Rock
Blueschist
Mavi şist
The blue rock coloured by glaucophane: it forms under high pressure but low temperature, only in subduction zones.
Also known as: Glaucophane schist
- Popularly: Common stone
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In short
Blueschist is a bluish rock formed by the metamorphism of basalt and similar rocks. It gets its colour mainly from the minerals glaucophane and lawsonite. It needs relatively low temperatures (200–500 °C) but very high pressure, equal to a depth of 15–30 km. These conditions arise only in subduction zones, where one plate dives beneath another. In Türkiye, the Sivrihisar area of Eskişehir is known for its blueschists.
Photos
How does it form?
The basalt of the ocean crust sinks quickly with a cold plate to depths of 15–30 km in subduction zones. There the pressure is very high and the temperature relatively low (200–500 °C). Under these conditions the minerals of the basalt turn into high-pressure minerals such as glaucophane, lawsonite or epidote. For blueschist to reach the surface, the rocks must rise quickly, before they warm up and turn into other minerals; that is why blueschist is a rare rock.
What is it used for?
- A trace of subduction: Blueschist shows where and how an ocean plate sank in the past; it is used to unravel the history of mountain belts.
- Paving stone in history: In Minoan Crete it was used to pave streets and courtyards.
In Türkiye
Eskişehir’s Sivrihisar massif is part of the Tavşanlı Zone and contains lawsonite and epidote blueschists together with eclogites. These rocks carry the traces of a subduction zone here and have been the subject of international research.
What determines its value?
Blueschist is not a commercial stone; its value is scientific. Because it records the deep, cold conditions of subduction zones, it matters to researchers and teaching collections. Collectors look for fresh specimens with vivid colour and clear glaucophane and lawsonite crystals.
This information is educational, not buying, selling or investment advice. Value varies enormously with quality and treatment.
How to recognise it
- 1
Blue, bluish grey or lavender blue; the colour comes from needle- and prism-shaped glaucophane crystals.
- 2
It can split into thin layers (schistosity); its surface shows a fine streaky texture.
- 3
It can contain white lawsonite or yellowish-green epidote crystals, and sometimes red garnet grains.
- 4
It is usually found together with eclogite, greenschist and serpentinite.
Fakes and imitations
What shops usually don’t tell you:
- Blue stones such as lapis lazuli or sodalite can be mistaken for blueschist; blueschist, however, is a layered rock whose colour comes from needle-shaped glaucophane crystals.
Myths and misconceptions
MythBlueschist forms in very hot conditions; metamorphism always means high temperature.
FactBlueschist forms at relatively low temperatures (200–500 °C) but very high pressure. Because a cold ocean plate dives deep quickly without heating up its surroundings, this unusual mix of temperature and pressure is found only in subduction zones.
Did you know?
No blueschist older than the Neoproterozoic (about a billion years ago) is known, which suggests rocks in earlier subduction zones never reached blueschist conditions.
In Minoan Crete, between 1650 and 1600 BC, blueschist and greenschist quarried at Agia Pelagia were used to pave streets and courtyards.
The kind of metamorphism these rocks represent is also called the “Franciscan facies series”, after rocks on the west coast of the United States.
Where is it found?
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- Sivrihisar Massif (Tavşanlı Zone), Eskişehir, Türkiye · Known localityGlaucophane in lawsonite blueschist and eclogite; the point marks the Sivrihisar area.
- Syros, Cyclades, Greece · Known localityAn island famous for its glaucophane–lawsonite schists.
- Tiburon Peninsula, Marin County, California, USA · Known localityPart of the Franciscan Complex, which holds blocks of blueschist and eclogite; the Handbook of Mineralogy lists glaucophane from here.
- Anglesey, Wales, United Kingdom · Known localityOne of the glaucophane localities listed by the Handbook of Mineralogy.
- Kodiak Islands, Alaska, USA · Known localityOne of the glaucophane localities listed by the Handbook of Mineralogy.
Locations are approximate to protect sites. Red dots mark places in Türkiye. Open in the big map →
Frequently asked questions
What is blueschist?
Blueschist is a bluish rock formed by the metamorphism of basalt and similar rocks. It gets its colour mainly from the minerals glaucophane and lawsonite. It needs relatively low temperatures (200–500 °C) but very high pressure, equal to a depth of 15–30 km. These conditions arise only in subduction zones, where one plate dives beneath another. In Türkiye, the Sivrihisar area of Eskişehir is known for its blueschists.
What is blueschist used for?
A trace of subduction: Blueschist shows where and how an ocean plate sank in the past; it is used to unravel the history of mountain belts. Paving stone in history: In Minoan Crete it was used to pave streets and courtyards.
How hard is blueschist?
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 blueschist found?
Its best-known localities: Sivrihisar Massif (Tavşanlı Zone), Eskişehir, Türkiye; Syros, Cyclades, Greece; Tiburon Peninsula, Marin County, California, USA; Anglesey, Wales, United Kingdom; Kodiak Islands, Alaska, USA.
Where is blueschist found in Türkiye?
Known localities in Türkiye: Sivrihisar Massif (Tavşanlı Zone), Eskişehir, Türkiye.
How can you tell if blueschist is real?
Blue, bluish grey or lavender blue; the colour comes from needle- and prism-shaped glaucophane crystals. It can split into thin layers (schistosity); its surface shows a fine streaky texture. It can contain white lawsonite or yellowish-green epidote crystals, and sometimes red garnet grains. Blue stones such as lapis lazuli or sodalite can be mistaken for blueschist; blueschist, however, is a layered rock whose colour comes from needle-shaped glaucophane crystals.
Is blueschist valuable?
On the “street to diamond” value ladder it sits on step 2 of nine: Raw material by the tonne. Blueschist is not a commercial stone; its value is scientific. Because it records the deep, cold conditions of subduction zones, it matters to researchers and teaching collections. Collectors look for fresh specimens with vivid colour and clear glaucophane and lawsonite crystals.
Sources
- Blueschist · Wikipedia
- Handbook of Mineralogy: Glaucophane · Mineralogical Society of America
- Franciscan Assemblage · Wikipedia
- Petrogenesis of lawsonite and epidote eclogite and blueschist, Sivrihisar Massif, Turkey (Davis, Whitney), Journal of Metamorphic Geology · Wiley, 2006
- Sivrihisar · Wikipedia
External links
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