BH250-93
Title
BH250-93
Subject
Gabbro (Stillwater Complex, Montana)
Description
Major Minerals: Plagioclase, Diopside (clinopyroxene), Enstatite (orthopyroxene)
Location: Stillwater Complex, Montana, USA
BH250-93 is one of the best teaching samples in this collection for illustrating early plagioclase crystallization in mafic magma systems. Collected from the famous Stillwater Complex of Montana, this specimen represents a classic layered mafic intrusive environment where crystal settling, magma differentiation, and phase equilibria can be studied exceptionally well. The Stillwater Complex is one of the world’s best examples of a large layered mafic intrusion and is often compared to South Africa’s Bushveld Complex because both preserve remarkable records of magmatic differentiation.
In thin section, BH250-93 displays a framework of interlocking euhedral plagioclase feldspar crystals that dominate the rock. These feldspars show well-developed crystal faces and clear polysynthetic twinning, indicating that they crystallized early when space was readily available within the magma chamber. The pyroxenes—both diopside (clinopyroxene) and enstatite (orthopyroxene)—occur primarily in interstitial spaces between the feldspar crystals and are noticeably more anhedral. Their irregular crystal boundaries indicate that they crystallized later, after much of the plagioclase framework had already formed.
This textural relationship provides an elegant teaching example of a magma whose original composition likely began within the plagioclase primary crystallization field of a simplified plagioclase-pyroxene system. As cooling began, plagioclase nucleated first and grew freely, developing euhedral forms. The remaining melt gradually evolved toward eutectic conditions where both clinopyroxene and orthopyroxene began crystallizing in the remaining pore spaces. This sequence explains why the pyroxenes appear texturally constrained and anhedral.
For introductory students, BH250-93 demonstrates how crystal shape reflects crystallization order. For advanced petrology students, it becomes an excellent exercise in reconstructing parental magma composition, equilibrium crystallization pathways, and interpreting simplified phase diagrams. This specimen beautifully demonstrates that texture can preserve the cooling history of a magma long after solidification.
Location: Stillwater Complex, Montana, USA
BH250-93 is one of the best teaching samples in this collection for illustrating early plagioclase crystallization in mafic magma systems. Collected from the famous Stillwater Complex of Montana, this specimen represents a classic layered mafic intrusive environment where crystal settling, magma differentiation, and phase equilibria can be studied exceptionally well. The Stillwater Complex is one of the world’s best examples of a large layered mafic intrusion and is often compared to South Africa’s Bushveld Complex because both preserve remarkable records of magmatic differentiation.
In thin section, BH250-93 displays a framework of interlocking euhedral plagioclase feldspar crystals that dominate the rock. These feldspars show well-developed crystal faces and clear polysynthetic twinning, indicating that they crystallized early when space was readily available within the magma chamber. The pyroxenes—both diopside (clinopyroxene) and enstatite (orthopyroxene)—occur primarily in interstitial spaces between the feldspar crystals and are noticeably more anhedral. Their irregular crystal boundaries indicate that they crystallized later, after much of the plagioclase framework had already formed.
This textural relationship provides an elegant teaching example of a magma whose original composition likely began within the plagioclase primary crystallization field of a simplified plagioclase-pyroxene system. As cooling began, plagioclase nucleated first and grew freely, developing euhedral forms. The remaining melt gradually evolved toward eutectic conditions where both clinopyroxene and orthopyroxene began crystallizing in the remaining pore spaces. This sequence explains why the pyroxenes appear texturally constrained and anhedral.
For introductory students, BH250-93 demonstrates how crystal shape reflects crystallization order. For advanced petrology students, it becomes an excellent exercise in reconstructing parental magma composition, equilibrium crystallization pathways, and interpreting simplified phase diagrams. This specimen beautifully demonstrates that texture can preserve the cooling history of a magma long after solidification.
Coverage
Location: Stillwater, Montana, USA
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-93. Housed at Carleton College in Minnesota.
Type
Thin section
Relation
Collection
Citation
Bereket Haileab, “BH250-93,” BH250 Mineralogy Teaching Collection, accessed October 22, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/106.