BH250-222
Title
BH250-222
Subject
Coesite Xenolith
Description
Major Minerals: spinel, clinopyroxene, orthopyroxene, phlogopite
Minor Mineral: coesite
BH250-222 — Coesite-Bearing Xenolith (Retrogressed Ultra-High-Pressure Rock)
BH250-222 is an exceptionally rare xenolith that preserves evidence of ultra-high-pressure metamorphism, recording conditions reached deep within Earth where pressures were high enough to stabilize coesite, a dense polymorph of SiO₂. In hand sample, the rock appears massive and coarse-grained, with a greenish-gray weathered surface and darker mineral-rich patches scattered throughout. The green coloration likely reflects alteration of ferromagnesian minerals, while the overall coarse crystalline texture is consistent with deep crustal to upper mantle mineral equilibration. Xenoliths such as this are valuable because they provide direct physical samples of environments that are otherwise inaccessible to direct observation.
The thin sections reveal a complex and heavily overprinted mineral assemblage. Large crystals of clinopyroxene and orthopyroxene are visible throughout the thin section and preserve much of the rock’s original high-temperature assemblage. Several photomicrographs show large feldspar grains displaying well-developed polysynthetic twinning under crossed polars, producing striking repetitive lamellae. These feldspars likely formed during later decompression-related recrystallization or retrograde alteration, overprinting the earlier ultra-high-pressure mineral assemblage. Phlogopite occurs as brown pleochroic grains and suggests potassium-rich metasomatism by volatile-bearing fluids within the mantle or deep crust. Minor rutile is also present and is particularly important because rutile can retain trace element signatures useful for pressure-temperature reconstruction and geochronology.
The most important feature of this sample is not what is fully preserved—but what is missing. According to notes provided by John Goodge (Class of 1980), this rock originally contained coesite, but the mineral has since inverted to lower-pressure silica polymorphs during exhumation. This transformation is common because coesite is only stable at pressures typically above about 2.5–3.0 GPa, equivalent to depths of roughly 90–100 kilometers or greater. During uplift, coesite reverts to quartz or other low-pressure silica phases, often leaving behind textural evidence rather than intact crystals.
One of the clearest indicators of former coesite in this specimen is the presence of radial fractures surrounding garnet, visible in thin section. These fractures formed because coesite occupies less volume than quartz; when it transformed back to quartz during decompression, the resulting volume expansion generated stress that fractured surrounding minerals. These fracture halos are classic diagnostic textures used to identify former coesite-bearing rocks, even when the original coesite crystals are no longer preserved.
This sample is scientifically important because coesite-bearing rocks fundamentally changed how geologists understand plate tectonics and continental collision. Their discovery demonstrated that continental crust can be subducted to mantle depths and later returned to the surface. Similar coesite-bearing rocks have been documented in major ultra-high-pressure terranes such as the Western Alps, Dabie-Sulu Orogen, and Norwegian Caledonides. BH250-222 serves as an outstanding teaching specimen because it allows students to study both preserved high-pressure minerals and the retrograde textures that document the rock’s return journey to Earth’s surface.
The thin sections are especially valuable because the pyroxenes show unusual undulose, or wavy, extinction under crossed polars. This is a striking feature because wavy extinction is commonly discussed in quartz, but it is much less commonly emphasized in pyroxenes. In this sample, the wavy extinction suggests that the pyroxene crystals experienced crystal-plastic strain or internal lattice distortion during deep burial, deformation, or later exhumation. The pyroxenes also display clear exsolution lamellae, visible as fine, regular internal bands. These lamellae record cooling and re-equilibration of an originally higher-temperature pyroxene composition, as one pyroxene phase separated into two distinct compositions during slow cooling. Together, the wavy extinction and exsolution textures make this specimen an excellent teaching example of both deformation and cooling history in high-pressure pyroxene-bearing rocks.
This specimen was collected and generously donated by John Goodge (Class of 1980).
Minor Mineral: coesite
BH250-222 — Coesite-Bearing Xenolith (Retrogressed Ultra-High-Pressure Rock)
BH250-222 is an exceptionally rare xenolith that preserves evidence of ultra-high-pressure metamorphism, recording conditions reached deep within Earth where pressures were high enough to stabilize coesite, a dense polymorph of SiO₂. In hand sample, the rock appears massive and coarse-grained, with a greenish-gray weathered surface and darker mineral-rich patches scattered throughout. The green coloration likely reflects alteration of ferromagnesian minerals, while the overall coarse crystalline texture is consistent with deep crustal to upper mantle mineral equilibration. Xenoliths such as this are valuable because they provide direct physical samples of environments that are otherwise inaccessible to direct observation.
The thin sections reveal a complex and heavily overprinted mineral assemblage. Large crystals of clinopyroxene and orthopyroxene are visible throughout the thin section and preserve much of the rock’s original high-temperature assemblage. Several photomicrographs show large feldspar grains displaying well-developed polysynthetic twinning under crossed polars, producing striking repetitive lamellae. These feldspars likely formed during later decompression-related recrystallization or retrograde alteration, overprinting the earlier ultra-high-pressure mineral assemblage. Phlogopite occurs as brown pleochroic grains and suggests potassium-rich metasomatism by volatile-bearing fluids within the mantle or deep crust. Minor rutile is also present and is particularly important because rutile can retain trace element signatures useful for pressure-temperature reconstruction and geochronology.
The most important feature of this sample is not what is fully preserved—but what is missing. According to notes provided by John Goodge (Class of 1980), this rock originally contained coesite, but the mineral has since inverted to lower-pressure silica polymorphs during exhumation. This transformation is common because coesite is only stable at pressures typically above about 2.5–3.0 GPa, equivalent to depths of roughly 90–100 kilometers or greater. During uplift, coesite reverts to quartz or other low-pressure silica phases, often leaving behind textural evidence rather than intact crystals.
One of the clearest indicators of former coesite in this specimen is the presence of radial fractures surrounding garnet, visible in thin section. These fractures formed because coesite occupies less volume than quartz; when it transformed back to quartz during decompression, the resulting volume expansion generated stress that fractured surrounding minerals. These fracture halos are classic diagnostic textures used to identify former coesite-bearing rocks, even when the original coesite crystals are no longer preserved.
This sample is scientifically important because coesite-bearing rocks fundamentally changed how geologists understand plate tectonics and continental collision. Their discovery demonstrated that continental crust can be subducted to mantle depths and later returned to the surface. Similar coesite-bearing rocks have been documented in major ultra-high-pressure terranes such as the Western Alps, Dabie-Sulu Orogen, and Norwegian Caledonides. BH250-222 serves as an outstanding teaching specimen because it allows students to study both preserved high-pressure minerals and the retrograde textures that document the rock’s return journey to Earth’s surface.
The thin sections are especially valuable because the pyroxenes show unusual undulose, or wavy, extinction under crossed polars. This is a striking feature because wavy extinction is commonly discussed in quartz, but it is much less commonly emphasized in pyroxenes. In this sample, the wavy extinction suggests that the pyroxene crystals experienced crystal-plastic strain or internal lattice distortion during deep burial, deformation, or later exhumation. The pyroxenes also display clear exsolution lamellae, visible as fine, regular internal bands. These lamellae record cooling and re-equilibration of an originally higher-temperature pyroxene composition, as one pyroxene phase separated into two distinct compositions during slow cooling. Together, the wavy extinction and exsolution textures make this specimen an excellent teaching example of both deformation and cooling history in high-pressure pyroxene-bearing rocks.
This specimen was collected and generously donated by John Goodge (Class of 1980).
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 222. Housed at Carleton College in Minnesota.
Contributor
John Goodge
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-222,” BH250 Mineralogy Teaching Collection, accessed October 22, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/293.