Chemistry Reference
In-Depth Information
3.2.3 Amphiboles
3.2.3.1 Structure and chemical composition
Amphiboles are inosilicates where tetrahedra (SiO
4
)
4−
are organized
in double chains (Si
4
O
11
)
6−
. The unit cell includes the equivalent of two
double chains and comprises 8 tetrahedral sites Z (Figure 3.17). These
chains are linked by 5 octahedral Y sites (6-fold coordination) and two
X sites which have 6- or 8-fold coordination. There is also a large A site
of 12-fold coordination, which may be full or empty. OH ions are at the
center of the hexagons that form the chain in the plane of the apexes of
the tetrahedra.
Thus general formula of amphiboles is:
A
0-1
X
2
Y
5
Z
8
O
22
(OH)
2
Z
=
site tetrahedral site: Si, Al
Y
=
site octahedral site: Mg, Fe
2+
, Fe
3+
, Al, Ti, Mn, (Cr, Li)
X
=
site of 8- or 6-fold coordination: Ca, Na, K, Fe, Mg (Mn, Li)
A
=
site of 12-fold coordination 12: Na, K
means misleading: under the very similar exter-
nal aspects, amphiboles shows a wide range of chemical composition. The
precise determination of amphibole requires microprobe. The composition
of amphiboles (other than ferromagnesian amphiboles) is deduced from the
composition of tremolite
Ca
2
Mg
5
Si
8
O
22
(OH)
2
(the A site is empty)by the
following substitutions:
The word
αμφιβολοσ
R
2+
Si
R
3+
Al
VI
Ca
x
R
2+
Na
x
R
3+
Ca
x
Na
x
Na
A
Si
Al
IV
Na
A
Fe
Mg
Mn Al
VI
R
3+
Na
K
OH
F
Cl
2 Al
VI
(Fe,Mg) Ti
2 (Fe, Mg)
Ti Si AlVI Al
IV
Ti
2 R
2+
Al
VI
Li Al
VI
R
2+
Li
where R
2+
is a divalent cation, R
3+
a trivalent cation, a vacant site
These (independent) substitutions form a 15 dimension space. There are
certainly miscibility gaps in this space but not all of them are investigated
here.
3.2.3.2 Classification of the amphiboles (Leake, 1978)
Amphiboles are classified according to the filling of the X site: Fe, Mg, Ca
or Na (Table 3.1 lists the end members).
