https://bereket-haileab.geology.sites.carleton.edu/files/original/fefd918074c0925ff54b30f0cc615128.jpg 1a0d604115ce231b3f65947b595eb34d 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 BH250-92 Subject The topic of the resource Graphic Granite Description An account of the resource BH250-92 — Graphic Granite Major Minerals: quartz, orthoclase Minor Minerals: muscovite BH250-92 is a beautiful example of graphic granite, one of the most visually striking igneous textures students encounter in mineralogy and petrology. In hand sample, large white potassium feldspar crystals host dark gray quartz intergrowths that form angular, wedge-shaped, and vermicular patterns resembling ancient cuneiform writing, hence the term graphic. This is not a separate rock type, but rather a distinctive texture most commonly found in granitic pegmatites and highly evolved felsic intrusive bodies. Your specimen is particularly useful because the texture is exceptionally well developed and easily visible even to introductory students, making it an ideal teaching sample. Graphic granite forms during the final stages of granitic magma crystallization, when only a small volume of residual melt remains. By this stage, most early-forming minerals such as plagioclase, biotite, amphibole, and other common granite minerals have already crystallized and removed many major elements from the magma. What remains is a silica-rich, potassium-rich, water-rich residual melt enriched in incompatible elements and volatiles such as H₂O, F, B, Li, and other trace constituents. These volatiles lower the viscosity of the melt and allow ions to diffuse rapidly, creating ideal conditions for unusual crystal growth textures. As cooling continues, the residual melt approaches the quartz–feldspar eutectic composition. At the eutectic point, quartz and potassium feldspar become stable at nearly the same temperature and begin crystallizing simultaneously. Rather than forming separate, independent crystals, both minerals grow together in an interlocking fashion. Quartz commonly develops wedge-shaped rods or vermicular blebs enclosed within the growing orthoclase crystal. This simultaneous growth produces the characteristic runic or script-like texture seen in BH250-92. For intro to geology-level students, this specimen is an excellent introduction to the idea that texture records cooling history. They can easily observe that this is very different from ordinary coarse-grained granite and begin asking why quartz appears to be “written” inside feldspar. It opens discussion about intrusive cooling, magma evolution, and pegmatites. For upper-level petrology students, BH250-92 becomes much more sophisticated. It provides an opportunity to discuss: • eutectic crystallization • residual melt evolution • volatile enrichment in granitic systems • diffusion-controlled crystal growth • pegmatite formation • simultaneous nucleation of mineral phases • disequilibrium versus equilibrium crystallization textures Some researchers have proposed that portions of graphic texture may also be enhanced by exsolution processes, where quartz separates from feldspar during late cooling. However, the dominant interpretation remains simultaneous crystallization near eutectic conditions. The minor muscovite present in this sample further supports crystallization from a late-stage evolved granitic melt because muscovite commonly forms in volatile-rich pegmatitic environments. In many systems, graphic granite may occur adjacent to even more evolved pegmatites containing tourmaline, beryl, spodumene, topaz, or rare-element minerals. Source A related resource from which the described resource is derived From the rock collection of Bereket Haileab. Sample BH250-92. 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=554ae8287e3c45089c00f7a16f748e3c" 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=554ae8287e3c45089c00f7a16f748e3c">here</a><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1xS6iBZeQk28Z0D3Ri91iR3Fate5rV5qX/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1PW9CJjcXgWPJ8A42xcanjxaEFp-VYUtj/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1Ky7gUPWM8xH1FvHlmA9Ne-86of1xwsQo/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1OgTWbyEy-05riMgdPfwcuy6O1av1E-nh/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1zWNGxbmW6Xd4o0eDWmfnm4QY7NYzpVMw/preview" allow="autoplay"></iframe> Creator An entity primarily responsible for making the resource Bereket Haileab Cuneiform Texture Eutectic crystallization felsic granite Granite Pegmatite Granitic Differentiation Graphic granite Graphic Texture igneous Late-Stage Magmatism muscovite orthoclase Pegmatite Formation Pegmatite texture Pegmatitic Texture phase diagram Phase Equilibria plutonic Potassium Feldspar Quartz Quartz-Feldspar Eutectic Quartz-feldspar intergrowth Residual Melt Evolution Subject Graphic Granite Vermicular quartz Volatile-Rich Melt