BH250-82
Title
Subject
Description
Minor Mineral: magnetite
Optics:
Olivine and pyroxene exhibit strong birefringence, commonly displaying second-order, rainbow-like interference colors. Some olivine grains are cut perpendicular to the optic axis and are good for obtaining a biaxial optic axis figure with a 2V angle of approximately 90°as seen in the video included. Undulatory extinction (strain shadow) is oalso bserved in some olivine crystals, indicative of strain. Olivines are occasionally elongated.
Reaction rim: Reaction rims are present around both olivine and pyroxene grains. hese rims reflect chemical changes between the crystal and melt. Olivine (Mg rich) may react with a more evolved (silica-rich) or oxidized melt during late-stage cooling or eruption. A simplified reaction:
Olivine (Mg,Fe)₂SiO₄ + SiO₂ (melt) → Orthopyroxene (Mg,Fe)SiO₃
Texture:
The rock and thin sections contains euhedral phenocrysts of olivine and pyroxene enclosed in groundmass evidence of two stage cooling.
Hand Sample Description:
Dark grey to black, fine-grained basalt exhibiting a distinct porphyritic texture. Prominent greenish olivine phenocrysts (1–4 mm), subhedral to euhedral in shape, are embedded within a dark, aphanitic groundmass. Scattered vesicles are present throughout the rock, contributing to its vesicular character.
This mineral sample was collected from a trip to Hawaii in 2018. A large hand specimen is also available in the miscellaneous rock collection in Anderson 129.
Coverage
GPS Coordinates: 18°56'4.42"N, 155°38'45.89"W