Aragonite and prehnite are widely associated in low-grade sedimentary, volcanic, and metaplutonic rocks of the San Juan Islands, Washington. The abundance of prehnite and the apparent absence of the high-pressure mineral lawsonite were thought to indicate that the aragonite had formed metastably. Our work now shows that lawsonite, commonly turbid and with a fibrous habit, also occurs in some of these rocks. The discovery of lawsonite indicates that the San Juan aragonite is probably a stable phase that indicates high-pressure metamorphism. The association of aragonite and prehnite also implies that the stability range of prehnite may extend to pressures above those of the calcite-aragonite transition. The occurrence of lawsonite is compatible with the interpretation that the bed rock of some of the San Juan Islands was subjected to deep and rapid tectonic burial by subduction at a convergent plate boundary.

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