The mineral grains in question are sparse, small, <=0.1 mm, and bright yellow in thin section. Some are microgranular clumps; others resemble the normal thomsonite in habit. The yellow grains are closely associated with euhedral vesuvianite and with normal thomsonite which forms an intergranular matrix to the idiotopic vesuvianite. Other major minerals include ragged, residual grains of FeTi garnet, and poikiloblastic magnetite enclosing euhedral vesuvianite. Locally, the vesuvianite has been partly or wholly converted to hydrogarnet (hibschite). This rock was originally a sandy limestone block, probably a few hundred meters in diameter. The block was emplaced in a major volcanic vent zone and subsequently underwent extensive contact metamorphism and later hydrothermal metamorphism. The yellow grains return a thomsonitelike microprobe analysis except for 1 to 4 wt % MgO and 5 to 6 wt % iron as FeO. Good agreement with zeolite chemical parameters is attained by assuming that the iron is Fe3+ in tetrahedral coordination. Assuming the iron to be Fe2+ negates this good agreement. Observation in transmitted light, reflected light, and the electron absorption image observed prior to the microprobe analysis suggest that the yellow grains are a single phase. If so, these grains are the zeolite thomsonite in which Fe3+ comprises 1/12 or about 8 % of the cations in tetrahedral coordination.