The omphacite in this thin section is variably sized. Larger crystals are interspersed with granular aggregates; all are dusty due to alteration to clays. Interstitial muscovite is common and variation in birefringence indicates differing compositions or degrees of alteration of the micaceous phase.
The groundmass of this basalt is composed of plagioclase microlites with tiny interstitial clinopyroxenes. Vesicles are infilled with chlorite and quartz. The few larger plagioclase phenocrysts have been partially altered to calcite.
According to the sample information sheet, JPN-21 is a crossite-epidote-amphibole-bearing blueschist. Though mineral identification is somewhat complicated by the fine-grained nature of this foliated green rock, the dominant minerals appear to be quartz, chlorite, and calcite or aragonite, all of which are elongated and strongly aligned.
This rock grades from an omphacite-rich zone into a muscovite-rich zone, and yet again into a glaucophane and chlorite-rich zone. The transitions between each section is rather abrupt. Micaceous phases are strongly aligned to define a foliation. Sphene is noticeably less abundant than in other samples. A k-feldspar porphyroclast surrounded by radiating chlorite is observed in the chlorite-glaucophane region.
Randomly oriented, irregularly-shaped omphacite crystals comprise a boudin within the blueschist. In cracks within this boudin are aligned muscovite, glaucophane, and chlorite crystals. In one corner of the thin section, a titanite aggregate forms a reddish inclusion.
The predominant mineral in this thin section is opaque red chert which is cross-cut by veins infilled with chlorite and serpentine. Towards the margins of the thin section is a metamorphosed basalt with a groundmass plagioclase mircolites and quartz, calcite, and chlorite in-filled vesicles and veins. According to collector David Sawyer, the interpillow red chert is an important indicator of deep sea origins.
Allotriomorphic omphacite and clinozoisite are intergrown with one another, with clinozoisite occurring frequently as inclusions within the omphacite crystals. The occurrence of muscovite is typically contained to polycrystalline nodes with radiating crystals. The conspicuous rutile grain in the center with radiating veinlets has a rim of plagioclase buffering it from the surrounding material. It contains inclusions of chlorite and sphene. Sphene is abundant as large, subhedral clasts.
This crenulated foliation is defined by elongate omphacite interleaved with clinozoisite, sphene, chlorite, muscovite, and the rarer glaucophane crystals. Euhedral though fractured garnets contain chlorite, muscovite, and omphacite inclusions. Hexagonal porphyroclasts of randomly-oriented chlorite are scattered throughout.