BH250-94
Title
BH250-94
Subject
Olivine-Bearing Gabbro (Stillwater Complex, Montana)
Description
Major Minerals: plagioclase, olivine
Minor Minerals: serpentine, oxides
Location: Stillwater Complex, Montana, USA
BH250-94 is a particularly valuable teaching specimen from the Stillwater Complex of Montana, one of the world’s classic layered mafic intrusions. While BH250-93 emphasizes early plagioclase crystallization, BH250-94 records a contrasting magmatic pathway in which the original melt composition favored the crystallization of mafic minerals first. This makes the two samples an exceptional paired teaching set for demonstrating how subtle differences in melt composition can produce very different textures while still generating broadly similar gabbroic rocks.
In thin section, BH250-94 is dominated by plagioclase and olivine. The most striking feature is that the mafic minerals—particularly olivine—are more euhedral and better formed than the surrounding plagioclase. These early-formed olivine crystals had sufficient space to develop well-defined crystal boundaries during the initial stages of crystallization. In contrast, plagioclase commonly appears more anhedral and interstitial, occupying the spaces left between earlier mafic minerals. This textural relationship strongly suggests that the parental magma began within the olivine primary crystallization field, where olivine nucleated first during cooling.
Many olivine grains now show partial alteration to serpentine, a common secondary mineral produced during low-temperature hydration of olivine. This alteration provides an excellent opportunity to discuss both primary igneous processes and later post-magmatic alteration. Minor oxide minerals likely represent Fe-Ti oxides that crystallized during later stages of cooling.
From a petrologic perspective, BH250-94 is particularly effective for teaching simplified phase equilibria in mafic systems. Students can compare this sample directly with BH250-93 and recognize that the original melts occupied different positions on a phase diagram. BH250-93 began in a plagioclase-dominated crystallization field, whereas BH250-94 likely began in a more mafic field favoring early olivine crystallization. Both melts evolved toward more complex crystallization pathways as cooling progressed.
For introductory students, this sample demonstrates how crystal shape preserves the order of crystallization. For advanced petrology students, it becomes a powerful exercise in reconstructing parental magma chemistry, interpreting phase diagrams, and understanding layered intrusion processes. Together with BH250-93, this specimen beautifully illustrates why textures often preserve information that bulk rock names alone cannot reveal.
Minor Minerals: serpentine, oxides
Location: Stillwater Complex, Montana, USA
BH250-94 is a particularly valuable teaching specimen from the Stillwater Complex of Montana, one of the world’s classic layered mafic intrusions. While BH250-93 emphasizes early plagioclase crystallization, BH250-94 records a contrasting magmatic pathway in which the original melt composition favored the crystallization of mafic minerals first. This makes the two samples an exceptional paired teaching set for demonstrating how subtle differences in melt composition can produce very different textures while still generating broadly similar gabbroic rocks.
In thin section, BH250-94 is dominated by plagioclase and olivine. The most striking feature is that the mafic minerals—particularly olivine—are more euhedral and better formed than the surrounding plagioclase. These early-formed olivine crystals had sufficient space to develop well-defined crystal boundaries during the initial stages of crystallization. In contrast, plagioclase commonly appears more anhedral and interstitial, occupying the spaces left between earlier mafic minerals. This textural relationship strongly suggests that the parental magma began within the olivine primary crystallization field, where olivine nucleated first during cooling.
Many olivine grains now show partial alteration to serpentine, a common secondary mineral produced during low-temperature hydration of olivine. This alteration provides an excellent opportunity to discuss both primary igneous processes and later post-magmatic alteration. Minor oxide minerals likely represent Fe-Ti oxides that crystallized during later stages of cooling.
From a petrologic perspective, BH250-94 is particularly effective for teaching simplified phase equilibria in mafic systems. Students can compare this sample directly with BH250-93 and recognize that the original melts occupied different positions on a phase diagram. BH250-93 began in a plagioclase-dominated crystallization field, whereas BH250-94 likely began in a more mafic field favoring early olivine crystallization. Both melts evolved toward more complex crystallization pathways as cooling progressed.
For introductory students, this sample demonstrates how crystal shape preserves the order of crystallization. For advanced petrology students, it becomes a powerful exercise in reconstructing parental magma chemistry, interpreting phase diagrams, and understanding layered intrusion processes. Together with BH250-93, this specimen beautifully illustrates why textures often preserve information that bulk rock names alone cannot reveal.
Coverage
Location: Stillwater, Montana, USA
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-94. Housed at Carleton College in Minnesota.
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-94,” BH250 Mineralogy Teaching Collection, accessed October 22, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/107.