BH250-60
Title
BH250-60
Subject
Basalt; Basanite
Description
Major Minerals: hauyne, pyroxene, titanaugite, plagiocalse
Minor Minerals: olivine, aegirine, oxide, biotite, apatite
Mineralogy Comments: hauyne, biotite is groundmass
Texture: porphyritic, vesicular, zoned titanaugite
Optical Features: Titanaugite displays anomalous interference colors. Aegirine-augite zoned by titanaugite crystals. Altered pyroxenes to iddingsite (clay minerals, iron oxides) due hydration and oxidation processes.
In these thin sections, the early-formed aegirine-augite becomes unstable and is rimmed by or replaced with augite. This transformation may result from fractional crystallization, where silica increases in the melt, and/or from changes in oxygen fugacity. Under the microscope, this is evident as zoning, with greenish aegirine-augite cores surrounded by brownish augite rims. You may also observe reaction textures, such as partial replacement or corona structures, a decrease in pleochroism toward the rim (as augite is less pleochroic), and changes in extinction angles and interference colors across the grain.
Note on Anomalous Interference Colors:
Titanaugite contains significant titanium (titaniume oxides of 3.7 wt. % in this rock), which enhances pleochroic absorption. This strong absorption alters the behavior of light as it travels through the crystal, causing deviations from the typical interference colors expected from its birefringence. These anomalous interference colors are primarily due to intense absorption linked to titanium and iron content, a phenomenon known as anomalous interference coloration.
Minor Minerals: olivine, aegirine, oxide, biotite, apatite
Mineralogy Comments: hauyne, biotite is groundmass
Texture: porphyritic, vesicular, zoned titanaugite
Optical Features: Titanaugite displays anomalous interference colors. Aegirine-augite zoned by titanaugite crystals. Altered pyroxenes to iddingsite (clay minerals, iron oxides) due hydration and oxidation processes.
In these thin sections, the early-formed aegirine-augite becomes unstable and is rimmed by or replaced with augite. This transformation may result from fractional crystallization, where silica increases in the melt, and/or from changes in oxygen fugacity. Under the microscope, this is evident as zoning, with greenish aegirine-augite cores surrounded by brownish augite rims. You may also observe reaction textures, such as partial replacement or corona structures, a decrease in pleochroism toward the rim (as augite is less pleochroic), and changes in extinction angles and interference colors across the grain.
Glomerocryst, an aggregate or clump of two or more crystals (usually phenocrysts) that appear to have grown together or accumulated before or during crystallization.
BH250-60 was specifically collected and prepared for the study of the mineral hauyne. The sample was collected by Sarah Greene Hoffmann ’05 during a University of Wisconsin petrology field trip to the Canary Islands. There are 15 thin sections available, each containing beautiful crystals of hauyne.
Hauyne is a sodium-rich, sulfate-bearing feldspathoid belonging to the sodalite group. It plays an important role in the petrology of silica-undersaturated igneous rocks. Common rock types in which hauyne occurs include phonolite, nephelinite, tephrite, and hauyne-bearing basanite.
Note on Anomalous Interference Colors:
Titanaugite contains significant titanium (titaniume oxides of 3.7 wt. % in this rock), which enhances pleochroic absorption. This strong absorption alters the behavior of light as it travels through the crystal, causing deviations from the typical interference colors expected from its birefringence. These anomalous interference colors are primarily due to intense absorption linked to titanium and iron content, a phenomenon known as anomalous interference coloration.
Coverage
Location: Canary Islands
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-60. Housed at Carleton College in Minnesota.
Contributor
Bereke Haileab
Sarah Greene Hoffmann, '05
Sarah Greene Hoffmann, '05
Type
Thin section
Relation
Collection
Citation
Bereket Haileab, “BH250-60,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/69.