Chemistry Reference
In-Depth Information
Hornblende is a very common constituent of basic (or even ultrabasic),
intermediate and more rarely, acidic
igneous rocks
. It is more common than
in the corresponding volcanic rocks because in igneous rocks the evolution
of the magma occurs under water pressures higher than in volcanic rocks.
Amphibole reflects the composition of magmatic evolution: magnesian
hornblende in gabbros, ferrohastingsite in granites and nepheline syenite.
Amphiboles of the arfvedsonite-riebeckite series appear in the acidic
alkaline rocks, under-saturated or not in silica. Barkevicite is a rare con-
stituent of alkaline basic to intermediate rocks under-saturated in silica like
theralite (nepheline gabbros) essexite (nepheline monzogabbros), nepheline
syenite or highly under-saturated rocks like jacupirangites (rocks made of
titaniferous augite, magnetite and minor nepheline).
Amphibole as a secondary mineral
Ouralite is a common alteration by hydratation of pyroxene into amphibole
(actinolite or hornblende).
Kelyphite is an alteration of a pyrope-rich garnet into amphibole
plagi-
oclase, as a retromophic reaction from eclogite facies to amphibolite facies.
+
3.2.4 Pyroxenes
Pyroxenes are most important ferromagnesian minerals: they are major con-
stituents of many igneous rocks and also occur in varied metamorphic rocks.
3.2.4.1 Structure and chemical composition
Pyroxenes are single chain inosilicates [(SiO
3
)
2-
]
n
(Figure 3.21). The chains
are linked by chains of octahedra (6-fold coordinated Y sites) and 8-fold
coordinated X sites. The general formula of pyroxene is thus:
X
1-p
Y
1+p
Z
2
O
6
Z
=
tetrahedral site: Si, Al
Y
=
octahedral site: Mg, Fe
2+
, Mn, Fe
3+
, Al, Ti, (Ni, Cr)
X
=
8-fold coordinated site: Ca, Na, (K), Li, Mg, Fe
2+
, Mn
Pyroxenes of the enstatite-ferrosilite series are orthorhombic (orthopy-
roxenes or Opx); all others are monoclinic (clinopyroxenes or Cpx).
Pyroxenes are classified according to the filling of the X site X:
ferromagnesian pyroxenes
calcic pyroxenes
