Chemistry Reference
In-Depth Information
Introduction of aluminum in the network occurs by the substitutions:
Mg Si
Al
VI
Al
IV
Mg Si
Fe
3+
Al
IV
The corresponding end members are respectively CaAl
2
SiO
6
(Tscher-
mak molecule) and Ca Fe
3+
AlSiO
6
(esseneite).
Al
2
O
3
content of common augites of igneous rocks ranges from 1.5-4
wt%. High temperatures and high pressures favor the introduction of alu-
minum in the lattice.
Introduction of titanium in the lattice of pyroxenes occurs by
substitution:
Mg Si
2
Ti Al
IV
2
(corresponding end member CaTiAl
IV
O
6
).
In the above substitutions aluminum enters mostly in tetrahedral site.
Augites containing 1-2 wt% titanium are called titaniferous augites;
those with a content of 3-5 wt% (or more) are called titanaugites.
Common augite contain 0.3-0.7 wt% of sodium in the Xsite. This con-
tent may reach up to 2-3% in the sodic augite.
Aluminous diopsides
(also called
fassaites
,) calcic aluminous non sodic
pyroxenes, are derived from diopside by the above substitutions. These min-
erals are distinguished from aluminous augites by their high calcium content
(Ca0 around 25 wt%) so that in the structural formula Ca
1 (in augite
CaO content is generally below 21 wt% and Ca in the structural formula is
less than 0.9).
The general formula of aluminous diopside is thus:
=
Ca (Mg, Fe
2+
)
1-x
(Al
VI
, Fe
3+
)
x
Si
2-x
, Al
IV
x
O
6
Some aluminous diopsides contain several percent of titanium.
Sodic pyroxenes
These form two groups that are distinguished by their chemical composition
and occurrences:
1
sodic aluminous pyroxene whose formula is derived from that of diop-
side by substitution:
Ca Mg
Na Al
VI
Aluminum is then in the octahedral site.
