BH250-96
Title
Subject
Description
Major Minerals: orthoclase, quartz
Minor Minerals: chlorite, oxides, perthite, microcline, zircon, monazite
BH250-96 is a visually striking pink granite that provides an excellent example of how granitic rocks can preserve both primary igneous textures and later deformation-related overprints. In hand sample, the rock displays the classic appearance of a felsic intrusive rock, dominated by abundant pink potassium feldspar that gives the specimen its characteristic salmon to reddish color. Intergrown quartz contributes lighter gray translucent patches, while scattered dark minerals and minor alteration products provide subtle textural contrast. At first glance, the specimen appears to be a relatively typical coarse-grained granite, but thin section analysis reveals a far more dynamic geological history.
The dominant feldspar phase is orthoclase, accompanied by microcline and abundant perthitic textures, where potassium feldspar hosts exsolved sodium-rich lamellae. These perthitic intergrowths indicate slow cooling at depth, allowing alkali feldspars to unmix during subsolidus cooling. In several grains, feldspars display well-developed tartan twinning characteristic of microcline, while others preserve perthitic exsolution textures that are particularly useful for teaching feldspar crystallography and cooling histories in granitic systems. Accessory zircon and monazite occur in minor amounts but are especially important because they can preserve geochronologic information and provide insight into the timing of crystallization and later thermal events.
Quartz grains in BH250-96 are particularly noteworthy because they exhibit strong undulose extinction (wavy extinction) under crossed polarized light. Rather than going extinct uniformly, individual quartz grains extinguish in sweeping patterns as the microscope stage rotates. This feature reflects internal lattice strain caused by tectonic deformation after the granite had fully crystallized. The quartz behavior suggests that the rock experienced significant stress during later regional tectonic events, likely associated with deformation at relatively low to moderate metamorphic conditions.
Plagioclase feldspars also preserve evidence of deformation. Several grains show bent twin lamellae and subtle kink-like textures, indicating that the crystals were mechanically strained after crystallization. These deformation features are especially valuable because feldspars are often more brittle than quartz and preserve different responses to tectonic stress. Together, the strained quartz and deformed feldspars demonstrate that this granite records both its original magmatic history and a later deformational episode.
Biotite occurs in minor amounts and shows partial alteration to chlorite. Some biotite grains display a distinct green pleochroism in plane-polarized light, suggesting intermediate Fe³⁺/Mg²⁺ compositions. This green coloration may reflect either original compositional variation or partial chemical alteration during later fluid interaction. Chloritization further suggests low-temperature hydrothermal alteration or retrograde metamorphism following emplacement and deformation. Minor opaque oxides are also present throughout the rock.
From a teaching perspective, BH250-96 is an exceptional specimen because it allows students to examine multiple geological processes within a single rock sample: primary felsic magma crystallization, slow cooling and feldspar exsolution, accessory mineral growth, tectonic deformation, and later hydrothermal alteration. Many granites are introduced simply as “quartz + feldspar + mica” rocks, but this specimen demonstrates that granites often preserve far more complex histories. BH250-96 serves as an excellent reminder that intrusive rocks are not always geologically static—they may continue to evolve long after crystallization through deformation, recrystallization, and fluid-rock interaction