Anthophyllite is an
orthorhombic amphibole mineral: ☐Mg
2Mg
5Si
8O
22(OH)
2 (☐ is for a vacancy, a
point defect in the crystal structure),
magnesium iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
inosilicate hydroxide. Anthophyllite is
polymorphic with
cummingtonite
Cummingtonite ( ) is a metamorphic amphibole with the chemical composition , magnesium iron silicate hydroxide.
Monoclinic cummingtonite is compositionally similar and polymorphic with orthorhombic anthophyllite, which is a much more common fo ...
. Some forms of anthophyllite are lamellar or fibrous and are classed as
asbestos. The name is derived from the Latin word ''anthophyllum'', meaning ''clove'', an allusion to the most common color of the mineral. The Anthophyllite crystal is characterized by its perfect
cleavage along directions 126 degrees and 54 degrees.
Occurrence
Anthophyllite is the product of
metamorphism of
magnesium-rich rocks, especially ultrabasic
igneous rocks and impure
dolomitic
Dolomite () is an anhydrous carbonate mineral composed of calcium magnesium carbonate, ideally The term is also used for a sedimentary carbonate rock composed mostly of the mineral dolomite. An alternative name sometimes used for the 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
serpentinite.
Occurrence in ultramafic rocks
Anthophyllite is formed by the breakdown of
talc in ultramafic rocks in the presence of water and
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
as a prograde
metamorphic reaction. The partial pressure of carbon dioxide (XCO
2) in aqueous solution favors production of anthophyllite. Higher partial pressures of CO
2 reduces the temperature of the ''anthophyllite-in''
isograd.
Ultramafic rocks in purely hydrous, CO
2-free environments will tend to form
serpentinite-
antigorite
Antigorite is a lamellated, monoclinic mineral in the phylosilicate serpentine subgroup with the ideal chemical formula of (Mg,Fe2+)3Si2O5(OH)4. It is the high-pressure polymorph of serpentine and is commonly found in metamorphosed serpentini ...
-
brucite
Brucite is the mineral form of magnesium hydroxide, with the chemical formula Mg( OH)2. It is a common alteration product of periclase in marble; a low-temperature hydrothermal vein mineral in metamorphosed limestones and chlorite schists ...
-
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)
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
Fibrous anthophyllite is one of the six recognised types of
asbestos. It was mined in
Finland
Finland ( fi, Suomi ; sv, Finland ), officially the Republic of Finland (; ), is a Nordic country in Northern Europe. It shares land borders with Sweden to the northwest, Norway to the north, and Russia to the east, with the Gulf of Bot ...
and also in
Matsubase, Japan where a large-scale open-cast asbestos mine and mill was in operation between 1883 and 1970.
In Finland anthophyllite asbestos was mined in two mines, the larger one Paakkila in the
Tuusniemi commune started in 1918 and closed in 1975 due to the dust problems. The smaller mine, Maljasalmi in the commune of
Outokumpu, was mined from 1944 to 1952. The anthophyllite was used in asbestos cement and for insulation, roofing material etc.
Anthophyllite is also known as azbolen asbestos.
References
{{Commons category, Anthophyllite
Amphibole group
Magnesium minerals
Iron minerals
Asbestos
Orthorhombic minerals
Minerals in space group 62
Luminescent minerals