https://bereket-haileab.geology.sites.carleton.edu/files/original/f8f6bc145617a96285e6040407a7d394.jpg eedc89c44dde57240bae87e0b3f80719 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 BH250-209a Subject The topic of the resource Nepheline-Bearing Syenite Description An account of the resource Major Minerals: sodic plagioclase, orthoclase, biotite (Fe-rich and Mg-rich varieties), nepheline Minor Minerals: aegirine-augite (deep green), opaque oxides Secondary Mineral: calcite BH250-209a is a coarse-grained nepheline-bearing syenite collected from Blue Mountain in Methuen Township, Ontario, part of the well-known alkaline igneous complexes of the Bancroft region. This rock is dominated by feldspars and feldspathoids and represents a silica-undersaturated intrusive igneous rock. The presence of nepheline confirms that the magma from which this rock crystallized was deficient in silica; if sufficient silica had been available, nepheline would have reacted to form quartz or additional feldspar. In hand sample, the rock displays a coarse phaneritic texture with abundant light-colored feldspar and nepheline crystals contrasted by dark mafic minerals. Dark brown to green biotite flakes are visible throughout the specimen, along with scattered deep green pyroxene crystals interpreted as aegirine-augite. Minor opaque oxides are also present. Late-stage calcite occurs as a secondary mineral, likely introduced during post-magmatic hydrothermal alteration or fluid interaction. This sample is particularly useful for teaching because it demonstrates the mineralogical distinction between silica-saturated rocks such as granite and silica-undersaturated rocks such as nepheline syenite. It also provides excellent examples of feldspar textures, nepheline optics, and alkaline pyroxene mineralogy. Optical Mineralogy Nepheline Nepheline in this thin section is exceptionally fresh and commonly occurs as anhedral to subhedral grains interstitial to feldspar. Many crystals appear to be cut perpendicular to the c-axis, producing classic uniaxial negative interference figures. Nepheline exhibits very low birefringence and poor cleavage, and beginning mineralogy students often mistake it for quartz. However, its occurrence in a silica-undersaturated assemblage and its optical behavior clearly distinguish it from quartz. Plagioclase Feldspar The plagioclase is predominantly sodic and displays well-developed albite twinning, which is easily visible in thin section. Several grains appear oriented perpendicular to the b-axis, making them useful for estimating compositional variation using the Michel-Lévy method. The feldspars form much of the framework of the rock and show typical interlocking igneous textures. Orthoclase Orthoclase occurs as large feldspar crystals intergrown with plagioclase and nepheline. Together, the alkali feldspars dominate the felsic component of the rock and place it firmly within the syenite classification field. Biotite Biotite occurs as elongate flakes with perfect basal cleavage. Under crossed polars, many grains display classic bird’s-eye maple interference colors, making them excellent teaching examples. In plane-polarized light, pleochroism ranges from deep reddish-brown to pale yellow in Fe-rich varieties. Some crystals display greenish pleochroism and are likely more magnesium-rich, approaching phlogopite composition. The coexistence of both Fe-rich and Mg-rich biotite varieties suggests subtle chemical variation during crystallization. Aegirine-Augite Deep green pyroxene grains in this section are interpreted as aegirine-augite, a sodium-rich clinopyroxene commonly associated with alkaline igneous rocks. These crystals typically appear dark green to nearly black in plane-polarized light and may exhibit strong pleochroism from green to yellow-green. Although aegirine is birefringent, strong absorption and low interference colors can sometimes make it appear nearly isotropic in thin section. Its presence is consistent with the alkaline, silica-undersaturated nature of this rock. Classification and Geological Significance Syenite is a coarse-grained intrusive igneous rock composed primarily of alkali feldspar with lesser plagioclase and minor mafic minerals. It is chemically similar to granite but contains little to no quartz (typically less than 5%). Instead, in silica-undersaturated systems such as this one, feldspathoids like nepheline may crystallize. The name syenite is derived from Syene (modern-day Aswan), Egypt, where similar quartz-poor feldspar-rich rocks were historically quarried and used in ancient monuments and obelisks. Although modern petrologic classification has refined the term, Syene remains the historical type locality associated with the name. This specimen is particularly valuable for teaching IUGS igneous rock classification, especially the distinction between quartz-bearing granitoids and feldspathoid-bearing alkaline rocks. Students can use this sample to understand how mineral assemblages reflect magma chemistry and silica saturation. Classification of Syenites <ul> <li>Quartz syenite: contains 5–20% quartz</li> <li>Quartz-free syenite: contains less than 5% quartz</li> <li>Nepheline syenite: contains feldspathoids such as nepheline and forms in silica-undersaturated systems</li> <li>Potassic syenite: dominated by potassium feldspar</li> <li>Sodic syenite: richer in sodium feldspar such as albite</li> </ul> This sample is best classified as a nepheline-bearing sodic syenite.   <iframe width="1800" height="100" src="https://docs.google.com/spreadsheets/d/e/2PACX-1vS4nyqCmZQGn3t6vX4g4nhDPwHMBaKrigt1LaY2pykWbBGXB1wNb6FYArx625I-JTtD7lyg4uz1OS1r/pubhtml?gid=266054396&single=true&widget=true&headers=false"></iframe> Source A related resource from which the described resource is derived From the rock collection of Bereket Haileab. Sample BH250-209a. 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=b8e6b1f95ed742799b5a9cc47ae42629" frameborder="0" allowfullscreen="allowfullscreen">iFrames are not supported on this page.</iframe> View on ArcGIS Online <a href="https://carleton.maps.arcgis.com/home/item.html?id=b8e6b1f95ed742799b5a9cc47ae42629">here</a> <iframe width="100%" height="480" src="https://drive.google.com/file/d/1cY1b9urTVAQGJMzMslRmkUP-Eorj9WZF/preview" allow="autoplay"></iframe> <iframe width="100%" height="480" src="https://drive.google.com/file/d/1LxVoH5SoWgLQfTTM717u1bAEOZ4C4Oyu/preview" allow="autoplay"></iframe> <iframe width="100%" height="480" src="https://drive.google.com/file/d/1QavUbL5hLqkif-honNK5SmVPrDPOQfkr/preview" allow="autoplay"></iframe> <iframe width="100%" height="480" src="https://drive.google.com/file/d/1-ZqlcrwWk3qfHj2xtM_BxMkBLwAyagdX/preview" allow="autoplay"></iframe> <iframe width="100%" height="480" src="https://drive.google.com/file/d/1L95BljYJ4vIieTVXTLQ3G3fEEW7OJ9UO/preview" allow="autoplay"></iframe> <iframe width="100%" height="480" src="https://drive.google.com/file/d/1UQoQBFXcdDp23lAqCAqRDh9jDitaftmI/preview" allow="autoplay"></iframe> <iframe width="100%" height="480" src="https://drive.google.com/file/d/1zocXZO7YqMwqubbPyzUljTLXIY_hFj-0/preview" allow="autoplay"></iframe> <iframe width="100%" height="480" src="https://drive.google.com/file/d/1AtfaIgoKK0_4hUESxZG4G1qJ-STtBDXp/preview" allow="autoplay"></iframe> <iframe width="100%" height="480" src="https://drive.google.com/file/d/1Ay5A7vLRu2FQXGXt-cbMuD8T8OANSHMp/preview" allow="autoplay"></iframe> TAS diagram of sample BH250-209a. Creator An entity primarily responsible for making the resource Bereket Haileab Coverage The spatial or temporal topic of the resource, the spatial applicability of the resource, or the jurisdiction under which the resource is relevant Blue Mountain, Methuen Township, Ontario, Canada Aegirine-augite Albite twinning alkali feldspar Alkaline magmatism Biotite (Fe-Mg varieties) Bird’s eye maple interference colors Egypt Feldspathoid-bearing rock felsic igneous IUGS classification Michel-Lévy method modern-day Aswan Nepheline Plagioclase (sodic) Pleochroism (green plutonic Potassic syenite Quartz syenite Quartz-free syenite red Silica-poor magma Sodic syenite Syene syenite Uniaxial negative interference figure yellow)