Anthophyllite is formed by the breakdown of
talc in ultramafic rocks in the presence of water and
carbon dioxide as a prograde
metamorphic reaction. The partial pressure of carbon dioxide (XCO2) in aqueous solution favors production of anthophyllite. Higher partial pressures of CO2 reduces the temperature of the
anthophyllite-in isograd. Ultramafic rocks in purely hydrous, CO2-free environments will tend to form
serpentinite-
antigorite-
brucite-
tremolite assemblages (dependent on MgO content) or at amphibolite to granulite metamorphic grade, metamorphic pyroxene or olivine. Thus, metamorphic assemblages of ultramafic rocks containing anthophyllite are indicative of at least
greenschist facies metamorphism in the presence of carbon dioxide bearing metamorphic fluids. The typical metamorphic assemblage reactions for low-magnesian (25% MgO) ultramafic rocks are; • Olivine + Tremolite + Talc → Olivine + Tremolite + Anthophyllite (low MgO, >550 °C, XCO2 500 °C, XCO2 >0.6) • Talc + Magnesite + Tremolite → Anthophyllite + Tremolite + Magnesite (Low MgO, >500 °C, XCO2 >0.6) Retrogressive anthophyllite is relatively rare in ultramafic rocks and is usually poorly developed due to the lower energy state available for metamorphic reactions to progress and also the general dehydration of rock masses during metamorphism. Similarly, the need for substantial components of carbon dioxide in metamorphic fluid restricts the appearance of anthophyllite as a retrograde mineral. The usual metamorphic assemblage of retrograde-altered ultramafic rocks is thus usually a
serpentinite or
talc-
magnesite assemblage. Retrograde anthophyllite is present most usually in
shear zones where
fracturing and shearing of the rocks provides a conduit for carbonated fluids during retrogression. == Fibrous anthophyllite ==