BH250-210
Title
BH250-210
Subject
Eclogite
Description
Major Minerals: garnet, clinopyroxene, omphacite, sphene
BH250 Eclogite is a classic example of a high-pressure metamorphic rock that records burial of crustal material to extraordinary depths within a subduction-zone environment. In hand sample, the rock is striking because of its dense, dark green matrix of clinopyroxene surrounding conspicuous red to reddish-brown garnet porphyroblasts. This color contrast makes eclogite one of the most visually recognizable metamorphic rocks in teaching collections. The rock is notably heavy for its size due to the abundance of dense mafic minerals and lacks plagioclase, which is one of the defining mineralogical characteristics of true eclogite-facies metamorphism.
The dominant minerals in this specimen are Garnet and Clinopyroxene, most likely omphacite, the sodium-rich clinopyroxene typical of many eclogites. Garnet commonly forms large porphyroblasts and may preserve compositional zoning that records changing pressure-temperature conditions during burial. The green clinopyroxene matrix typically shows moderate to high relief in thin section and may display pale green pleochroism depending on composition. Accessory Sphene occurs as minor grains and is important because it can preserve trace elements and geochronologic information useful for reconstructing metamorphic histories. Some samples may also contain rutile, quartz, amphibole, or retrograde minerals formed during exhumation, although your specimen appears dominated by the classic garnet-clinopyroxene assemblage.
Eclogite forms when basaltic or gabbroic protoliths are subducted to depths commonly exceeding 50–90 km, where pressures are high enough that plagioclase becomes unstable. Under these conditions, plagioclase breaks down and new high-pressure minerals such as garnet and omphacite form. The transformation can be summarized as a shift from lower-pressure mafic assemblages toward denser mineral phases stable in subduction zones. This makes eclogite one of the best rocks for teaching plate tectonics because it preserves direct evidence of crust being carried deep into Earth’s mantle before later returning to the surface.
The occurrence in Sonoma County is geologically significant because northern California contains fragments of subduction-related metamorphic terranes associated with the larger Franciscan Complex. The Franciscan assemblage is famous for preserving blueschist- and eclogite-facies rocks that formed during subduction along the ancient western margin of North America. These rocks provide evidence for long-lived convergent margin tectonics, oceanic crust subduction, and the complex exhumation processes that return deeply buried rocks back to the surface.
BH250 Eclogite is a classic example of a high-pressure metamorphic rock that records burial of crustal material to extraordinary depths within a subduction-zone environment. In hand sample, the rock is striking because of its dense, dark green matrix of clinopyroxene surrounding conspicuous red to reddish-brown garnet porphyroblasts. This color contrast makes eclogite one of the most visually recognizable metamorphic rocks in teaching collections. The rock is notably heavy for its size due to the abundance of dense mafic minerals and lacks plagioclase, which is one of the defining mineralogical characteristics of true eclogite-facies metamorphism.
The dominant minerals in this specimen are Garnet and Clinopyroxene, most likely omphacite, the sodium-rich clinopyroxene typical of many eclogites. Garnet commonly forms large porphyroblasts and may preserve compositional zoning that records changing pressure-temperature conditions during burial. The green clinopyroxene matrix typically shows moderate to high relief in thin section and may display pale green pleochroism depending on composition. Accessory Sphene occurs as minor grains and is important because it can preserve trace elements and geochronologic information useful for reconstructing metamorphic histories. Some samples may also contain rutile, quartz, amphibole, or retrograde minerals formed during exhumation, although your specimen appears dominated by the classic garnet-clinopyroxene assemblage.
Eclogite forms when basaltic or gabbroic protoliths are subducted to depths commonly exceeding 50–90 km, where pressures are high enough that plagioclase becomes unstable. Under these conditions, plagioclase breaks down and new high-pressure minerals such as garnet and omphacite form. The transformation can be summarized as a shift from lower-pressure mafic assemblages toward denser mineral phases stable in subduction zones. This makes eclogite one of the best rocks for teaching plate tectonics because it preserves direct evidence of crust being carried deep into Earth’s mantle before later returning to the surface.
The occurrence in Sonoma County is geologically significant because northern California contains fragments of subduction-related metamorphic terranes associated with the larger Franciscan Complex. The Franciscan assemblage is famous for preserving blueschist- and eclogite-facies rocks that formed during subduction along the ancient western margin of North America. These rocks provide evidence for long-lived convergent margin tectonics, oceanic crust subduction, and the complex exhumation processes that return deeply buried rocks back to the surface.
Coverage
Location: Sonoma County, California, USA
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 210. Housed at Carleton College in Minnesota.
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-210,” BH250 Mineralogy Teaching Collection, accessed October 22, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/270.