https://bereket-haileab.geology.sites.carleton.edu/files/original/62c85998cd25f10692061c06d632eb1e.jpg 93ded0da55051e6091983ac6949c6546 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 BH250-90 Subject The topic of the resource Diorite Description An account of the resource <p data-start="0" data-end="80"><strong data-start="0" data-end="78">BH250-90 — Diorite (Quartz-Bearing Diorite / Transitional to Granodiorite)</strong></p> <p data-start="82" data-end="267"><strong data-start="82" data-end="101">Major Minerals:</strong> plagioclase (albite twinning visible), quartz, orthoclase, hornblende, biotite<br data-start="180" data-end="183" /><strong data-start="183" data-end="202">Minor Minerals:</strong> titanite, zircon, apatite, Fe-Ti oxides, sericite (alteration)</p> <p>BH250-90 is one of the best teaching examples of diorite in this collection because it works beautifully at multiple scales—from hand sample to thin section to geochemistry. The sample was collected from a quarry near<span>&nbsp;</span><strong data-start="582" data-end="611">Little Chicago, Minnesota</strong>, and like much of Minnesota’s diverse bedrock assemblage, it was likely transported and deposited by Pleistocene glaciation rather than representing bedrock exposed at its original emplacement site. This glacial history adds another layer of geological interest, reminding students that hand samples collected from quarries, roadcuts, and glacial deposits may not always reflect the local bedrock geology. In hand sample, the specimen displays the classic<span>&nbsp;</span><strong data-start="1068" data-end="1100">“salt-and-pepper” appearance</strong><span>&nbsp;that makes diorite such a recognizable intrusive igneous rock in introductory geology courses. Light-colored feldspar and quartz are intergrown with darker hornblende and biotite, producing a coarse-grained phaneritic texture that clearly indicates slow cooling beneath Earth’s surface. The crystals are large enough to be easily identified by students in hand sample, making this specimen an excellent introduction to plutonic textures and intermediate igneous compositions</span>.</p> <p data-start="993" data-end="2029">Chemically, BH250-90 confirms what students first observe visually. With <strong data-start="1066" data-end="1085">61.42 wt.% SiO₂</strong>, the rock falls within an <strong data-start="1112" data-end="1140">intermediate composition</strong>, though it sits near the boundary between classic diorite and more silica-rich granodioritic compositions. The relatively high <strong data-start="1268" data-end="1286">Al₂O₃ (16.57%)</strong>, moderate <strong data-start="1297" data-end="1312">CaO (5.01%)</strong>, <strong data-start="1314" data-end="1330">Na₂O (3.93%)</strong>, and lower <strong data-start="1342" data-end="1357">K₂O (2.01%)</strong>reflect abundant plagioclase feldspar as the dominant felsic phase. Meanwhile, <strong data-start="1437" data-end="1457">Fe₂O₃(T) (6.87%)</strong>, <strong data-start="1459" data-end="1474">MgO (2.41%)</strong>, and <strong data-start="1480" data-end="1496">TiO₂ (0.69%)</strong> record the contribution of hornblende, biotite, and oxide minerals. The presence of <strong data-start="1581" data-end="1595">210 ppm Zr</strong>, elevated <strong data-start="1606" data-end="1622">Ba (619 ppm)</strong>, and <strong data-start="1628" data-end="1644">Sr (529 ppm)</strong> is consistent with crystallization from a relatively evolved calc-alkaline magma that retained accessory zircon, titanite, and apatite. This chemistry makes the sample particularly useful for teaching students that igneous rock classification is not always perfectly rigid—this specimen visually resembles diorite, but chemically trends toward <strong data-start="1989" data-end="2028">quartz diorite or even granodiorite</strong>.</p> <p data-start="2031" data-end="2659">Under plane-polarized and cross-polarized light, the thin section reveals a classic hypidiomorphic granular texture. Large plagioclase crystals dominate the rock and show well-developed <strong data-start="2217" data-end="2250">polysynthetic albite twinning</strong>, making them ideal teaching examples for feldspar identification. Some feldspar grains show minor alteration to sericite along fractures and cleavage planes, recording late-stage hydrothermal alteration. Quartz occurs as interstitial anhedral grains with low first-order gray interference colors and lacks strong deformation features, suggesting the rock remained relatively undeformed after crystallization.</p> <p data-start="2661" data-end="3476">The mafic minerals are especially instructive. Hornblende displays moderate to strong pleochroism in plane-polarized light, ranging from greenish-brown to olive tones depending on orientation. Under crossed polars it exhibits moderate birefringence and characteristic amphibole cleavage intersections. Biotite appears as brown pleochroic flakes and shows bright second-order interference colors. Several photomicrographs beautifully capture the relationship between these mafic minerals and the surrounding feldspars. One particularly elegant feature is the presence of <strong data-start="3231" data-end="3285">small apatite inclusions enclosed within amphibole</strong>, an excellent example of mineral inclusion relationships and crystallization sequence. Students can use this texture to discuss which minerals formed early versus later during magma cooling.</p> <p data-start="3478" data-end="3945">Accessory minerals add another layer of petrographic interest. Titanite occurs as high-relief grains with bright interference colors, zircon appears as small accessory crystals, and opaque Fe-Ti oxides contribute to the darker mineral assemblage. These minerals provide excellent opportunities to discuss trace element partitioning during crystallization and why elements such as zirconium, phosphorus, and titanium become concentrated in late-stage accessory phases.</p> <p data-start="3947" data-end="4419" data-is-last-node="" data-is-only-node="">BH250-90 is an outstanding specimen because it bridges introductory and advanced teaching. Beginners immediately recognize its salt-and-pepper hand sample texture, intermediate students can identify minerals optically, and advanced petrology students can connect its mineralogy to whole-rock chemistry and magmatic evolution. It is a specimen that demonstrates how hand sample observations, petrography, and geochemistry all work together to tell a coherent igneous story.</p> <br /><br /><br /><iframe width="1800" height="100" src="https://docs.google.com/spreadsheets/d/e/2PACX-1vS4nyqCmZQGn3t6vX4g4nhDPwHMBaKrigt1LaY2pykWbBGXB1wNb6FYArx625I-JTtD7lyg4uz1OS1r/pubhtml?gid=1710241944&amp;single=true&amp;widget=true&amp;headers=false"></iframe> 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: Little Chicago, Minnesota, USA Source A related resource from which the described resource is derived From the rock collection of Bereket Haileab. Sample BH250-90. 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=96dae91d9256400bb865c7303f320506" 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=96dae91d9256400bb865c7303f320506">here</a><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1pzCSVm-hgUmMbd9zgY-_PBWp3hqy_aW6/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1OlcPTjTX0rWnApvqkq1cyvspG3lX3Daq/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1FjJdM2NyE2wo2pQoUFcIUEiiNuL6UiBx/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/19yd1at-_lWaskZiQceuyPM8rLhK4O1Ab/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1QuLPRYmGr5dq7mbBryySMZiEwdQ6_eyJ/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1VnnJod_Ecb69s3E745XFJAwHPqYLN9yj/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/177QLvv4x0CxMKt4o8Vbz3tJAQ9esrZY6/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1BsEG-ZU5wrHNs28V8Ih4RgB3_EQsNInW/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1p_UpXqotg9UGngb7xFzAgUwI_k3rFasb/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1sZZyDk3V4DKiUk_7B4PYPdkUelfFi2v5/preview" allow="autoplay"></iframe>&nbsp;&nbsp;<br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1Kx_PT_cz8iNM25RnFiECuBwVclfUphvd/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1lWUqcYIGa-aIFoqYiOwh4o8u2EZqxPMN/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1gXvDH1Y2vguoRNaT6BEv_mkSBoafJtpx/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1oXA7W5fSkZWQGDZTLb6q1upWyn9iMP1X/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/15_K8U9rEkg_jbZNxXvVEU9IvTWCINL1P/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1xb5GMMAbT0zrtVJYsWWH9ff9ZPS7KYLJ/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1uCDQlJ3XgFPKV6_btQJqTpRIZ6Y7Sn3X/preview" allow="autoplay"></iframe><br />TAS diagram of sample BH250-90. Creator An entity primarily responsible for making the resource Bereket Haileab Accessory minerals Albite twinning Apatite Apatite in Amphibole Biotite Calc-Alkaline Magma Coarse-grained igneous rock Continental crust magmatism diorite Fe-Ti oxides Granodiorite Transition Hornblende igneous intermediate Intermediate composition Intermediate Igneous Rock Intermediate Magma Intrusive Rock Mineral inclusions orthoclase Phaneritic texture Plagioclase plutonic Plutonic Igneous Rock Quartz Quartz Diorite Salt and pepper texture sericite alteration Sphene (Titanite) Zircon