https://bereket-haileab.geology.sites.carleton.edu/files/original/366fb43e58bc282662c994af3e4152fa.jpg c58e310cf460fd4a4f34f54ecab0ded8 Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Igneous Rocks Subject The topic of the resource This collection covers both volcanic and intrusive rocks, including rhyolites, granites, diorites, andesites, basalts, gabbros, diabases, pumices, obsidians, syenites, phonolites, peridotites, carbonatites, dacites, trachyte, dunites, xenoliths, ignimbrites, and pegmatites. Creator An entity primarily responsible for making the resource Bereket Haileab Source A related resource from which the described resource is derived Housed in Anderson Hall, Carleton College. Format The file format, physical medium, or dimensions of the resource Thin sections, thin section scans, hand samples, hand sample images, photomicrographs, chemical analyses (major elements, trace elements and REE analyses for selected samples). Thin section and hand sample Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource BH250-94 Subject The topic of the resource Olivine-Bearing Gabbro (Stillwater Complex, Montana) Description An account of the resource 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. Coverage The spatial or temporal topic of the resource, the spatial applicability of the resource, or the jurisdiction under which the resource is relevant Location: Stillwater, Montana, USA Source A related resource from which the described resource is derived From the rock collection of Bereket Haileab. Sample BH250-94. Housed at Carleton College in Minnesota. Type The nature or genre of the resource Thin section and hand sample Relation A related resource <iframe width="100%" height="600" style="border: 0;" src="https://carleton.maps.arcgis.com/apps/instant/media/index.html?appid=7c3dcc78747d4d3ba2ef22b06adc6d2c" frameborder="0" allowfullscreen="allowfullscreen">iFrames are not supported on this page.</iframe><br />View on ArcGIS Online <a href="https://carleton.maps.arcgis.com/home/item.html?id=7c3dcc78747d4d3ba2ef22b06adc6d2c">here</a><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1RMCfYCpX9_K2TXHGM1gVW6vd6WJyhu3A/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/15CqhXOAxCZLv0wYc6mADIpkAFlIo1w30/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1gcxeALY0bII9SmBLUrWm554Q3GkHZC_y/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1CA54ITE1qH8B9-RN3esJpEF09lTOM7a4/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1D9gBcaZVRxvP_YsiW6a3vpFDuBAjuTc4/preview" allow="autoplay"></iframe><iframe width="100%" height="480" src="https://drive.google.com/file/d/1j9MQaW0X3aNybyQFsAMOARvJ7Q_xuJLv/preview" allow="autoplay"></iframe> Creator An entity primarily responsible for making the resource Bereket Haileab Anhedral Plagioclase Anhedral plagioclase feldspars Bowen’s Reaction Series Crystal Growth Sequence Cumulate texture Equilibrium crystallization euhedral olivine euhedral pyroxenes Fractional crystallization gabbro igneous Igneous Texture Interstitial pyroxene Interstitial Texture Layered Intrusions Layered mafic intrusion mafic Mafic Igneous Petrology Mafic Magma Evolution Magma crystallization sequence Magmatic Differentiation Montana geology olivine Olivine Crystallization Olivine Gabbro Olivine-Bearing Gabbro Oxide minerals Phaneritic texture Phase diagram modeling Phase Equilibria Plagioclase plutonic Serpentine alteration Serpentinization Stillwater Complex Stillwater Complex Petrology Thin Section Teaching Sample