Chemistry Reference
In-Depth Information
Pyroxenes of the
alkaline series
that occur in basic to intermediate rocks,
augites close to the diopside-hedenbergite series or salites. These augite are
richer in aluminium and titanium than in the subalkaline series, all the more
aluminous and titaniferous as the series is more alkaline. In differentiated
rocks, the pyroxene is aegirine and/or aegirine augite. In plutonic rocks
(syenite, nepheline syenite, alkali granite), aegirine is commonly associated
with riebeckite.
Hedenbergite appears with fayalite in rare alkali granites and quartz
syenite, oversaturated in silica and highly under-saturated in alumina.
Basic and intermediate rocks of the
subalkaline series
contain two
pyroxenes: an augite and calcium-poor pyroxene.
In the differentiated
lavas
(and hypabyssal rocks) of the
tholeiitic series
,
this pyroxene is a pigeonite, forming both microlites and phenocrysts (in
smaller quantities and more rare). In little or no differentiated lavas of these
aegyrine
peralkaline
phonolites
80
60
40
phonolites
20
0
0
20
40
60
80
diopside
hedenbergite
50
40
30
Figure 3.25
Pyroxenes of the Velay volcanic series, French Massif Central (unpublished
data by E. Berger).
