Images
Hornblende is a series between
ferro-hornblende: ☐Ca2(Fe2+4Al)
(Si7Al)O22(OH)2 and
magnesio-hornblende: ☐Ca2(Mg4Al)
(Si7Al)O22(OH)2
Both are inosilicates (chain silicates) amphiboles
Specific gravity: 3.00 to 3.47
Hardness: 5 to 6
Streak: White
Colour: Green, black, brown
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Common impurities: Ti,Mn,Na,K
Environments:
Plutonic igneous environments
Volcanic igneous environments
Pegmatites
Metamorphic environments
Hornblende is an important and widely
distributed primary, rock-forming mineral, occurring both in
igneous and in metamorphic rocks;
it is particularly characteristic of
amphibolite
in which hornblende and
associated plagioclase feldspar are the major constituents.
In metamorphic environments hornblende is found both in
contact and in
regional metamorphic
environments.
Hornblende is an essential constituent of
ultramafic rocks
It is a common constituent of
granite,
syenite,
diorite and
gabbro.
It also may be found in
trachyte,
andesite,
basalt and
gneiss.
Hornblende is characteristic of the
hornblende-hornfels and
amphibolite facies, and it is
also a mineral of the
greenschist,
granulite,
blueschist and
albite-epidote-hornfels
facies.
Localities
The Two Mile and Three Mile deposits, Paddy's River, Paddys River District, Australian Capital Territory, Australia,
are skarn deposits at the contact between
granodiorite and volcanic rocks.
Hornblende is a major primary silicate in silicate
skarn at the Two Mile deposit, associated with
grossular, diopside,
hastingsite and actinolite,
and less commonly anthophyllite. It is also common in
magnetite-rich skarn, where
it is associated with actinolite,
chlorite and scheelite
(AJM 22.1.35).
Alteration
Hornblende characteristically alters from pyroxene both during the late
stages of crystallisation of igneous rocks and during metamorphism.
anorthite, enstatite,
spinel, K2O and H2O to Al-rich hornblende, Mg-rich
sapphirine and phlogopite
2.5Ca(Al2Si2O8) + 10MgSiO3 + 6MgAl2O4 +
K2O + 3H2O →
Ca2.5Mg4Al(Al2Si6)O22(OH)2 +
3Mg2Al4SiO10 + 2KMg3(AlSi3O10)(OH)2
This reaction occurs in the granulite to
amphibolite facies
(DHZ 2A p631).
epidote and chlorite
to hornblende and anorthite
6Ca2Al3(SiO4)3(OH) +
Mg5Al2Si3O18(OH)8 →
Ca2Mg5Si8O22(OH)2 +
10CaAl2Si2O8
This reaction represents changes when the metamorphic grade increases from the
greenschist facies to
the amphibolite facies
(KB p429 diagram p430).
hornblende, calcite and quartz to
Fe-rich diopside, anorthite,
CO2 and H2O
Ca2(Mg,Fe2+)3(Al4Si6)O22(OH)2 +
3CaCO3 + 4SiO2 = 3Ca(Mg,Fe2+)Si2O6 +
2Ca(Al2Si2O8) + 3CO2 + H2O
Fe-rich diopside occurs commonly in
regionally metamorphosed calcium-rich sediments and basic igneous rocks
belonging to the higher grades of the amphibolite facies. The
above reaction is typical
(DHZ 2A p272).
hornblende, grossular and quartz to
Fe-rich diopside, anorthite,
almandine and H2O
2Ca2(Mg,Fe2+)3(Al4Si6)O22(OH)2 +
Ca3Al2Si3O12 + 2SiO2 =
3Ca(Mg,Fe2+)Si2O6 +
4CaAl2Si2O8 + (Mg,Fe2+)Al2Si3O12
+ 2H2O
Fe-rich diopside occurs commonly in
regionally metamorphosed calcium-rich sediments and basic igneous rocks
belonging to the higher grades of the amphibolite facies. The
above reaction is typical
(DHZ 2A p272).
Al-rich hornblende, spinel, quartz,
K2O and H2O to anorthite,
Mg-rich sapphirine and phlogopite
Ca2.5Mg4Al(Al2Si6)O22(OH)2 +
4 MgAl2O4 + 6SiO2 + K2O +
H2O → 2.5Ca(Al2Si2O8) +
Mg2Al4SiO10 + 2KMg3(AlSi3O10)(OH)2
(DHZ 2A 631)
spinel and
tremolite to forsterite and
magnesio-hornblende
MgAl2O4 + Ca2Mg5Si8O22(OH)2 ⇌
Mg2SiO4 +
Ca2(Mg4Al)(Si7Al)O22(OH)2
This reaction occurs in some strongly metamorphosed serpentinite
(DHZ 1A p261).
Back to Minerals