Chemistry Reference
In-Depth Information
colusite has the ideal formula Cu
24
[V
2
As
6
]S
32
, with two vacancies at the Cu sites.
In both cases, all the copper present is in the
1 oxidation state.
The ideal formula for germanite is Cu
26
[Fe
4
Ge
4
]S
32
, the Fe and Ge being
disordered on the 8e site of the space group in a 1:1 ratio, but again there is
considerable variation in the reported formulae: Tettenhorst and Corbato [
35
]
as [Cu
1+
]
20
[Cu
2+
]
6
[Fe
3+
]
2
Ge
6
S
32
. Both reported structures are almost identical with
the arrangement of the V(1), V(2), Li(1), Li(2), Li(4), N(1) and N(2) atoms
in Li
7
VN
4
, as the diagrams above clearly show, so we would claim these two
structures as genuine substructures of Li
7
VN
4
: only Li(3) and Li(5) of the full
Li
7
VN
4
structure are missing, so we can describe the Li
7
VN
4
structure as both
“Li-stuffed colusite” and “Li-stuffed germanite”.
In attempting a chemical rationalization of these close relationships, we must
first relate the colusite composition, V
2
Cu
26
(As
4
Sn
2
)S
32
, to that of the parent
Li
56
V
8
N
32
. We would have to propose a hypothetical parental analogue Li
30
Cu
(I)
26
[V
2
As
4
Sb
2
]P
32
, substituting 26 Cu(I) atoms for 26 of the 56 Li atoms, and then
apply the
EZKC
approach as follows: the 30 Li atoms donate one electron each to
convert 30 P atoms to 30 [
þ
-S] atoms, and the 2 Sb atoms donate 1 electron each to
the remaining 2 P atoms, thereby becoming 2 [
C
C
-Sn] and 2 [
C
-S], respectively. The
nett overall result is the pseudo-compound
-He.
In similar fashion, we might try to relate the germanite composition, Cu
26
[Fe
4
Ge
4
]
S
32
, to the hypothetical parent Li
30
Cu
26
[V
4
As
4
]P
32
, except that only 6 of these 26
Cu atoms are Cu(I), able to substitute directly for Li(I). If, however, we allow the full
substitution, and consider the 20 Cu(II) atoms as 20 Cu(I) atoms plus 20 “free”
electrons, these being free to donate to the rest of the structure as required, we can
progress. We require 32 electrons to convert the 32 P atoms to 32
C
-{Cu
26
[V
2
As
4
Sn
2
]S
32
}
þ
30
C
C
-S. The V atoms
require three electrons each to convert to
C
-Fe, and the As atoms donate one electron
each to become
-Ge. The nett electron requirement is achieved from the Li atoms
and the free electrons. Things look better if we take the Spiridinov ideal formula,
[Cu(I)]
20
[Cu(II)]
6
[Fe(III)]
2
Ge
6
S
32
. In general terms, however, the variable composi-
tions of complex minerals would seem to make any chemical rationalizations of the
structures extremely complicated, and sometimes even impossible.
C
3.2.5 ZnS sphalerite
And while still focussing on these sphalerite-related structures, we now consider the
sphalerite structure itself, taking
Zn
S as the archetype. The struc
tu
re is shown in
Figure 11a: the space group is
. Figure
11b
shows the corresponding tetrahedra of the Li
7
VN
4
structure. From the Li
7
VN
4
structure, half of the V(1)N
4
tetrahedra (one such) plus half of the Li(3)N
4
(three
such) correspond with the Zn atoms, and half of the Li(4)N
4
tetrahedra (four such)
correspond with the S atoms, so the sphalerite structure is clearly a substructure of
Li
7
VN
4
. Once again we are equating single atoms of one phase with the larger
tetrahedra of the other to explain the different cell edges, but in this case we have
F
43
m
(216), a supergroup of
P
43
n
