Glaucophane
Encyclopedia
Glaucophane is a mineral
belonging to the amphibole
group, chemical formula [][ Na
2][ Mg
3Al2][(OH)2| Si
8O22].
, glaucophane means "blue appearing"). Glaucophane crystallizes in the monoclinic system
.
are the only common amphibole minerals that are typically blue. Glaucophane forms a solid solution series with ferroglaucophane; Na2(Fe,Mg)3Al2Si8O22(OH)2. Glaucophane is the magnesium
-rich member and ferroglaucophane is the iron
-rich member. Ferroglaucophane is similar to glaucophane but is denser, generally darker in color with a diminished pearly luster
. Glaucophane's hardness is 5 - 6, and its specific gravity
is approximately 3 - 3.2.
s that are either particularly rich in sodium
or that have experienced low temperature-high pressure metamorphism
such as would occur along a subduction
zone. This material has undergone intense pressure and moderate heat as it was subducted downward toward the mantle
. It is glaucophane's color that gives the blueschist
facies its name. Glaucophane is also found in eclogite
s that have undergone retrograde metamorphism.
There is also a rare amphibole called holmquistite
, chemical formula Li2Mg3Al2Si8O22(OH)2, which occurs only in lithium-rich continental rocks. For many years, holmquistite was mistaken for glaucophane, as the two look identical in thin section.
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...
belonging to the amphibole
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:...
group, chemical formula []
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
2]
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...
3Al2][(OH)2
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
8O22].
Name
It is named from its typical blue color (in GreekGreek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
, glaucophane means "blue appearing"). Glaucophane crystallizes in the monoclinic system
Monoclinic crystal system
In crystallography, the monoclinic crystal system is one of the 7 lattice point groups. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal length, as in the orthorhombic system. They form a rectangular prism with a...
.
Characteristics
The blue color is very diagnostic for this species. It, along with the closely related mineral riebeckiteRiebeckite
Riebeckite is a sodium-rich member of the amphibole group of silicate minerals, chemical formula [][Na2][32][2|Si8O22]. It forms a series with magnesioriebeckite. It crystallizes in the monoclinic system, usually as long prismatic crystals showing a diamond-shaped cross section, but also in...
are the only common amphibole minerals that are typically blue. Glaucophane forms a solid solution series with ferroglaucophane; Na2(Fe,Mg)3Al2Si8O22(OH)2. Glaucophane is the 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 member and ferroglaucophane is the 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...
-rich member. Ferroglaucophane is similar to glaucophane but is denser, generally darker in color with a diminished pearly luster
Lustre (mineralogy)
Lustre is a description of the way light interacts with the surface of a crystal, rock, or mineral. The word lustre traces its origins back to the Latin word lux, meaning "light", and generally implies radiance, gloss, or brilliance....
. Glaucophane's hardness is 5 - 6, and its specific gravity
Specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
is approximately 3 - 3.2.
Occurrence
Glaucophane forms in metamorphic rockMetamorphic 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...
s that are either particularly rich in sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
or that have experienced low temperature-high pressure metamorphism
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
such as would occur along a subduction
Subduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
zone. This material has undergone intense pressure and moderate heat as it was subducted downward toward the mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
. It is glaucophane's color that gives the blueschist
Blueschist
Blueschist is a rock that forms by the metamorphism of basalt and rocks with similar composition at high pressures and low temperatures, approximately corresponding to a depth of 15 to 30 kilometers and 200 to ~500 degrees Celsius....
facies its name. Glaucophane is also found in eclogite
Eclogite
Eclogite is a mafic metamorphic rock. Eclogite is of special interest for at least two reasons. First, it forms at pressures greater than those typical of the crust of the Earth...
s that have undergone retrograde metamorphism.
There is also a rare amphibole called holmquistite
Holmquistite
Holmquistite is a lithium magnesium aluminium inosilicate mineral with chemical formula: Li23Al2Si8O222. It crystallizes in the orthorhombic crystal system as prismatic crystals up to 10 cm or as massive aggregates...
, chemical formula Li2Mg3Al2Si8O22(OH)2, which occurs only in lithium-rich continental rocks. For many years, holmquistite was mistaken for glaucophane, as the two look identical in thin section.