo-
chemistry of germanium in detail. Just like in silicates, even in germanates, we can
have two types: (1) Germanates containing rare earth elements and (2) germanates
without rare earth elements. The phase formation and crystal chemical elucidation
of the germanates have been discussed in general here.
The natural compounds of germanium and rare earth elements are practically
absent and only their synthesis allows one to study their structures, crystal chemis-
try, and physical and chemical characteristics in detail. Under such conditions, ger-
manium can occupy both tetrahedral and octahedral positions in the structure. The
fixation of Si in octahedra (coordination number 6) demands much higher pressure
(
.
10 kbar). This fact reveals an important aspect of the study of rare earth germa-
nate crystallization, because the germanates obtained under lower temperatures
and pressures may be the analogs of silicate phases formed in the earth crust at
large depth (high-pressures phases). The classification of germanates containing
rare earth elements differ slightly from that of the silicates containing rare earth
elements.
For germanium, like silicon, a tendency to form chains has been noticed. For sil-
icon, both hydrides and halogenides have been obtained, but germanium forms
only halogenides with the bond Ge
.
Ge.
GeO
2
is largely analogous to SiO
2
, and it may form glass while cooling from
the melt. One of the polymorphic modifications of GeO
2
at 1033
C has a quartz
structure. This GeO
2
modification becomes metastable at less than 1053
C. The
stable modification of GeO
2
has the characteristic rutile, TiO
2
-type structure.
Germanium may exist in solutions as a negatively charged ion and forming
sulfides and tiosalts differs from silicon, but at the same time germanates and
fluorogermanates are often isostructural with the corresponding silicates. Among
rare earth germanates, we have alkali-free germanates and alkali rare earth
germanates.
a
