BH250-10
Title
BH250-10
Subject
Garnet Sillimanite Schist
Description
Major Minerals: garnet, sillimanite, quartz
Minor Mineral: biotite, orthoclase, plagioclase
Garnet sillimanite schist is a medium to high-grade metapelitic rock that forms under amphibolite to granulite facies metamorphic conditions. It is derived from aluminous shale or mudstone protoliths and reflects deep crustal metamorphism.
In this sample, the major minerals are:
Garnet – Large, porphyroblastic, now deformed
Sillimanite – Typically occurs as fibrolite or prismatic needles
Quartz – Forms the matrix and reflects metamorphic recrystallization
Minor minerals:
Biotite – Aligned in the foliation, contributes to schistosity
Orthoclase and Plagioclase – Present as felsic phases, possibly from original sediment or late-stage metamorphic reaction
Microstructure & Deformation:
Porphyroblastic garnets are described as prograde and pre-kinematic they grew before the main deformation event and were subsequently deformed, suggesting syn- to post-tectonic strain after garnet crystallization. These garnets now exhibit strain shadows, pressure solution features, or boudinage, indicative of deformation during peak or retrograde metamorphism.
Sillimanite are present as fibrolite along foliation planes, reflecting high-temperature metamorphism.
Metamorphic Significance:
The garnet + sillimanite + quartz ± feldspar assemblage places this rock within the upper amphibolite to granulite facies, and its mineralogy is consistent with pelitic bulk composition.
Minor Mineral: biotite, orthoclase, plagioclase
Garnet sillimanite schist is a medium to high-grade metapelitic rock that forms under amphibolite to granulite facies metamorphic conditions. It is derived from aluminous shale or mudstone protoliths and reflects deep crustal metamorphism.
In this sample, the major minerals are:
Garnet – Large, porphyroblastic, now deformed
Sillimanite – Typically occurs as fibrolite or prismatic needles
Quartz – Forms the matrix and reflects metamorphic recrystallization
Minor minerals:
Biotite – Aligned in the foliation, contributes to schistosity
Orthoclase and Plagioclase – Present as felsic phases, possibly from original sediment or late-stage metamorphic reaction
Microstructure & Deformation:
Porphyroblastic garnets are described as prograde and pre-kinematic they grew before the main deformation event and were subsequently deformed, suggesting syn- to post-tectonic strain after garnet crystallization. These garnets now exhibit strain shadows, pressure solution features, or boudinage, indicative of deformation during peak or retrograde metamorphism.
Sillimanite are present as fibrolite along foliation planes, reflecting high-temperature metamorphism.
Metamorphic Significance:
The garnet + sillimanite + quartz ± feldspar assemblage places this rock within the upper amphibolite to granulite facies, and its mineralogy is consistent with pelitic bulk composition.
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 10. Housed at Carleton College in Minnesota.
Type
Thin section
Relation
Collection
Citation
Bereket Haileab, “BH250-10,” BH250 Mineralogy Teaching Collection, accessed April 26, 2026, https://www.bereket-haileab.geology.sites.carleton.edu/items/show/10.