BH250-173c
Title
BH250-173c
Subject
Amygdaloidal Basalt
Description
Major Minerals: orthopyroxene, clinopyroxene, plagioclase
Minor (secondary) minerals (vesicle fills): epidote, zeolite (notably exhibiting distinctive fan-shaped to radial crystal habits), chlorite (pennite variety), quartz
Textyre: amygdaloidal basalt:
Optics: Chlorite (pennite) displays characteristic berlin blue interference colors under cross-polarized light. Zeolites often appear as radiating or fan-shaped aggregates within vesicles, showing low birefringence and subtle extinction zoning.
Amygdules and amygdaloidal basalt
Vesicles in basalt filled with concentrically zoned secondary minerals are a striking result of post-eruption processes, recording interactions between fluids and solidified rock. Vesicles form when volatiles (e.g., H₂O, CO₂, SO₂) escape from magma during eruption. As lava cools rapidly, gas bubbles become trapped, leaving spherical or elongated cavities. After solidification, hydrothermal fluids or groundwater enter the basalt and deposit minerals inside the vesicles — a process known as amygdaloid formation. The infilled vesicles, called amygdules, contain secondary minerals. Amygdules are rounded, almond-shaped mineral deposits. These minerals often form concentric bands due to stepwise precipitation as fluid conditions change (e.g., temperature, pH, or chemistry). Early minerals coat the walls; later ones fill the center. Supersaturation of minerals like calcite or zeolites leads to layered deposition.
Typical textures include:
• Concentric banding – Alternating mineral layers
• Botryoidal texture – Grape-like mineral masses
• Zonation – Color or compositional variation from rim to core
These features not only stand out visually but also reflect the history of fluid-rock interaction and may indicate low-grade metamorphism or hydrothermal alteration. Common secondary minerals include zeolites (see Mara MacDonell '17 comps), analcime, calcite, chlorite, and quartz, each marking shifts in post-volcanic conditions.
Minor (secondary) minerals (vesicle fills): epidote, zeolite (notably exhibiting distinctive fan-shaped to radial crystal habits), chlorite (pennite variety), quartz
Textyre: amygdaloidal basalt:
Optics: Chlorite (pennite) displays characteristic berlin blue interference colors under cross-polarized light. Zeolites often appear as radiating or fan-shaped aggregates within vesicles, showing low birefringence and subtle extinction zoning.
Amygdules and amygdaloidal basalt
Vesicles in basalt filled with concentrically zoned secondary minerals are a striking result of post-eruption processes, recording interactions between fluids and solidified rock. Vesicles form when volatiles (e.g., H₂O, CO₂, SO₂) escape from magma during eruption. As lava cools rapidly, gas bubbles become trapped, leaving spherical or elongated cavities. After solidification, hydrothermal fluids or groundwater enter the basalt and deposit minerals inside the vesicles — a process known as amygdaloid formation. The infilled vesicles, called amygdules, contain secondary minerals. Amygdules are rounded, almond-shaped mineral deposits. These minerals often form concentric bands due to stepwise precipitation as fluid conditions change (e.g., temperature, pH, or chemistry). Early minerals coat the walls; later ones fill the center. Supersaturation of minerals like calcite or zeolites leads to layered deposition.
Typical textures include:
• Concentric banding – Alternating mineral layers
• Botryoidal texture – Grape-like mineral masses
• Zonation – Color or compositional variation from rim to core
These features not only stand out visually but also reflect the history of fluid-rock interaction and may indicate low-grade metamorphism or hydrothermal alteration. Common secondary minerals include zeolites (see Mara MacDonell '17 comps), analcime, calcite, chlorite, and quartz, each marking shifts in post-volcanic conditions.
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-173c. Housed at Carleton College in Minnesota.
Type
Thin section
Relation
Collection
Citation
Bereket Haileab, “BH250-173c,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/220.