Chemistry Reference
In-Depth Information
3.5.5.1 Pyroxenes
Pyroxenes have the general formula X(Y)(SiO
3
)
2
. They crystallize in a monoclinic
(clinopyroxenes) or orthorhombic (orthopyroxenes) structure. The Y cations are
located in the M1 sites which are moderately distorted octahedra whereas X
represents cations in highly distorted sites with six-, seven- or eight-fold coordi-
nation according to the kind of mineral. Generally, the structure is orthorhombic
when X is Mg or Fe
2+
and is monoclinic when X is Ca or Na.
Orthopyroxenes have compositions close to the enstatite-ferrosilite MgSi
2
O
6
-
FeSi
2
O
6
tie line. The spectra consist of two ferrous doublets, which can be clearly
resolved (Fig.
3.25
). The one with the small quadrupole splitting (see Table
3.16
)
can be assigned to the distorted M2 sites [
208
]. In the (Mg,Fe)Si
2
O
6
series this
doublet remains rather intense evoking the preference of iron for the M2 sites. [
209
].
A larger variety of crystal chemistry is possible in the clinopyroxenes. Besides
the compositions on and close to the diopside-hedenbergite CaMgSi
2
O
6
-CaFe-
Si
2
O
6
tie line (augites) there exist sodium pyroxenes such as jadeite NaAlSi
2
O
6
,
aegirine NaFe
3+
Si
2
O
6
(also called acmite), kosmochlor NaCrSi
2
O
6
and jervisite
NaScSi
2
O
6
, lithium pyroxenes as spodumene LiAlSi
2
O
6
, monoclinic forms of
enstatite and ferrosilite (clinoenstatite and clinoferrosilite) and of course all
compositions between them of which some received mineral names like ompha-
cites (Ca,Na)(Fe
2+
,Fe
3+
,Mg,Al)Si
2
O
6
and pigeonites (Ca,Fe
2+
,Mg)(Fe,Mg)Si
2
O
6
.
Ca-free clinopyroxenes along the MgSiO
3
-FeSiO
3
tie line only exist under high
pressure in the earth crust [
210
] (Fig.
3.26
).
The Mössbauer spectra of the diopside-hedenbergite series are straightforward
and predominantly consist of a ferrous doublet (Fig.
3.27
a) for which the quad-
rupole splitting D varies between 2.30 and 1.85 mm/s (Table
3.16
), decreasing on
going from hedenbergite to diopside. [
211
-
213
]. Deviations from stoichiometry
introduce a small amount of an Fe
2+
doublet from M2 sites and a weak Fe
3+
doublet. Because the quadrupole splitting of the M1 doublet is strongly temper-
ature dependent whereas the one of the M2 doublet remains invariant at about
2.0 mm/s, measurements at various temperatures are usually necessary to resolve
both doublets. Trivalent iron can also be introduced to some extent in diopside,
thus substituting for Mg and Si.
Table 3.16
Table Representative hyperfine parameters at RT for some pyroxenes
Mineral
Formula
Fe site
d
Fe
(mm/s)
D (mm/s)
Fe
2+
Ferrosilite
Fe
2
Si
2
O
6
M1
1.17
2.48
Fe
2+
M2
1.13
1.93
(Mg,Fe
2+
,Mn)
2
Si
2
O
6
Fe
2+
Enstatite- ferrosilite
M1
1.15-1.18
2.35-2.69
Fe
2+
M2
1.12-1.16
1.91-2.13
CaFe
2+
Si
2
O
6
Fe
2+
Hedenbergite
M1
1.19
2.20
(Fe
3+
M1)
0.34
0.68
Ca(Mg,Fe
2+
)Si
2
O
6
Fe
2+
Diopside-hedenbergite
M1
1.19
1.85-2.30
(Na,Li)Fe
3+
Si
2
O
6
Fe
3+
Aegerine
M1
0.39
0.30
