BH250-82
Title
Subject
Description
Major Minerals: olivine (forsterite-rich), augite, plagioclase feldspar, volcanic glass
Minor Minerals: magnetite and other Fe–Ti oxides
BH250-82 is one of the finest olivine basalts in the BH250 Teaching Collection and among the most popular samples used in introduction to geology, mineralogy and petrology laboratories. Collected during a visit to the Big Island of Hawaii, the specimen contains abundant green olivine phenocrysts enclosed within a dark vesicular basaltic matrix. Several hand specimens and thin sections are available, including large samples that beautifully display the characteristic green olivine crystals for which Hawaiian basalts are famous.
This specimen provides an outstanding introduction to basaltic volcanism, mantle-derived magmas, volcanic textures, mineral optics, and igneous petrology. Students can examine the same minerals in hand sample and thin section and relate crystal growth, cooling history, and optical properties to the evolution of basaltic magma.
Geological Significance
BH250-82 originated from a primitive basaltic magma generated within the Hawaiian mantle plume. Olivine was among the first minerals to crystallize from the hot magma at depth. These early-formed crystals remained suspended within the melt as it ascended toward the surface and were eventually erupted as lava.
The abundant vesicles throughout the rock record the escape of dissolved volcanic gases during eruption and cooling. The combination of olivine phenocrysts and vesicular texture is characteristic of many Hawaiian basalts and provides evidence for both deep crystallization and shallow volcanic emplacement.
The high abundance of olivine indicates crystallization from a magnesium-rich magma and reflects the mantle origin of Hawaiian hotspot volcanism.
Hand Sample Description
The rock is dark gray to black and displays a strongly porphyritic texture. Numerous green olivine phenocrysts, typically 1–4 mm in diameter, are scattered throughout a dark aphanitic groundmass. The olivine crystals are generally euhedral to subhedral and are readily visible without magnification.
Abundant vesicles occur throughout the specimen and vary in size from less than a millimeter to several millimeters in diameter. These vesicles formed as volcanic gases escaped from the lava during eruption and cooling. The striking contrast between the green olivine crystals and the dark volcanic matrix makes this one of the most visually appealing volcanic rocks in the collection.
Petrography and Texture
Thin sections reveal a classic porphyritic volcanic texture consisting of large olivine and pyroxene phenocrysts surrounded by a very fine-grained groundmass composed of plagioclase microlites, pyroxene, volcanic glass, and opaque oxide minerals.
The texture records a two-stage cooling history. The large olivine and pyroxene crystals formed during relatively slow cooling within a magma reservoir beneath the volcano. Following eruption, the remaining melt cooled rapidly, producing the fine-grained to partly glassy groundmass.
The groundmass is dominated by abundant plagioclase microlites and tiny pyroxene crystals that exhibit a pilotaxitic texture typical of rapidly cooled basaltic lava. Vesicles are present in both hand sample and thin section, preserving evidence of volatile-rich magma during eruption.
Several olivine crystals contain fractures and small inclusions that developed during crystal growth and cooling. The crystal boundaries are generally sharp and well preserved, indicating that the olivine remained stable throughout the final stages of crystallization.
Optical Mineralogy
BH250-82 is an exceptional teaching specimen for optical mineralogy.
Olivine exhibits high relief, no cleavage, irregular fractures, and bright second-order interference colors under crossed polars. Several grains are cut nearly perpendicular to an optic axis and produce excellent biaxial interference figures. These grains display a large 2V angle approaching 90°, making them ideal for demonstrating optical orientation and interference figure analysis.
Some olivine crystals show subtle undulatory extinction and internal strain features visible during stage rotation. These features likely reflect crystal-growth stresses, magma transport, or cooling-related deformation rather than regional metamorphism.
Clinopyroxene (augite) displays moderate to high relief, characteristic cleavage, inclined extinction, and bright interference colors. The contrast between olivine and pyroxene provides an excellent opportunity for students to learn mineral identification and optical techniques.
Educational Value
BH250-82 is one of the most versatile volcanic rocks in the BH250 Collection and is particularly useful for teaching:
- Basalt petrology
- Hawaiian hotspot volcanism
- Mantle-derived magmas
- Olivine crystallization
- Porphyritic texture
- Two-stage cooling histories
- Vesicular volcanic rocks
- Volcanic gas exsolution
- Optical mineralogy
- Biaxial interference figures
- Determination of 2V angles
- Igneous textures
- Volcanic glass and microlitic groundmasses
- Ocean-island basalt (OIB) petrogenesis
The combination of spectacular hand specimens, excellent thin sections, abundant olivine phenocrysts, and high-quality optical features makes BH250-82 one of the premier volcanic teaching samples in the collection.
Coverage
GPS Coordinates: 18°56'4.42"N, 155°38'45.89"W