BH250-209c
Title
BH250-209c
Subject
Syenite
Description
Major Minerals: perthite, nepheline, hornblende
Minor Minerals: biotite, sphene, zircon, apatite
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: BH250-209c is a perthite-rich rock and thin section, distinct from the perthites in BH250-209b. Perthite is a texture or intergrowth in feldspar minerals rather than a distinct mineral species. It forms when potassium feldspar (K-feldspar) exsolves sodium-rich plagioclase feldspar (albite) during slow cooling.
Perthite consists 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.
Minor Minerals: biotite, sphene, zircon, apatite
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: BH250-209c is a perthite-rich rock and thin section, distinct from the perthites in BH250-209b. Perthite is a texture or intergrowth in feldspar minerals rather than a distinct mineral species. It forms when potassium feldspar (K-feldspar) exsolves sodium-rich plagioclase feldspar (albite) during slow cooling.
Perthite consists 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.
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 209c. Housed at Carleton College in Minnesota.
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-209c,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/269.