Anthophyllite
Encyclopedia
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.
of magnesium
-rich rocks, especially ultrabasic igneous rock
s and impure dolomitic
shale
s. It also forms as a retrograde product rimming relict orthopyroxenes and olivine
, and as an accessory mineral in cordierite
-bearing gneiss
es and schist
s. Anthophyllite also occurs as a retrograde metamorphic mineral derived from ultramafic rocks along with serpentinite
. 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.
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) and high-magnesian (>25% MgO) ultramafic rocks are;
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.
Amphibole
Amphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...
mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
: (Mg, Fe)7Si8O22(OH)2, magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
inosilicate
Silicate minerals
The silicate minerals make up the largest and most important class of rock-forming minerals, constituting approximately 90 percent of the crust of the Earth. They are classified based on the structure of their silicate group...
hydroxide
Hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...
. Anthophyllite is polymorphic
Polymorphism (materials science)
Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...
with cummingtonite
Cummingtonite
Cummingtonite is a metamorphic amphibole with the chemical composition 7Si8O222, magnesium iron silicate hydroxide.Monoclinic cummingtonite is compositionally similar and polymorphic with orthorhombic anthophyllite, which is a much more common form of magnesium-rich amphibole, the latter being...
. Some forms of anthophyllite are lamellar or fibrous and are used as asbestos
Asbestos
Asbestos is a set of six naturally occurring silicate minerals used commercially for their desirable physical properties. They all have in common their eponymous, asbestiform habit: long, thin fibrous crystals...
. 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 metamorphismMetamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
of magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
-rich rocks, especially ultrabasic igneous rock
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
s and impure dolomitic
Dolomite
Dolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg2. The term is also used to describe the sedimentary carbonate rock dolostone....
shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
s. It also forms as a retrograde product rimming relict orthopyroxenes and olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
, and as an accessory mineral in cordierite
Cordierite
Cordierite or iolite is a magnesium iron aluminium cyclosilicate. Iron is almost always present and a solid solution exists between Mg-rich cordierite and Fe-rich sekaninaite with a series formula: 2 to 2...
-bearing gneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
es and schist
Schist
The schists constitute a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is...
s. Anthophyllite also occurs as a retrograde metamorphic mineral derived from ultramafic rocks along with serpentinite
Serpentinite
Serpentinite is a rock composed of one or more serpentine group minerals. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle...
. 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 talcTalc
Talc is a mineral composed of hydrated magnesium silicate with the chemical formula H2Mg34 or Mg3Si4O102. In loose form, it is the widely-used substance known as talcum powder. It occurs as foliated to fibrous masses, its crystals being so rare as to be almost unknown...
in ultramafic rocks in the presence of water and carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
as a prograde metamorphic reaction
Metamorphic reaction
A metamorphic reaction is a chemical reaction that takes place during the geological process of metamorphism wherein one assemblage of minerals is transformed into a second assemblage which is stable under the new temperature/pressure conditions resulting in the final stable state of the observed...
. 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
Isograd
In geology, an isograd is a plane of constant metamorphic grade in the field; it separates metamorphic zones of different metamorphic index minerals. On geologic maps focusing on metamorphic terranes , the boundaries between rocks of different metamorphic grade are commonly demarcated by isograd...
.
Ultramafic rocks in purely hydrous, CO2-free environments will tend to form serpentinite
Serpentinite
Serpentinite is a rock composed of one or more serpentine group minerals. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle...
-antigorite-brucite
Brucite
Brucite is the mineral form of magnesium hydroxide, with the chemical formula Mg2. It is a common alteration product of periclase in marble; a low-temperature hydrothermal vein mineral in metamorphosed limestones and chlorite schists; and formed during serpentinization of dunites...
-tremolite
Tremolite
Tremolite is a member of the amphibole group of silicate minerals with composition: Ca2Mg5Si8O222. Tremolite forms by metamorphism of sediments rich in dolomite and quartz. Tremolite forms a series with actinolite and ferro-actinolite. Pure magnesium tremolite is creamy white, but the color grades...
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
Greenschist
Greenschist is a general field petrologic term applied to metamorphic or altered mafic volcanic rock. The term greenstone is sometimes used to refer to greenschist but can refer to other rock types too. The green is due to abundant green chlorite, actinolite and epidote minerals that dominate the...
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
Serpentinite
Serpentinite is a rock composed of one or more serpentine group minerals. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle...
or talc
Talc
Talc is a mineral composed of hydrated magnesium silicate with the chemical formula H2Mg34 or Mg3Si4O102. In loose form, it is the widely-used substance known as talcum powder. It occurs as foliated to fibrous masses, its crystals being so rare as to be almost unknown...
-magnesite
Magnesite
Magnesite is magnesium carbonate, MgCO3. Iron substitutes for magnesium with a complete solution series with siderite, FeCO3. Calcium, manganese, cobalt, and nickel may also occur in small amounts...
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.