Phenocrysts of resorbed plagioclase and k-feldspar give this otherwise glassy rock its porphyritic texture. A couple of resorbed pyroxene phenocrysts are present. Aligned needles of an opaque phase give the glassy groundmass a trachytic texture.
The phenocrysts in this basalt are plagioclase, orthopyroxene and clinopyroxene. Most pyroxenes are gathered in glomeroporphyroclasts with plagioclase, though the latter phase is most commonly found as isolated, irregularly twinned and shaped phenocrysts. The groundmass is glass and plagioclase.
Phenocrysts of plagioclase, clinopyroxene, and iddingsite pseudomorphs after olivineâ€“presumablyâ€“are surrounded by a groundmass of unoriented plagioclase microlites, glass, and localized calcite. Plucking of the phenocrysts is noticable and vesicles are present.
Amidst the groundmass of glass, randomly oriented plagioclase laths and equant to anhedral patches of secondary serpentine, are strewn subhedral phenocrysts of plagioclase and twinned clinopyroxene.
Subhedral phenocrysts of plagioclase, clinopyroxene, and olivine are scattered randomly throughout this basalt. Plagioclase has a sieve texture and is frequently zoned. The corners of most grains are rounded. Both clinopyroxene and olivine are generally equant in shape and olivine is more abundant than the pyroxene. All three phases show some degree of resorption. The groundmass contains moderately-sized crystals of all three phases. This sample is very fresh and unaltered.
The olivine that is so conspicuous in the JPN-3 handsample is conspicuously absent in thin section and represented by a mere 1 or 2 grains. Rather, phenocrysts of plagioclase and pyroxenes dominate the phaneritic phases. Both phases are subhedral and the plagioclase in particular indicates several different generations of growth. Some plagioclase is concentrically zoned, though most grains contain alternating inclusion-rich and poor zones. Inclusions are of pyroxenes and glass blebs. The groundmass contains glass, plagioclase microlites, and tiny pyroxenes.
This seems to be a porphyritic basalt though the phenocrystic phases, presumably augite, are plucked enough as to be virtually nonexistent. The remainder of the thin section is a glassy groundmass with unoriented microlites. The microlitic phases are probably pyroxenes and plagioclase, though color, relief, birefringence, and extinction angles seem incongruent with these minerals.
Two minerals, olivine and augite, comprise the phenocrystic population. Olivine is larger and more abundant. Both are present in the glassy groundmass, along with trace plagioclase laths, and abundant opaques.
The plagioclase crystals in this vesicular basalt display a continuous range of sized from groundmass microlite to sieve-textured, sometimes zoned blocky phenocrysts. The groundmass contains more blocky plagioclase than other lavas from the Bali suite. The pyroxenes are generally subhedral and form glomeroporphyroclasts.
The groundmass of this porphyritic basalt is comprised chiefly of glass, plagioclase, and clinopyroxene, in decreasing proportions respectively. Some plagioclase phenocrysts have sieve textures, some inclusions of glass blebs, others are concentrically zoned, and still more are relatively fresh. Clinopyroxene phenocrysts frequently form glomeroporphyroclasts, sometimes with olivine; several grains have coronas of differing composition growing around them. Both olivine and clinopyroxene are heavily fractured and irregularly shaped.
The groundmass of this porphyritic basalt is predominantly glass with less abundant plagioclase microlites. The most readily-identifiable mineral of the phenocryst population is plagioclase. Of greater abundance are euhedral, opaque pseudomorphs. Rare inclusions of pyroxene are observed within these opaques. Pockets of chalcedony disrupt the otherwise uniform groundmass.
Based on grain size, two populations of pyroxenes and plagioclase exist in this sample; the first are the fine-grained, equant crystals of the glassy groundmass, and the second, the subhedral to anhedral phenocrysts visible in handsample. The inconsistently-shaped plagioclase grains are sometimes zoned and frequently contain inclusions of glass.