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Figure F66. Thin section photomicrographs of altered olivine and groundmass. A. Phenocryst of olivine completely altered to green clay (outer part) and calcite in moderately olivine-phyric lava lobe. (Sample 330-U1377A-6R-2, 64–66 cm; Thin Section 275) (with crossed polars). B. Groundmass highly altered to green and brown clay and slightly altered phenocrysts of plagioclase in aphyric basalt lava body or intrusive sheet (Sample 330-U1377A-3R-1, 68–70 cm; Thin Section 270) (with crossed polars). C. Vesicles (sparsely olivine-phyric lava lobe) filled with thin layer of brown clay at rim and later by white clay and carbonates (Sample 330-U1377A-6R-2, 13–15 cm; Thin Section 274) (with crossed polars). D. Vuggy vein filled with mix of clay minerals, Fe oxyhydroxides, and goethite in aphyric basalt lava body or intrusive sheet. (Sample 330-U1377A-3R-1, 68–70 cm; Thin Section 270) (plane-polarized light).

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