Anthophyllite

inosilicate

cummingtonite

Formula: ☐Mg2Mg5Si8O22(OH)2 inosilicate (chain silicate) amphibole
Anthophyllite is a dimorph of cummingtonite.
Specific gravity: 2.8 to 3.2
Hardness: 5½
Streak: White
Colour: Brownish, yellowish grey
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Metamorphic environments

Anthophyllite is a metamorphic product of magnesium-rich rocks such as ultramafic igneous rocks and impure dolomite shale. It is common in cordierite-bearing gneiss and schist. It may also form as a retrograde product rimming relict orthopyroxenes such as enstatite, and olivine.
Anthophyllite may be found in gneiss and schist.
It is a mineral of the hornblende-hornfels facies.

Alteration

anthophyllite to enstatite, quartz and H2O
2☐Mg2Mg5Si8O22(OH)2 ⇌ 7Mg2Si2O6 + 2SiO2 + 2H2O
At 10 kbar pressure the equilibrium temperature is about 810oC (granulite facies). The equilibrium moves to the right at higher temperatures and to the left at lower temperatures (SERC).

anthophyllite and forsterite to enstatite and H2O
2☐Mg2Mg5Si8O22(OH)2 + 2Mg2SiO4 ⇌ 9Mg2Si2O6 + 2H2O
At 2 kbar pressure the equilibrium temperature is about 690oC (pyroxene-hornfels facies). The equilibrium moves to the right at higher temperatures and to the left at lower temperatures (SERC).

forsterite and talc to anthophyllite and H2O
4Mg2SiO4 + 9Mg3Si4O10(OH)2 ⇌ 5Mg2Mg5Si8O22(OH)2 + 4H2O
At 2 kbar pressure the equilibrium temperature is about 650oC (pyroxene-hornfels facies), with equilibrium to the right at higher temperatures and to the left at lower temperatures (for the same pressure) (DHZ 1A p258, SERC).

talc to anthophyllite, quartz and H2O
7Mg3Si4O10(OH)2 ⇌ 3☐Mg2Mg5Si8O22(OH)2 + 4SiO2 + 4H2O
At 10 kbar pressure the equilibrium temperature is about 790oC (granulite facies). The equilibrium moves to the right at higher temperatures and to the left at lower temperatures (http://www.tulane.edu/~sanelson/eens212/metaminerals.htm, SERC).

talc and enstatite to anthophyllite
Mg3Si4O10(OH)2 + 2Mg2Si2O6 → ☐Mg2Mg5Si8O22(OH)2
At 10 kbar pressure the equilibrium temperature is 750oC granulite facies.

Common impurities: Ti,Al,Mn,Ca,Na

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