BH250-81
Title
BH250-81
Subject
Granite
Zircon-rich, REE-bearing perthitic granite
Zircon-rich, REE-bearing perthitic granite
Description
Major Minerals: perthite, antiperthite, albite, hornblende, quartz, fluorite
Minor Minerals: microcline, zircon, biotite, sphene (titanite), spinel, monazite, allanite
Mineralogy Comments: large zoned zircons; hornblende altered to chlorite; allanite mostly altered; euhedral monazite common
BH250-81 is one of the finest accessory-mineral-rich granitoids in the BH250 Collection and serves as an exceptional teaching specimen for mineralogy, igneous petrology, geochronology, and crustal evolution. Although the rock is dominated by feldspar, quartz, hornblende, and biotite, its greatest scientific value lies in the remarkable abundance of zircon, monazite, titanite, allanite, fluorite, and other trace minerals preserved throughout the thin section.
The rock exhibits a coarse-grained phaneritic texture characteristic of slow crystallization at depth. Large feldspar crystals dominate the rock and display spectacular perthitic and antiperthitic exsolution textures. Quartz occurs as anhedral interstitial grains occupying spaces between feldspars, while hornblende and biotite form scattered mafic aggregates. Some hornblende crystals show alteration to chlorite, indicating late-stage fluid interaction after primary crystallization.
The feldspars are among the most instructive features of the sample. Numerous crystals display both coarse patch perthite and fine string perthite produced during slow cooling. These exsolution textures formed when high-temperature alkali feldspars became unstable during cooling and unmixed into potassium-rich and sodium-rich phases. Some crystals display antiperthitic textures in which potassium feldspar occurs as exsolved blebs within plagioclase hosts. Together these textures provide a textbook example of feldspar re-equilibration below the solvus and record the prolonged cooling history of the intrusion.
The most remarkable feature of BH250-81 is its extraordinary population of zircon crystals. Many zircons are unusually large, euhedral, and exceptionally abundant. Several preserve internal growth zoning, while others display extensive networks of fractures and mosaic textures produced by metamictization. Metamictization occurs when the decay of uranium and thorium damages the zircon crystal lattice over geologic time. The resulting crackled appearance provides direct petrographic evidence of radioactive decay and makes these zircons ideal teaching examples for geochronology courses.
Many zircon grains occur in clusters associated with biotite-rich domains and late-stage interstitial phases. Some crystals preserve concentric growth zones that record changing magma chemistry during crystallization. Others display varying degrees of radiation damage, illustrating how crystal structures evolve through time as radioactive isotopes decay. Few teaching collections contain zircon populations that so clearly demonstrate both magmatic growth and post-crystallization radiation damage.
Monazite and allanite are also present and contribute to the rare-earth-element budget of the rock. Monazite occurs as euhedral grains and represents an important host for thorium and light rare-earth elements. Allanite is commonly altered but remains recognizable as a rare-earth-rich accessory mineral associated with late-stage magmatic differentiation. Titanite (sphene) is also present and provides an additional repository for uranium, thorium, and rare-earth elements.
The abundance of zircon, monazite, allanite, titanite, and apatite makes BH250-81 an outstanding natural laboratory for discussing modern geochronologic techniques. These minerals are routinely used for U-Pb, U-Th-Pb, and isotopic studies that constrain crystallization ages, metamorphic events, cooling histories, and crustal evolution. Students examining this thin section can directly observe the minerals that geochronologists separate, analyze, and interpret to reconstruct Earth history.
BH250-81 beautifully illustrates the connection between classical petrography and modern geochemistry. The sample demonstrates how textures observed under the microscope—from feldspar exsolution and amphibole alteration to zircon zoning and metamictization—can reveal processes operating over hundreds of millions to billions of years. It is therefore one of the most valuable geochronology and accessory-mineral teaching specimens in the BH250 Collection.
Minor Minerals: microcline, zircon, biotite, sphene (titanite), spinel, monazite, allanite
Mineralogy Comments: large zoned zircons; hornblende altered to chlorite; allanite mostly altered; euhedral monazite common
BH250-81 is one of the finest accessory-mineral-rich granitoids in the BH250 Collection and serves as an exceptional teaching specimen for mineralogy, igneous petrology, geochronology, and crustal evolution. Although the rock is dominated by feldspar, quartz, hornblende, and biotite, its greatest scientific value lies in the remarkable abundance of zircon, monazite, titanite, allanite, fluorite, and other trace minerals preserved throughout the thin section.
The rock exhibits a coarse-grained phaneritic texture characteristic of slow crystallization at depth. Large feldspar crystals dominate the rock and display spectacular perthitic and antiperthitic exsolution textures. Quartz occurs as anhedral interstitial grains occupying spaces between feldspars, while hornblende and biotite form scattered mafic aggregates. Some hornblende crystals show alteration to chlorite, indicating late-stage fluid interaction after primary crystallization.
The feldspars are among the most instructive features of the sample. Numerous crystals display both coarse patch perthite and fine string perthite produced during slow cooling. These exsolution textures formed when high-temperature alkali feldspars became unstable during cooling and unmixed into potassium-rich and sodium-rich phases. Some crystals display antiperthitic textures in which potassium feldspar occurs as exsolved blebs within plagioclase hosts. Together these textures provide a textbook example of feldspar re-equilibration below the solvus and record the prolonged cooling history of the intrusion.
The most remarkable feature of BH250-81 is its extraordinary population of zircon crystals. Many zircons are unusually large, euhedral, and exceptionally abundant. Several preserve internal growth zoning, while others display extensive networks of fractures and mosaic textures produced by metamictization. Metamictization occurs when the decay of uranium and thorium damages the zircon crystal lattice over geologic time. The resulting crackled appearance provides direct petrographic evidence of radioactive decay and makes these zircons ideal teaching examples for geochronology courses.
Many zircon grains occur in clusters associated with biotite-rich domains and late-stage interstitial phases. Some crystals preserve concentric growth zones that record changing magma chemistry during crystallization. Others display varying degrees of radiation damage, illustrating how crystal structures evolve through time as radioactive isotopes decay. Few teaching collections contain zircon populations that so clearly demonstrate both magmatic growth and post-crystallization radiation damage.
Monazite and allanite are also present and contribute to the rare-earth-element budget of the rock. Monazite occurs as euhedral grains and represents an important host for thorium and light rare-earth elements. Allanite is commonly altered but remains recognizable as a rare-earth-rich accessory mineral associated with late-stage magmatic differentiation. Titanite (sphene) is also present and provides an additional repository for uranium, thorium, and rare-earth elements.
The abundance of zircon, monazite, allanite, titanite, and apatite makes BH250-81 an outstanding natural laboratory for discussing modern geochronologic techniques. These minerals are routinely used for U-Pb, U-Th-Pb, and isotopic studies that constrain crystallization ages, metamorphic events, cooling histories, and crustal evolution. Students examining this thin section can directly observe the minerals that geochronologists separate, analyze, and interpret to reconstruct Earth history.
BH250-81 beautifully illustrates the connection between classical petrography and modern geochemistry. The sample demonstrates how textures observed under the microscope—from feldspar exsolution and amphibole alteration to zircon zoning and metamictization—can reveal processes operating over hundreds of millions to billions of years. It is therefore one of the most valuable geochronology and accessory-mineral teaching specimens in the BH250 Collection.
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-81. Housed at Carleton College in Minnesota.
Type
Thin section
Relation
Collection
Citation
Bereket Haileab, “BH250-81,” BH250 Mineralogy Teaching Collection, accessed October 22, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/91.