anthophyllite

(mineralogy) A clove-brown orthorhombic mineral of the amphibole group, a variety of asbestos occurring as lamellae, radiations, fibers, or massive in metamorphic rocks. Also known as bidalotite.

A magnesium-rich orthorhombic amphibole with perfect {210} cleavage and a color which varies from white to various shades of green and brown. It is a comparatively rare metamorphic mineral which occurs as slender prismatic needles, in fibrous masses, and sometimes in asbestiform masses. Anthophyllite may occur together with calcite, magnesite, dolomite, quartz, tremolite, talc, or enstatite in metacarbonate rocks; with plagioclase, quartz, orthopyroxene, garnet, staurolite, chlorite, or spinel in cordierite-anthophyllite rocks; and with quartz and hematite in metamorphosed iron formations, and with talc, olivine, chlorite, or spinel in metamorphosed ultrabasic rocks. Anthophyllite is distinguished from other amphiboles by optical examination or by x-ray diffraction, and from other minerals by its two cleavage directions at approximately 126° and 54°.

Anthophyllite has the general formula

with x<1.0. For aluminum-poor varieties, up to about 40% of the Mg may be replaced by Fe²⁺; higher iron contents result in the formation of the monoclinic amphibole cummingtonite. Increasing the aluminum content in anthophyllite beyond x = 1.0 results in the formation of the orthohombic amphibole gedrite; aluminous anthrophyllite can accommodate more Fe²⁺ than Al-poor varieties. See also Amphibole; Cummingtonite.

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Environment

An uncommon mineral of metamorphic rocks, one usually associated with ore minerals emplaced during metamorphism.

Individual crystals not likely, usually in compactly intergrown masses with a fibrous structure.

Generally brown to black, sometimes with grayish or greenish tints. Luster glassy; hardness 5Ɖ-6 (but usually splinters and appears softer); specific gravity 2.9-3.4; cleavage prismatic. Silvers translucent; polished surfaces sometimes exhibit blue, golden or yellow schiller reflections.

Alkaline magnesium iron silicate (27.8% MgO, 16.6% FeO, 55.6% SiO ₂ , plus water).

Fuses with some difficulty to a black magnetic glass ball. Insoluble in acid.

Though not easily confirmed, it is a good guess for a compact, felted brown silicate mineral with the proper cleavage, hardness, and field associations.

Anthophyllite is thought to be a secondary mineral forming from olivine under conditions of moisture and pressure. Since more moisture and, perhaps, less pressure would have produced serpentine, the two should not be associated.

Well developed in the U.S. in the metamorphic rocks of Franklin Co., North Carolina, and in feathery masses in Delaware Co., Pennsylvania. Fairly common in schists. A closely related species, cummingtonite, forms brown fibrous "starsprays" at Cummington, Massachusetts. Beautiful blue schiller reflections reminiscent of labradorite characterize an unusual anthophyllite amphibolite from near Butte, Montana. Anthophyllite with a similarly coarsely felted mass of yellow and brown blades was brought to the U.S. from Greenland under the name nummite, as a possible decorative stone; it has also been found in Wyoming, Norway, and Finland.

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.

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.

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.

References

• Klein, Cornelius., 2002, The Manual of Mineral Science, 22nd ed., John Wiley & Sons, Inc. ISBN 0-471-25177-1
• Mindat
• Webmineral
• Mineral galleries

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