Hornblende

inosilicate

primary mineral

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
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.

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).

Common impurities: Ti,Mn,Na,K

Back to Minerals