Anthophyllite

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Anthophyllite

General
Category	Mineral
Chemical formula	Hydrous Magnesium Iron silicate (Mg,Fe)₇Si₈O₂₂(OH)₂
Identification
Color	gray to green, brown, and beige
Crystal habit	Rarely as distinct crystals. Commonly lamellar or fibrous.
Crystal system	orthorhombic; 2/m2/m2/m
Cleavage	{210} perfect 55°
Mohs Scale hardness	5.5 - 6
Luster	Vitreous
Refractive index	Optically (-) α=1.60 - 1.69, β=1.61 - 1.71, γ=1.62 - 1.72; 2V = 70° - 100° Indices increase with Fe content
Specific gravity	2.85 - 3.2
Diagnostic Features	Characterized by clove brown color, but unless in crystals, difficult to distinguish from other amphiboles without optical and/or X-ray tests

Anthophyllite is an amphibole mineral: (Mg, Fe)₇Si₈O₂₂(OH)₂, magnesium iron inosilicate hydroxide. Anthophyllite is polymorphic with cummingtonite. Some forms of anthophyllite are lamellar or fibrous and are used as asbestos. The name is derived from the Latin word anthophyllum, meaning clove, an allusion to the most common color of the mineral.

[edit] Occurrence

Anthophylite is the product of metamorphism of magnesium-rich rocks especially ultrabasic igneous rocks and impure dolomitic shales. It also forms as a retrograde product rimming relict orthopyroxenes and olivine, and as an accessory mineral in cordierite-bearing gneisses and schists. Anthophyllite also occurs as a retrograde metamorphic mineral derived from ultramafic rocks along with serpentine. Geographically, it occurs in Pennsylvania, southwestern New Hampshire, central Massachusetts, Franklin, North Carolina, and in the Gravelly Range and Tobacco Root Mountains of southwest Montana.

[edit] Occurrence in ultramafic rocks

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 envronments will tend to form serpentine-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) and high-magnesian (>25% MgO) ultramafic rocks are;

Olivine + Tremolite + Talc → Olivine + Tremolite + Anthophyllite (low MgO, >550°C, XCO2 <0.6)
Talc + Tremolite + Magnesite → Tremolite + Anthophyllite + Magnesite (High MgO, >500°C, XCO2 >0.6)
Talc + Magnesite + Tremolite → Anthophyllite + Tremolite + Magnesite (Low MgO, >500°C, XCO2 >0.6)

Anthophyllite in serpentinised komatiite, Maggie Hays Ni Mine, Western Australia

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 anthophllite 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.