BH250-90
Title
Subject
Description
BH250-90 — Diorite (Quartz-Bearing Diorite / Transitional to Granodiorite)
Major Minerals: plagioclase (albite twinning visible), quartz, orthoclase, hornblende, biotite
Minor Minerals: titanite, zircon, apatite, Fe-Ti oxides, sericite (alteration)
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 Little Chicago, Minnesota, 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 “salt-and-pepper” appearance 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.
Chemically, BH250-90 confirms what students first observe visually. With 61.42 wt.% SiO₂, the rock falls within an intermediate composition, though it sits near the boundary between classic diorite and more silica-rich granodioritic compositions. The relatively high Al₂O₃ (16.57%), moderate CaO (5.01%), Na₂O (3.93%), and lower K₂O (2.01%)reflect abundant plagioclase feldspar as the dominant felsic phase. Meanwhile, Fe₂O₃(T) (6.87%), MgO (2.41%), and TiO₂ (0.69%) record the contribution of hornblende, biotite, and oxide minerals. The presence of 210 ppm Zr, elevated Ba (619 ppm), and Sr (529 ppm) 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 quartz diorite or even granodiorite.
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 polysynthetic albite twinning, 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.
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 small apatite inclusions enclosed within amphibole, 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.
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.
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.