Mineral
Glaucophane
Na₂(Mg,Fe)₃Al₂Si₈O₂₂(OH)₂Glokofan
The blue of blueschist: a blue-grey amphibole formed in subduction zones at low temperature and very high pressure.
- Popularly: Common stone
- Important in Türkiye
In short
Glaucophane is a blue-grey amphibole mineral containing sodium, magnesium and aluminium; its name is Greek for “blue appearing”. Its importance lies not in its beauty but in what it tells: finding glaucophane in a rock shows that the rock was once transformed in a subduction zone, at low temperature and high pressure. Such rocks are called blueschist.
Photos
How does it form?
Glaucophane is the mineral of low-temperature, high-pressure metamorphism. When an oceanic plate dives beneath another, rocks of the sea floor are subjected to great pressure as they sink, before they reach the hot mantle; under these conditions glaucophane forms and the rock takes on a bluish colour (blueschist). Glaucophane also appears when eclogites are later transformed under lower pressures. During mountain building these rocks are brought to the surface.
What is it used for?
Glaucophane has no important known industrial or ornamental use; the interest is scientific:
- Trace of subduction: Blueschists show that an oceanic plate once dived beneath a region. Glaucophane therefore helps geologists unravel the past of mountain belts.
- Reading age and conditions: Minerals found with glaucophane-bearing rocks, such as micas, can be dated by radiometric methods to work out when subduction took place.
- Collecting: Blue prismatic crystals and blueschist specimens find a place in mineral and rock collections.
In Türkiye
In Türkiye glaucophane occurs in lawsonite blueschists and eclogites of the Sivrihisar Massif (Eskişehir), in the Tavşanlı Zone. There, the margins of the eclogite pods show veins of lawsonite, garnet and phengite (a mica) rimmed by glaucophane; the peak metamorphic age of these rocks has been measured at 90–93 million years.
What determines its value?
Glaucophane has almost no commercial value; its real worth is geological knowledge. Collectors give a small value to distinctly blue, well-formed crystals and fine blueschist specimens, especially from well-known localities such as Syros.
This information is educational, not buying, selling or investment advice. Value varies enormously with quality and treatment.
How to recognise it
- 1
Blue-grey, lavender-blue or bluish-black prismatic crystals; in the rock it often occurs as fine needles or granular clusters, and its colour varies in zones.
- 2
Its hardness is 6–6.5: a steel knife scratches it only with difficulty. It has perfect cleavage in two directions; its streak (powder colour) is bluish grey.
- 3
In the rocks called blueschist it is seen together with minerals such as lawsonite, garnet, epidote and omphacite.
- 4
Other blue amphiboles (riebeckite, for example) and other blue minerals can look similar; certain identification often needs a microscope.
Fakes and imitations
What shops usually don’t tell you:
- Not every blue rock or mineral is glaucophane: stones such as lazurite, sodalite and blue kyanite, or other blue amphiboles, can look similar; an expert is needed for certain identification.
Myths and misconceptions
MythIt is always high temperature that transforms rocks, so the glaucophane in blueschist formed near magma or a volcano.
FactGlaucophane is on the contrary a mineral of low temperature and very high pressure: it forms in subduction zones, where one oceanic plate dives beneath another. That is why blueschist is taken as the sign of an ancient subduction zone.
Did you know?
Its name comes from the Greek words for “blue” and “to appear”; it means “blue appearing”.
Age measurements on the lawsonite blueschists and eclogites of Sivrihisar (Eskişehir) show that these rocks went through peak metamorphism deep in a subduction zone about 90–93 million years ago.
Where is it found?
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- Syros, Cyclades, Greece · Known localityAn island famous for its glaucophane–lawsonite schists
- Tiburon Peninsula, Marin County, California, USA · Known locality
- Kodiak Islands, Alaska, USA · Known locality
- Saint-Marcel, Aosta Valley, Italy · Known locality
- Anglesey, Wales, United Kingdom · Known locality
- Tokushima Prefecture, Japan · Known locality
- Sivrihisar Massif (Tavşanlı Zone), Eskişehir, Türkiye · Known localityGlaucophane in lawsonite blueschist and eclogite; the point marks the Sivrihisar area
Locations are approximate to protect sites. Red dots mark places in Türkiye. Open in the big map →
Frequently asked questions
What is glaucophane?
Glaucophane is a blue-grey amphibole mineral containing sodium, magnesium and aluminium; its name is Greek for “blue appearing”. Its importance lies not in its beauty but in what it tells: finding glaucophane in a rock shows that the rock was once transformed in a subduction zone, at low temperature and high pressure. Such rocks are called blueschist.
What is glaucophane used for?
Glaucophane has no important known industrial or ornamental use; the interest is scientific: Trace of subduction: Blueschists show that an oceanic plate once dived beneath a region. Glaucophane therefore helps geologists unravel the past of mountain belts. Reading age and conditions: Minerals found with glaucophane-bearing rocks, such as micas, can be dated by radiometric methods to work out when subduction took place. Collecting: Blue prismatic crystals and blueschist specimens find a place in mineral and rock collections.
How hard is glaucophane?
It has a hardness of 6–6.5 on the Mohs scale. It can scratch glass; a steel knife scratches it only with difficulty. Its specific gravity is 3–3.2 g/cm³.
Where is glaucophane found?
Its best-known localities: Syros, Cyclades, Greece; Tiburon Peninsula, Marin County, California, USA; Kodiak Islands, Alaska, USA; Saint-Marcel, Aosta Valley, Italy; Anglesey, Wales, United Kingdom.
Where is glaucophane found in Türkiye?
Known localities in Türkiye: Sivrihisar Massif (Tavşanlı Zone), Eskişehir, Türkiye.
How can you tell if glaucophane is real?
Blue-grey, lavender-blue or bluish-black prismatic crystals; in the rock it often occurs as fine needles or granular clusters, and its colour varies in zones. Its hardness is 6–6.5: a steel knife scratches it only with difficulty. It has perfect cleavage in two directions; its streak (powder colour) is bluish grey. In the rocks called blueschist it is seen together with minerals such as lawsonite, garnet, epidote and omphacite. Not every blue rock or mineral is glaucophane: stones such as lazurite, sodalite and blue kyanite, or other blue amphiboles, can look similar; an expert is needed for certain identification.
Is glaucophane valuable?
On the “street to diamond” value ladder it sits on step 2 of nine: Raw material by the tonne. Glaucophane has almost no commercial value; its real worth is geological knowledge. Collectors give a small value to distinctly blue, well-formed crystals and fine blueschist specimens, especially from well-known localities such as Syros.
Sources
- Handbook of Mineralogy: Glaucophane · Mineralogical Society of America
- Glaucophane · Webmineral
- Glaucophane · Wikipedia
- Tracking the timing of subduction and exhumation using 40Ar/39Ar phengite ages in blueschist- and eclogite-facies rocks (Sivrihisar, Turkey) (Fornash, Cosca, Whitney), Contributions to Mineralogy and Petrology · U.S. Geological Survey (publication record), 2016
- Sivrihisar · Wikipedia
External links
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