Formula: Mg4(Mg3Al9O4)[Si3Al9O36] inosilicate (chain silicate), non-pyroxene
Specific gravity: 3.4 to 3.5
Hardness: 7½
Streak: Colourless
Colour: Light blue, blue-gray, green, greenish gray, rarely yellow-brown or pink


Plutonic igneous environments
Metamorphic environments

Sapphirine is found in high temperature metamorphic rocks or xenoliths with abundant aluminium and magnesium and low silicon. It may occur as a primary magmatic mineral
It is a mineral of the granulite facies.


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.

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

spinel-hercynite, sillimanite and quartz to sapphirine
7(Mg,Fe2+)Al2O4 + 2Al2SiO5 + SiO2 → 4(Mg,Fe)1.75Al4.5Si0.75O10

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