The rare mineral cymrite, BaAl₂Si₂O₈·H₂O, was discovered in Nevada in a Cambrian bedded barite sequence that exhibits low-grade metamorphism. The mineral occurs exclusively in thin-bedded siliceous rock containing anhedral pyrite crystals up to 1 cm. Cymrite forms rectangular grains ca 40 m across, distributed throughout the chalcedonic quartz matrix. An SEM image of one such blocky grain shows that it is filled by tiny aggregates, instead of a single crystal of cymrite. This cymrite may have replaced a pre-existing rectangular mineral, most likely barite. The Nevada occurrence of cymrite prompted a restudy of its stability relations. Conventional hydrothermal techniques were adopted in the experimental work with run durations up to 7 months. The univariant curve for the dehydration reaction: BaA1₂Si₂O₈· H₂O -BaA1₂Si₂O₈ +H₂O passes the following reversed brackets: 300–315° C at 3 kbar, 290–300° C at a 2 kbar, 270–285° C at 1 kbar, and 240–270° C at 0.5 kbar. These results indicate that cymrite can be stable at much lower pressures than those previously reported. The replacement of barite by cymrite was experimentally demonstrated with an alkaline solution as depicted by the reaction: BaSO₄+2OH^-+A1₂O₃-2SiO₂=BaA1₂Si₂O₈·H₂O+SO ₄ ²