BH250-209b
Title
BH250-209b
Subject
Syenite
Description
Major Minerals: Perthite, nepheline, riebeckite, hornblende
Minor Minerals: Biotite (altering to chlorite), sphene, zircon, garnet, augite
Alteration: These thin sections clearly illustrate biotite alteration to chlorite.
Pleochroism: Amphiboles and biotite exhibit pleochroism. Hornblende shows strong pleochroism, shifting from brown or olive-green to yellow-green or pale green depending on crystal orientation. This color variation reflects its iron (Fe²⁺/Fe³⁺) and magnesium content—brown tones indicate higher iron, while green tones suggest higher magnesium or varying iron oxidation states. Hornblende is common in many igneous and metamorphic rocks.
Optics: BH250-209b is a perthite-rich rock and thin section. Perthite is a texture or intergrowth in feldspar minerals, not a distinct mineral species. It forms when potassium feldspar (K-feldspar) exsolves sodium-rich plagioclase feldspar (albite) during slow cooling.
Perthite is an intergrowth of K-feldspar (usually orthoclase or microcline) containing thin lamellae or exsolution veins of albite (Na-rich plagioclase). This texture develops through unmixing (exsolution) of a once homogeneous alkali feldspar solid solution as the rock cools.
Perthite appears as striations, veins, or patchy bands of lighter-colored albite within a darker potassium feldspar matrix. It can be seen under the microscope or sometimes with the naked eye.
Types of Perthite:
Perthite: K-feldspar with albite lamellae (common)
Antiperthite: Albite with exsolved K-feldspar lamellae (less common)
Mesoperthite: Roughly equal intergrowth of K-feldspar and albite
Occurrence:
Perthite is common in granites and syenites, where alkali feldspars crystallize from magma and cool slowly, allowing exsolution to occur.
Minor Minerals: Biotite (altering to chlorite), sphene, zircon, garnet, augite
Alteration: These thin sections clearly illustrate biotite alteration to chlorite.
Pleochroism: Amphiboles and biotite exhibit pleochroism. Hornblende shows strong pleochroism, shifting from brown or olive-green to yellow-green or pale green depending on crystal orientation. This color variation reflects its iron (Fe²⁺/Fe³⁺) and magnesium content—brown tones indicate higher iron, while green tones suggest higher magnesium or varying iron oxidation states. Hornblende is common in many igneous and metamorphic rocks.
Optics: BH250-209b is a perthite-rich rock and thin section. Perthite is a texture or intergrowth in feldspar minerals, not a distinct mineral species. It forms when potassium feldspar (K-feldspar) exsolves sodium-rich plagioclase feldspar (albite) during slow cooling.
Perthite is an intergrowth of K-feldspar (usually orthoclase or microcline) containing thin lamellae or exsolution veins of albite (Na-rich plagioclase). This texture develops through unmixing (exsolution) of a once homogeneous alkali feldspar solid solution as the rock cools.
Perthite appears as striations, veins, or patchy bands of lighter-colored albite within a darker potassium feldspar matrix. It can be seen under the microscope or sometimes with the naked eye.
Types of Perthite:
Perthite: K-feldspar with albite lamellae (common)
Antiperthite: Albite with exsolved K-feldspar lamellae (less common)
Mesoperthite: Roughly equal intergrowth of K-feldspar and albite
Occurrence:
Perthite is common in granites and syenites, where alkali feldspars crystallize from magma and cool slowly, allowing exsolution to occur.
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-209b. Housed at Carleton College in Minnesota.
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-209b,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/268.