In hand specimen, euhedral pyrite fills vesicles and forms thin (<0.5 mm) veins in slightly altered aphyric, moderately vesicular rhyodacite. Some of this "pyrite" identified with the hand lens may actually be marcasite (see next paragraph) but, unless known otherwise from thin section, is referred to as pyrite. The pyrite is brass colored where euhedral, and it is steel gray on broken surfaces. It is coated in places with a thin film of silica. Pyrite is also intergrown with silica and zeolites in vesicles or is precipitated on silica that lines vesicle walls. Commonly, the core was broken along the veins. Veins for which both walls are present are vuggy, with euhedral pyrite. Vesicles within a millimeter of a pyrite vein commonly contain pyrite and silica, and there is one example (Section 193-1191A-2R-2 [Piece 16]) of pyrite, marcasite, and silica in a 1-cm-wide halo of alteration on either side of a marcasite vein. This marcasite was identified in thin section (see next paragraph). Anhydrite is not observed in the veins or vesicles. Even though some veins contain close to 100% pyrite or marcasite, they are not sufficiently numerous to raise the sulfide content of the samples to the 5% required to be considered a sulfide rock.
Figure F7 shows that the euhedral vein marcasite (thought to be pyrite in hard specimen) in Section 193-1191A-2R-2 (Piece 16) has cores of framboids. The framboids are 0.01-0.05 mm in diameter and individual granules are 0.5-1 µm in diameter. The framboids look like pyrite, but pyrite framboids are typically associated with organic-rich sediments and form in the water column just above the seafloor at a redox boundary (Wilkin and Barnes, 1997). Therefore, these framboids are possibly made of the iron sulfide spinel, greigite (Fe3S4), which is indistinguishable from pyrite in thin section. It is also possible that greigite framboids recrystallized into pyrite, preserving the framboidal texture.